[Comments or questions]
Copyright © 1997-2001 jsd
17 Multi-Engine Flying
Q: In an underpowered twin, what is the role of the
A: It doubles your chance of engine failure, and
it will fly you to the scene of the accident.
17.1 Normal OperationsIn normal conditions, operating a twin is not very
different from operating a heavy, high-powered, complex single.
Even so, the weight, power, and complexity may be more than you
are accustomed to. In particular:
Normal multi-engine takeoff procedures are discussed
in section 13.6. Most of the rest of this chapter
is devoted to the procedures and principles of engine-out flying.
A typical twin has a much higher stalling
speed than a typical single.1 Other critical speeds are increased in
proportion. One consequence is that you will notice that normal turns
in the traffic pattern require considerably more room.
- The rate of climb will be considerably higher,
so you will reach pattern altitude sooner. The pitch
attitude during climb will be higher.
- A higher-performance airplane will typically operate over a
wider range of speeds. For example: in a Cessna 172, you might climb
at 78 knots and cruise at 105 knots (a ratio of 1.3), whereas in a
Seneca II you might climb at 90 knots and cruise at 170 knots (a ratio
of 1.9). When you level off following a climb, the Seneca will take
much longer to accelerate to
its cruising speed.
After the level-off, you let it accelerate for a few seconds, and then
trim it — but then it
will accelerate some more and you will have to trim it some more. You
should plan on prolonged acceleration and repeated
17.2 Engine Out ScenariosThis section discusses some of the things you might observe when an
engine fails, and what you can do in response.
17.2.1 Rejected Takeoff; Balanced vs. Unbalanced FieldAirliners are not allowed to take off unless the available
runway exceeds the
balanced field length;
that is, the runway must be sufficiently long that even if an engine
fails at the most critical point during takeoff, the airplane can
either stop on the remaining runway, or continue the takeoff and get
safely airborne on the remaining engine(s).
In contrast, general-aviation light twins are not required to restrict
themselves to balanced field takeoffs. Suppose you lose an engine
during the takeoff roll on a short runway. Even if your plane would
have done fine on a long runway, on a short runway there will be a
period during the middle of the takeoff roll where you can neither
stop nor continue safely on one engine. In such
a case you must shut down the good engine and apply the brakes. It is much
better to hit the trees at the end of the runway when you are
``almost'' stopped then to hit them when you are ``almost'' at full
flying speed. This seems obvious on paper, but when you are in the
cockpit it takes a lot of willpower to actually shut down the good
engine. Think about this. Promise yourself that you will do it
For our next scenario, suppose you are at a reasonable
altitude, climbing with full power on both engines. Then one
engine fails. One of the first things that you will notice is
that the single-engine rate of climb
is not half of the two-engine rate of climb. No, indeed! The
reason is simple: as shown in figure 17.1, when an engine
is shut down, you are not splitting the difference between two-engine
performance and level flight; you are splitting the difference
between two-engine performance and a zero-power descent.
The power curves in the figure are roughly
representative of a Piper Apache, a well-known light twin trainer.
Point A corresponds to the two-engine best rate of climb, 1150 fpm at
86 knots. Point B corresponds to the single-engine best rate of
climb, 160 fpm at 82 knots. (These numbers apply to a fully-loaded
aircraft at sea level in the clean configuration.) We see that the
single-engine rate of climb is less than 15% of the two-engine rate
At density altitudes above 5000 feet the Apache cannot
climb at all on one engine. Also, if the engines, propellers,
and paint job are not quite factory-new, the performance will
be even less than these book values suggest.
You must not allow yourself to think that just because airliners can
climb with an engine out, your favorite light twin can climb with an
It is legal to operate a light twin with anemic or nonexistent
single-engine climb performance. In such cases, engine failure at low
altitude is perhaps the most critical situation that arises in general
aviation with any appreciable frequency. Like a single-engine
aircraft with partial power failure, you need to make a forced
landing. The problem with the twin is that (because of the asymmetric
thrust) if you mishandle the situation, your chance of getting into a
spin is much higher than it would be in a single.
On the other hand, even if you are not climbing, you are probably not
descending very fast. You can treat it as another ``noisy
glider'' situation as mentioned in section 15.1. If
you start out several thousand feet above the ground, you can probably
travel dozens of miles while gradually descending. Look around and
find a nice place for a forced landing.
Generally, the best way to fly any airplane is to keep the airflow
aligned with the fuselage. Alas, in a multi-engine airplane with
asymmetric thrust, it can be rather tricky to perceive just what the
slip angle is. The most direct way to get information about this
angle is to use a slip string.
To create a slip string, tape a piece of yarn to the nose of the
airplane, in front of the center of the window where you can see it.
Leave a foot or two of yarn dangling free, so it will align itself
with the airflow.
In typical aircraft, the fuselage accelerates the sideways flow, more
than it accelerates the fore-to-aft flow. This increases the
sensitivity of the string. That is, the angle of the string is larger
than the actual slip angle.
Even in ordinary single-engine airplane, slip strings work
surprisingly well. The straight-back component of propwash decreases
the sensitivity somewhat, and the helical
component of propwash biases the string slightly to one side.
The slip string is commonly referred to as a ``yaw string'', even
though it measures the slip angle, not the yaw angle (i.e. heading) or
yaw rate. As discussed in section 19.6.3, the slip angle
measures the angle between the fuselage and the relative wind, whereas
yaw is defined relative to some fixed spatial direction. Heading and
heading change (i.e. yaw rate) are easy to perceive by looking out the
window, while it is not easy to perceive slip angle except by
reference to a slip string. Heading can also be perceived using the
directional gyro (heading indicator), and yaw rate can be perceived
using the rate-of-turn gyro
or by observing the rate of motion
of the directional gyro.
The inclinometer ball is often referred to as the slip/skid
instrument, but that is a misnomer. It measures inclination. As we
shall see in section 17.2.5, it is quite possible for
the airplane to be inclined but not slipping. Again: there is no
good way to determine the slip angle without a slip string.
17.2.4 CoordinationFor our next scenario, imagine that you are in level flight at cruise
airspeed at a comfortable altitude. Let's also suppose that your
airplane has a slip string installed. Then (surprise!) an engine
fails. To simplify the discussion, let's suppose the right-hand
engine2 is the one that
quits. You will immediately notice that the airplane will develop a
slip angle. In this case, the airplane will yaw to the right,
shown in figure 17.2. This is because the drag of the
airplane is more or less symmetrically disposed, while the thrust is
now quite asymmetric. As always, two forces with a lever arm
between them create a pure torque.
: Engine Out — Torque due to Asymmetric Thrust
This torque will produce an initial heading change. This will be a
pure yaw; that is, it will change the direction the airplane is
pointing without any immediate change in the direction the
airplane is going. You will observe that the slip string is
off-center to the right, indicating an asymmetric airflow. Then,
after a short time (a second or so), the torques will come back into
equilibrium, because of the airplane's natural yaw-axis stability (as
discussed in section 8.2). That is, the uncoordinated
airflow hitting the rudder will create a torque that opposes the
At this point, you could either (i) sit there and
be a spectator, as shown in figure 17.3, or (ii)
press on the left rudder pedal to center the slip string, as shown
in figure 17.4. From the point of view of directional
control, your choice doesn't matter very much. That is, in case
(i) the airflow strikes the whole airplane (rudder included) at
a nonzero angle, while in case (ii) the airflow strikes the airplane
at a zero angle, with the rudder deflected. In either case, the
amount of tail force produced is approximately the same. The
most important difference is that the airplane will climb better
in case (ii), because the airflow will be aligned with the fuselage.
Let's see what happens after the yaw-axis torques have returned to
equilibrium. Let's assume for now that you are keeping the wings
level. In case (i), the airplane will make a steady turn toward the
dead engine. This is obviously a boat turn, due to
the uncoordinated airflow striking the fuselage, as discussed in
section 8.10. This will be a genuine CM-turn (as defined in section 8.8), changing the
direction of motion of the center of mass; the heading will follow
the CM-turn in order to maintain a constant slip angle.
What is perhaps more surprising is that even in case (ii), if you keep
the wings level the plane will make a CM-turn toward the dead engine
(toward the right in this case). It will not turn as rapidly as in
case (i), but it will turn nonetheless. The reason is that the rudder
force, in addition to creating a torque, is creating an unbalanced
force. This force is changing the direction of motion of the center
of mass. A possible (but non-optimal) way to stop this turn would be
to apply even more pressure on the left rudder pedal, which would
create a wings-level non-turning slip, as will be discussed below in
conjunction with figure 17.5. For now, though, let's
consider the correct strategy, which is to keep the slip string
centered and apply a bank (to the left, in this case) to stop
the turn. This is shown in figure 17.4. This uses a
leftward component of lift3
paired with the rightward rudder force. Once again we have a pair of
forces with a lever arm between them, i.e. a pure torque. This
lift/rudder pair cancels the thrust/drag pair discussed above.
To reiterate: when engine trouble develops, the first result of the
asymmetric thrust is to make the airplane yaw toward the dead engine.
The airplane changes its heading immediately (whereas only later
does it gradually change the direction it is going). That is, a
slip angle develops immediately. If you don't deflect the
rudder, the slip angle will grow until the uncoordinated airflow
striking the rudder develops enough torque to stop further yawing.
This is the basic yaw-axis stability mechanism as discussed in section 8.2. The result is that the airplane does not
spin around and around like a Frisbee — it just develops a few
degrees of slip angle and then stabilizes.
17.2.5 Perception and Initial ResponseIf an engine fails suddenly, the initial yaw toward
the dead engine is usually quite noticeable. On the other hand,
if it fails gradually, the initial yaw (heading change) may be
harder to perceive than you might have guessed. It is especially
hard to perceive if it occurs during a turn — the turn just proceeds
a little faster or slower than normal. The subsequent boat turn
may not be super-easy to perceive, either.4
There are three basic ways to perceive and deal with
the slip and yaw.
At this point, you will find yourself maintaining a rudder
deflection and a bank angle, both toward the side with
the working engine. Use the rudder deflection (not the bank!) to
identify which engine has failed. The mnemonic is: working foot,
working engine; dead foot, dead engine. Specifically, if the right
foot is not being used to deflect the rudder, bend your right
knee. Raise that knee an inch or two, pat it a couple of times, and
say ``right engine has failed''. (More on this later.)
If you happen to have a slip string installed,
the procedure is simple. If the string is deflected to one side,
step on the rudder pedal on the opposite side, until the string
is centered. The mnemonic is: ``Step away from the string''.
- A less-elegant, less-accurate technique is to
use the inclinometer ball. If the ball is deflected to one side,
step on the rudder pedal on the same side. The mnemonic is: ``Step
on the ball''. Center the ball, then (to establish zero slip,
as discussed below) relax the pedal force to let the ball go off
center about one-third to one-half of its width.
- A third technique (which is really the most common
technique) is to roll the wings level and then apply the rudder
as needed to stop the boat turn. The advantage of this procedure
is that it can be done without reference to instruments. The
main disadvantage is that it doesn't help you regain or retain
control in a turn.5 (There are rare situations where
even though an engine has failed you might want to be turning.)
Then, once you've got the wings level and the turn stopped,
you should establish the optimal zero-slip condition, by raising
the dead engine a few degrees and releasing some of the rudder
To maintain zero slip, you will need to bank the
plane very slightly toward the working engine. The mnemonic is: raise the dead. This also implies that the
inclinometer ball will be slightly displaced toward the working
engine. This correct procedure (figure 17.4) requires
slightly more aileron and slightly less rudder than
you would need for wings-level, ball-centered, non-turning, uncoordinated
flight (figure 17.5).
Having the slip string centered but the inclinometer
ball not centered may seem a bit counterintuitive, so let's examine
the aerodynamics of the situation a little more closely.
The asymmetric thrust produces a yaw-axis torque,
which cannot remain unopposed. The rudder is part of the solution,
but remember that while the rudder is producing the desired torque
it is also producing a force. We need the forces to be in balance,
as well as the torques.
Suppose you try to maintain zero bank instead of
raising the dead. Initially the airplane is not in equilibrium,
because the rudder is producing an unbalanced force toward the
dead-engine side. There are then two possibilities:
Suppose the sideways force remains unbalanced.
This will cause the airplane to turn. This will be a wings-level,
coordinated turn. I call this a pseudo boat turn. Unlike an
ordinary boat turn, the airflow is coordinated along the fuselage,
but unlike a regular turn, the horizontal force is not coming
from the wings.
- Suppose you push a little harder on the rudder pedal,
establishing a slip toward the dead engine. The ball is centered, the
wings are level, and the rate of turn is zero. (Any two of those
things implies the third, regardless of engine status.) The forces
are in balance because there is enough uncoordinated airflow over the
fuselage to create a sideways force that balances the rudder force.
In this situation, as shown in figure 17.5, you are using
the fuselage as an airfoil. The problem is that the fuselage has a
really poor lift-to-drag ratio. The sideways fuselage lift force is
accompanied by a huge drag force, which steals energy from you. It
would be much better to use the wings, as previously discussed in
conjunction with figure 17.4.
The proper technique is counterintuitive, because
in any normal situation, proper coordination implies that the
rate of turn will be proportional to the amount of bank. (See
section 11.5.2, for example.) With an engine
out, though, proper coordination requires a slight bank when you
are not turning.
The amount of bank for a typical airplane can be
estimated using the following argument: The lift-to-drag ratio
of the airplane is roughly ten-to-one. In level flight the thrust
must therefore be one tenth of the lift. The lever arm between
the wings and the rudder is typically about three times the lever
arm between the thrust and the drag. Since the torques must cancel,
the rudder force (and the horizontal component of lift) must be
one third of the thrust, and therefore one thirtieth of the lift.
We conclude that the horizontal component of the lift is one
thirtieth of the total lift. One thirtieth of a radian is two
degrees — not exactly a huge bank.
To find out exactly how much bank you need to maintain coordinated
airflow over the fuselage, it helps to use a slip string. At a safe
altitude, set up for single-engine flight at VYSE. Apply enough
rudder pressure so that the slip string indicates
zero slip angle. Bank as required to maintain
nonturning flight. Experiment with slightly greater or lesser rudder
pressure, to see what produces the best climb performance.
You will discover that in optimal single-engine flight,
the inclinometer ball is not centered, but the slip string
is centered. The airplane is inclined, but it has zero slip
angle. Make a note of how much inclination is indicated by the
inclinometer ball; typically it will be off-center by one-half
or one-third of its diameter. You can use this information to
set up a good approximation of engine-out coordinated flight during
subsequent flights when you don't have a slip string installed.
The inclinometer ball measures the inclination of the wings
relative to the E-down7
direction. The inclinometer is sometimes referred to as a slip/skid
ball, but that is a misnomer8
because the slip string (as discussed in section 17.2.3)
provides your only direct information about the slip angle.
Achieving zero slip is the key to optimal climb performance.9 The idea is to have the airflow aligned with the
fuselage. Centering the inclinometer ball is not what
determines performance. Practice with the slip string until you learn
how much inclination is required for a given amount of asymmetric
17.2.6 Yaw Control at Reduced SpeedsSo far, we have discussed engine-out climb rate (section 17.2.2) and discussed the value of maintaining coordinated
flight (section 17.2.4). We now begin a discussion
of airspeed. As you might imagine, this is rather important.
So, let's consider what happens when airspeed
variations are added to the previous scenario. Let's assume
you start out at cruise airspeed and gradually decelerate. Again,
to simplify the discussion, let's assume the right engine has
The amount of asymmetric thrust does not depend on airspeed; it
depends only on the power output of the engine. In contrast, the
amount of force the rudder produces depends on the airspeed squared,
and on the rudder's angle of attack. Therefore as you decelerate you
will need progressively more rudder deflection in order to maintain
zero slip. If you do it right, the sideways force developed by the
rudder will remain unchanged, and the bank angle will remain unchanged
At some point you will run out of rudder deflection. The pedal (or
the rudder itself) will hit the stops. You will be unable to maintain
Now, suppose you continue to decelerate beyond this point. As a slip
develops, the airflow hits the tail and rudder at an angle. This
gives the tail/rudder an angle of attack over and above whatever angle
of attack you created by deflecting the rudder. You are using the
slip angle as a substitute for additional rudder deflection. Up
to a point, this higher angle of attack allows the tail/rudder to
produce a higher coefficient of sideways lift, allowing it to produce
the required force in spite of the lower airspeed.
In addition to the air hitting the rudder, you now
have the uncoordinated airflow hitting the fuselage. You are
relying on the rudder to produce at least 100% of the torque needed
to oppose the asymmetric thrust. The air hitting the fuselage
makes a small unhelpful contribution to the torque budget, and
(more noticeably) contributes to the sideways force budget, producing
an undesirable boat turn. This boat turn is in addition to the
pseudo boat turn that the rudder is producing, so you will need
to increase the bank angle to maintain nonturning flight.
Obviously there is a limit to this process. If you
keep increasing the rudder's angle of attack, at some point the
rudder will stall. Remember,
the amount of asymmetric thrust
does not depend on airspeed, whereas the absolute maximum
amount of force the rudder can produce depends on the airspeed
squared. Therefore, for any nonzero amount of asymmetric thrust,
there must be some airspeed below which the rudder cannot develop
enough torque. At that point there will be an uncontrollable
yaw toward the dead engine. The airplane will spin like a Frisbee.
You might think you could improve the situation by
releasing the rudder pedal, thinking this would reduce the rudder
angle of attack. Alas, it won't work. It will just cause the
airplane to establish a greater slip angle. Remember the rudder
needs to produce a certain amount of force to oppose the asymmetric
thrust, and the airplane's natural yaw-axis stability will adjust
the tail/rudder's angle of attack, trying to create the necessary
If the rudder stalls, it will be about as unpleasant as anything you
can imagine. There will be a sudden uncontrollable yawing motion.
Because of the yawing motion, the wingtip on the side with the good
engine will have a higher airspeed than the wingtip on the other side.
Because of the difference in airspeed (plus the difference in propwash
patterns) the good-side wing will produce much more lift, so you will
get an uncontrollable roll. As the inside wing drops, it will
probably stall (since you were already at a low airspeed). You are
now in a spin. There is no guarantee that it will be possible to
recover from the spin; multi-engine airplane certification regulations
do not require spin recoveries.
On some planes (such as an Apache, a common trainer)
low-speed engine-out performance is limited by the rudder, as
described above. On some other planes (such as a Seneca, another
common trainer) you don't need to worry about the rudder because
the wings will stall first.11 This is not much of an improvement,
because a stall with asymmetric power is also rather likely to
result in a spin.
To prevent such nasty things from happening, you
need to maintain a safe airspeed. The manufacturer gives you
some guidance in this regard, as is discussed in the following
17.2.7 Minimum Control Speed — DefinitionsThe symbol VMC denotes ``minimum
control airspeed''. There are at least four different definitions
of this term, including:
Note that none of these definitions require that
the airplane exhibit a positive rate of climb at VMC.14 Also note that during a VMC demonstration,
the pilot is not required to optimize the climb rate or to maintain
zero slip — although zero slip may be an advantage if it can be achieved.
- FAR 23 (the certification requirements for typical
airplanes) gives a very specific definition of VMC,
FAR 23.149 Minimum control speed.
(a) VMC is the calibrated airspeed at which, when the
critical engine is suddenly made inoperative, it is
possible to maintain control of the airplane with that
engine still inoperative, and thereafter maintain
straight flight at the same speed with an angle of bank
of not more than 5 degrees. The method used to simulate
critical engine failure must represent the most critical
mode of powerplant failure expected in service with
respect to controllability.
(b) VMC for takeoff must not exceed 1.2 VS1, where
VS1 is determined at the maximum takeoff weight. VMC
must be determined with the most unfavorable weight and
center of gravity position and with the airplane airborne
and the ground effect negligible, for the takeoff
(1) Maximum available takeoff power initially on each
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff
- FAR 1 (the ``definitions'' section) defines
VMC as ``minimum control speed with the
critical engine13 inoperative''.
It does not specify any restrictions as to weight, configuration,
altitude, et cetera.
- The FAA Practical Test Standards for the multi-engine
rating call for demonstrating ``VMC''
in a particular way that emphasizes losing yaw control without
stalling the wing or rudder, even though (as discussed below)
for many airplanes VMC (under definition I or
II) is limited by wing stall and/or rudder stall.
- In common parlance, pilots apply the term VMC
to the airspeed where the airplane (multi-engine or otherwise)
becomes uncontrollable, no matter what the reason, no matter what
the configuration, and no matter whether any engine is inoperative.
The VMC number in the Pilot's Operating Handbook is determined
according to the FAR 23.149 definition. This airspeed is marked with
a red radial line on the airspeed indicator, and is sometimes called
the FAR 23.149 redline airspeed.15
There are various ways to lose control; whichever
happens first determines where the manufacturer sets the redline:
Possibility (c) is in some ways attractive, but you have no guarantee
that this is what will happen. Rudder stall depends on slip angle,
so you may be wondering why FAR 23.149 should mention a bank
angle as opposed to a slip angle. Bank does not cause
slip.16 If you want to establish
any connection between bank and slip, you must consider:
- In some airplanes, under some conditions, you
can maintain control, even with an engine out, right down to the
point where wing stalls. This is discussed below in conjunction
with figure 17.6. A stall with asymmetric thrust could
be rather sudden and nasty.
- In others (with a smaller rudder, larger wing,
and/or higher-thrust engines), there will be conditions under
which the rudder will stall before the wings do, as is discussed
below in conjunction with figure 17.8. A rudder stall
could be very sudden and very nasty.
In yet others (larger rudder: area but a shorter
tail-boom, so that the rudder is closer to the
wings), there will
be situations where neither the wing nor the rudder is stalled,
but the boat-turn forces are so large that it requires more than
5 degrees of bank to counteract them and maintain nonturning flight.
The airplane would be perfectly controllable if the bank were
not limited to 5 degrees. Since a bank of 15 or 20 degrees is
not particularly dangerous, the 5 degree limitation must be considered
arbitrary. If your airplane, at a given weight and altitude,
does run up against this limitation, the resulting ``loss
of control'' is neither sudden nor nasty. The airplane will
just make a gentle boat turn toward the dead engine, as is discussed
below in conjunction with figure 17.7.
If any three of these are zero, the fourth is guaranteed
to be zero. More generally, other things being roughly equal,
given any three of these you can estimate the fourth. The problem
is that other things are generally not equal — depending
on weight, airspeed, airplane design, et cetera, five degrees
of bank could correspond to a large slip angle or perhaps no slip
angle at all. So this regulation is not 100% logical.
bank angle (i.e. the angle between wings and
- slip angle (as indicated by the slip string)
- rate of turn
- asymmetric thrust
Some people seem to assign a near-religious significance
to the ``5 degree bank'' mentioned in
FAR 23.149. However, the real significance is quite limited:
One thing we learn from this is that you should not
use bank angle or anything else as a substitute for proper airspeed
This regulation applies to the manufacturer during
certain tests. It does not apply to you in your ordinary flying.
If you have a real engine failure, you are limited only by the
laws of aerodynamics.
- This regulation does not even apply to you during
the checkride for a multi-engine rating. In particular, the FAA
Practical Test Standard says you should bank for ``best performance
and controllability''. Alas, that's inconsistent; best
requires a lot more bank than best performance, and the PTS doesn't
tell you how to make the tradeoff.
- Five degrees is no guarantee of optimum performance.
The optimal bank could be five degrees, or more, or less (usually
- There is nothing in FAR 23.149 or
anywhere else that guarantees the airplane is well behaved at the
``official'' 5-degree bank angle. The maximum bank you can safely use
in nonturning engine-out flight could be five degrees, or
or less.18 In particular, there
is no guarantee that by limiting yourself to 5 degrees you will always
get aerodynamic warning (in the form of a nice, gentle boat turn)
before you get a nasty rudder stall or wing stall. If you want to
demonstrate a gentle warning, you might need to limit yourself to much
less than 5 degrees.
For that matter, airspeed control requires a little
thought, too. Perhaps because FAR 23.149 uses words like ``most
critical'' and ``most unfavorable'', people commonly
assume that it is always possible to control the airplane at
airspeed, no matter what. This assumption is wrong — dangerously
wrong — in many airplanes. For example, there are some airplanes
where the certified takeoff configuration19 calls for the flaps
to be extended, and the FAR 23.149 redline is essentially equal
to the stalling speed in the takeoff configuration. Then if you
operate with the flaps retracted, you will lose control of the
airplane at an airspeed well above redline.20
Specific procedures for dealing with engine failure
are discussed below, in section 17.3.
17.2.8 Effect of Altitude, Weight, etc.FAR 23 tells us that the airplane, when operated
under a particular set of circumstances, can maintain directional
control at redline airspeed. The question is, what happens under
Let's discuss an example. Consider a non-turbocharged
airplane for which the handbook calls for flaps retracted during
takeoff. Then, under standard conditions (takeoff configuration,
maximum weight, etc.), the situation is shown in figure 17.6.
The single-engine stall speed for the example airplane is shown
by a black vertical line in the middle of the figure. The FAR
23 redline is shown as a bright red tick mark on the airspeed
axis. The manufacturer had to set it a knot or two above the
stalling speed, since that is what limits the low-speed handling
for this airplane in this configuration.
: Speed & Altitude affect
Directional Control (basic)
Also, in this figure, the magenta curve shows the
airspeed below which the rudder cannot develop enough force to
oppose the asymmetric thrust. Thirdly, the dotted cyan curve
shows the airspeed below which the boat turn forces are so large
that it would require more than 5 degrees of bank to maintain
Since the example airplane is not turbocharged, as
altitude increases there is less thrust available on the good
engine. The required amount of rudder force declines accordingly.
This is why the magenta and cyan curves trend to the left as
they go up. Note that in this configuration, for this airplane,
rudder performance is not a limitation — the wing stall is the
only relevant limitation.
Now, suppose that several things change:
The new situation is shown in figure 17.7.
You fly at reduced weight, about half of the
- You extend the flaps.21
- You limit yourself to 3 (not 5) degrees of bank.
- You limit yourself to less than full rudder deflection.
Let's consider what happens under these new circumstances.
The reduced weight will lower the stalling speed. Similarly,
extending the flaps lowers the stalling speed. This is indicated
by the black line, which moves to the right as we go from figure 17.6 to figure 17.7.
The amount of torque developed by the engine depends
on altitude in the same way as before, and is unaffected by the
weight, flaps, and other variations. The amount of force the
rudder can produce is also unaffected. Therefore the magenta
curve is the same in the two figures.
At the reduced weight, less lift is needed for supporting
the weight of the airplane. As always, the horizontal component
of lift, at any particular bank angle, is proportional to the
weight of the airplane. Therefore, at any particular bank angle,
you have less ability to oppose a boat turn. This is one reason
why the cyan dotted curve moves to the right as we go from figure 17.6 to figure 17.7.
Limiting yourself to less than full rudder deflection does not
reduce the amount of torque that must be produced in order to oppose
the asymmetric thrust; it just means that the airplane will establish
a slip to create the necessary force.22 In this slipping condition, the
fuselage produces a boat turn on top of whatever pseudo boat turn the
rudder is producing, so you will need more bank to oppose the turn,
and you will run up against the bank limitation sooner. This is the
second reason that the dotted cyan curve (the bank limit) moves to the
And of course, if you limit yourself to a smaller
bank, you will run up against the bank limit sooner. This is
the third reason that the cyan curve moves to the right as we
go from figure 17.6 to figure 17.7. Conversely,
if you allow yourself a large bank (15 or 20 degrees) you can
push the dotted cyan curve very far to the left, as indicated
in figure 17.8.
Finally, let's consider what happens in different
airplanes. For example #2, let's take an airplane where the wing
has a very low stalling speed. For such a plane, figure 17.6
never applies; figure 17.7 (or figure 17.8,
depending on bank angle) applies even at max weight with the flaps
For example #3, let's go to the opposite extreme
and consider an airplane that has somewhat smaller wings. To
compensate, the manufacturer specifies that flaps are to be extended
in the certified takeoff configuration. The result is that the
certified performance of the new plane is identical to the performance
of example airplane #1, as shown in figure 17.6. The
interesting wrinkle is this: if you fly the new airplane with
flaps retracted, the performance is as shown in figure 17.9. Note the higher wing stall speed. The airplane
will become uncontrollable at an airspeed well above the FAR 23.149
Let's summarize this information into a form that
is perhaps more directly useful when you are actually in the cockpit,
planning or performing engine-out maneuvers.
For more information on engine-out procedures, see section 17.3.
Maintain a safe airspeed. This speed should
be above the wing's stalling speed in the current configuration
and above the FAR 23.149 redline, whichever is higher. Leave
yourself a reasonable margin of safety.
- The best procedure involves establishing zero
slip (or minimum slip, if full rudder deflection isn't enough
to establish zero slip), and banking to stop the turn. The amount
of bank increases as the weight decreases; use whatever bank
angle does the job. Remember, though, that maintaining a safe
airspeed is more important than getting exactly the right slip
angle or bank angle.
- In your multi-engine training, you were probably
given the chance to demonstrate ``loss of directional control''
under conditions where the ``loss'' resulted in a gentle
boat turn toward the dead engine. You absolutely must not assume
that the airplane will always behave this way. In other circumstances,
you might get a sudden rudder stall or wing stall, either of which
could result in a spin.
- If you want to demonstrate the gentle boat turn, you can
arrange that it occurs before any of the nastier alternatives,
as suggested in figure 17.7. You just have to put
sufficiently strict limits on the bank and rudder deflection. Reduced
weight helps, too. Turbocharging makes it easier to perform the
demonstration at a safe altitude.
- If you go exploring speeds below redline, things
get dicey. If you allow yourself unlimited23
bank, there is no doubt that you can maintain directional control
right down to the point where the wing stalls and/or the rudder
stalls. You can get a good estimate24 of the wing's stalling speed,
but I can't think of a safe way for you to find out whether or
not the rudder will stall before the wings.25 Please do not try to find this out
17.2.9 Effect of Center of MassWe know that we have to pay careful attention to the location of the
airplane's center of mass, since it has a big effect on the angle of
attack stability; see for example section 6.1.1.
This leads us to wonder what effect center-of-mass position has on
VMC. There are two possible answers:
In both cases, you need to create a torque to oppose
the asymmetric thrust. You create it using a pair of forces with
a lever arm between them. One force comes from the rudder.
CM location has no effect whatsoever if you use the unwise
wings-level technique depicted in figure 17.5.
- CM location does matter if you use the recommended procedure
depicted in figure 17.4. As the CM (or, more precisely,
the center of lift) moves aft, VMC increases.
In case (1), the rudder force is paired with a horizontal
force due to air hitting the side of the fuselage. This fuselage
horizontal lift depends on the shape of the airplane, but does
not at all depend on the CM location.
There is a deep theorem of physics that says that for any two axes
parallel to each other, the torque around one is the same as the
torque around the other (provided there are no overall unbalanced
forces on the system). In the zero-bank case, it means that
VMC can't depend on center of mass location (unless the airplane is
actually turning, i.e. being accelerated sideways).
To understand the basis of this theorem, refer again
to figure 17.5. Let's pick two pivot points A
and B somewhere along the rudder/wing lever arm, as shown in the
figure. (You can, if you wish, imagine them to be two possible
locations of the center of mass; the CM is no better or worse than any
other pivot point.)
When we calculate the total torque around each
The total torque around A is exactly the same
as the total torque around B. The total torque is the only
thing that affects VMC, and that is the same
no matter what pivot point is used.
The lever arm from A to the rudder is
long, but the lever arm from A to the other horizontal
force is short.
- The lever arm from B to the rudder is
short, but the lever arm from B to the other horizontal
force is long.
In case (2), the story is slightly different.
The rudder force is paired with the horizontal component
of lift from the wings, tail, et cetera. This component arises
because you are in a slight bank, as illustrated in
figure 17.4. The location of this force depends
indirectly on the CM location, according to the following
chain of reasoning:
Here's another way of saying the same thing: the location of the lift
vector depends directly on the shape of the airplane, but you have to
adjust the shape of the airplane in order to keep the center of lift
located very close to the center of mass. Note that we are not
talking about the lift of the wings alone, but the lift of the entire
airplane including the tail. In the particular example illustrated in
figure 17.4, the center of mass is located rather far
forward. The tail has been adjusted to produce a negative amount of
lift in order to maintain equilibrium in pitch. The horizontal
component of lift depends directly on this contribution from the tail,
which in turn depends on CM location.
- The large vertical component of lift must be
located very close to the center of mass, to oppose the force of
gravity; otherwise the airplane would be out of equilibrium in pitch.
- The small horizontal component of lift is located
at the same place as the large vertical component.
As the center of lift moves aft, the lever arm between it and the
rudder gets shorter. This means you need more rudder deflection and
more bank to oppose any given amount of asymmetric thrust.
17.2.10 Effect of Drag (e.g. Landing Gear)To reiterate: in engine-out flight you have two problems:
impaired rate of climb, and asymmetric thrust which can lead to
uncontrollable yaw if you're not careful.
You may be thinking that it is possible to counteract
the asymmetric thrust using asymmetric drag. Technically, that's
true, but as we shall see, it isn't particularly practical.
An unrealistically good type of asymmetric drag is shown
in figure 17.10. A source of additional drag (a small
parachute) is attached far out on the wing (on the working-engine
side). Because it has a long lever arm, a modest amount of drag
force will create a significant amount of yaw-axis torque. This
will help you maintain directional control. Of course, the drag
will exacerbate your rate-of-climb problems.
If the parachute is attached at a different point,
the results will be different. If it is attached near the working
engine, as shown in figure 17.11, its contribution
to the yaw budget will be exactly the same as if you had throttled
back the working engine; added drag is the same as reduced thrust.
The effect on climb performance is also the same as if you had
throttled back. Obviously, using the throttle is more convenient
and practical than adding asymmetric drag.
Now we can do a more detailed analysis of how the landing gear
contributes to the yaw-axis stability and equilibrium. Let's take the
gear-up situation as a starting point, and see what differences
arise when you put the gear down.
With the gear up, the forces are in equilibrium: thrust balances drag.
With the gear down, there is extra drag. Eventually equilibrium will
be restored somehow. Let's assume26 the airplane just
slows down, so that the extra drag of the gear will be balanced by
reduced drag on the rest of the airplane.
So we have two new forces: a rearward contribution from the gear, and
a forward contribution from the reduced drag on the rest of the airplane.
First, let's see what happens when the slip angle is zero. In that
case the two new forces are oriented right along the line between
them. This contributes nothing to the yaw-axis torque budget, because
the forces have no component perpendicular to the lever-arm between
Next, let's see what happens when (as shown in figure 17.12)
a slip angle has developed. Once again, the new force on the wheel
will be mostly a drag force, rearward in the direction of the relative
wind. The other new force (the reduced drag on the rest of the
airplane) will act in the opposite direction, centered at a place
called the center of lateral effort.
Now we have a pair of forces with a component perpendicular to the
lever arm. This will create a yaw-axis torque. The torque will grow
in proportion to the slip angle. On most airplanes the nose wheel is
far ahead of the center of lateral effort, so this will make a
negative contribution to the yaw-axis stability.
As a final refinement, we consider the fact that when the wheel meets
the air at an angle (as shown in figure 17.12), it acts a
little bit like an airfoil and produces a force perpendicular to
the relative wind, i.e. a sideways lift force. This force grows in
proportion to the slip angle and makes another negative contribution
to the yaw-axis stability.
To summarize this section:
Of course, during the descent and landing phases,
there are some obvious advantages to extending the landing gear.
Extending the landing gear always creates drag,
which impairs the rate of climb.
- To the extent that the landing gear creates symmetric
drag, it contributes nothing to yaw-axis equilibrium.
- The landing gear typically makes a negative contribution
to yaw-axis stability.27
- Usually the only contribution that is even
theoretically helpful comes from the asymmetry of having one landing
gear in the propwash of the working engine. However, this is
not a practical advantage since you could achieve better rate
of climb, the same equilibrium, and better stability by keeping
the landing gear retracted and slightly reducing power on the
* Other ScenariosThe foregoing assumed that you maintained level flight at constant
engine power, with non-constant airspeed. But this is just one of
The first three cases usually make a negative contribution to
yaw-axis stability; they create a torque when the slip angle is
nonzero. The fourth case is even worse: it creates an unhelpful
torque even when the slip angle is zero.
You might compensate for the added drag of the landing gear by a
reduction in other drag, by way of a speed change (or otherwise). As
discussed in detail above, this causes a torque that depends on the
lever arm between the added drag and the reduction in other drag.
- If/when the airplane is decelerating, while things are out of
equilibrium, there will be a torque that depends on the lever arm
between the added drag and the center of mass.
- You might decide to prevent any speed-change by lowering the
nose, compensating for the added drag by a new component of weight
along the direction of flight. Einstein's principle of equivalence
tells us this is just like the previous case; gravity is equivalent to
an acceleration. There will be a torque that depends on the lever arm
between the added drag and the center of mass.
- You might decide to prevent any speed-change by adding power on
the working engine. This creates a torque that depends on the lever
arm between the added drag and the engine.
Whenever one or more engines are producing power, propeller drag will
cause a rolling moment, as discussed in section 9.5.
You will need to deflect the ailerons to the right to compensate.
Losing an engine will cause additional roll-axis problems on top of
all your other problems. That's because the working engine creates
more propwash over its wing, producing more lift on that side.
You need to deflect the ailerons toward the working engine to
compensate. Many airplanes have aileron trim to help you deal
17.2.12 Critical EngineOn a typical twin, you will notice that the left engine causes more
yaw trouble than the right engine does. There are several reasons for
this, including helical propwash, twisted lift, and possibly P-factor.
First: Helical propwash was discussed in
section 8.4 in connection with single-engine
airplanes. The multi-engine story is partly the same and partly
different. To be specific, let's consider a plane where the engines
rotate clockwise as seen from behind.
In some airplanes, a lot of propwash hits the tail, and you need to
apply right rudder to compensate, just like in single-engine planes.
If your airplane requires right rudder during the initial takeoff
roll, it must be due to propwash; it can't be due to twisted lift
(because there is no lift yet), and it can't be due to P-factor (because
the prop disks are not inclined).
In other airplanes, in normal flight, most of the propwash misses the
vertical tail, as shown in figure 17.13. This causes no
problems, so no compensation is required. Even with one engine out,
as long as you are able to maintain zero slip, most of the propwash
misses the vertical tail, as shown in figure 17.14.
: Engine Out: Helical Propwash Still Has No
If you don't apply enough rudder to maintain zero slip, more of the
tail will move into the propwash, as shown in
figure 17.15. (At low airspeeds, you could easily have a
situation where you can't apply enough rudder to prevent this.)
Since the vertical tail sticks up, not down, the propwash from the
right engine will be rotating in such a way as to reduce rudder
effectiveness.28 If possible, you should apply additional right rudder to
It is a bit ironic that propwash affects the yaw-axis torque budget
more when you already have a big slip angle. Normally you don't
allow that to happen unless you are forced to, so this effect is
usually only noticeable at low airspeeds — such as a VMC
demonstration, or a crosswind takeoff (especially a crosswind from the
In a plane with four propellers, the tail will be much more
affected by the propwash from the inboard engines than from the
outboard engines. By using the engines one at a time, and in various
pairings, you can shed a lot of light on the effects discussed in this
Secondly: As mentioned in section 17.2.11, propeller drag creates
a rolling moment and requires right aileron no matter which engine is
running. This aileron deflection will produce a certain amount of
twisted lift, even though the magnitude of the
lift vector is the same on both sides, as discussed in
section 8.8.4. You will need to apply right rudder to
compensate. This will be most noticeable in high-power low-airspeed
Thirdly: P-factor (asymmetric disk loading) makes a small
contribution to the yaw-axis torque budget. I measured this in a
light twin, as discussed in section 8.5.4, using
both engines. The effect was small, but could be observed if you
looked closely. With only one engine, the effect would be half as
I also calculated from theory that when the airspeed decreases from
cruise to VMC, the corresponding increase in angle of attack causes
the center of effort of the propeller disk to move to the right by
about one inch. That's not zero, but it's not very much, either.
Most of the effects that people blame on P-factor are really mainly
due to a combination of adverse yaw and helical propwash.
To summarize: Some yaw contributions are unbiased, requiring rudder
deflection depending on which engine is out. Working foot, working
engine. These include the asymmetric thrust (as diagrammed in
figure 17.4) and the increased lift over the working
engine's wing (as mentioned in section 17.2.11).
Some other contributions are biased to the right, requiring right
rudder no matter which engine is out. These include helical propwash
acting on the tail, propeller drag acting via twisted lift, and
P-factor. These are what make one engine more critical than the
Terminology: The engine you most regret losing is called the
critical engine. In a twin where both engines rotate
clockwise, that will be the left engine. With the left engine out,
you will run out of rudder authority sooner, because the biased
contributions add to the unbiased contributions. (If the right engine
were out, the biased contributions would work in your favor, reducing
the amount of left rudder required.)
Some twins have counter-rotating propellers. (That is, one engine
rotates clockwise while the other rotates counterclockwise.) In that
case both engines cause equally much yaw trouble, and either (or
neither) can be considered the critical engine.
17.3 Engine Out ProceduresEngine failure is an emergency. You might want to
review the general discussion of emergencies in chapter 15.
Make sure you know the emergency checklist for your airplane.
Not all airplanes are the same. The following discussion applies to a
``generic'' airplane, and serves to illustrate some important
concepts, but should not be taken as a substitute for
During takeoff, it is
important to be able to detect
any problems promptly. Early in the takeoff roll, you should
glance at the gauges (RPM, manifold pressure, and fuel flow) to
make sure the readings are normal — and that both engines are
the same. Make sure the airplane ``feels'' like it is
pulling straight, i.e. no unusual steering effort is required
to keep it going straight.
If anything funny happens while there is adequate
runway remaining ahead of you, close both throttles immediately
and stop straight ahead. In a high-powered airplane, such as
an airliner, there will be a point where it is not possible to
stop on the runway but it is possible to accelerate and fly away
safely on one engine.
In contrast to airliners, typical light twins use
a smaller fraction of the runway for a normal takeoff, yet have
worse single-engine performance. As a consequence, there is a
time even after liftoff when it is better to close the throttles
and re-land on the remaining runway. Indeed, even if the remaining
runway is not quite enough, you might want to land on it: Suppose
that because of density altitude or whatever, your aircraft has
poor single-engine climb performance. You will sustain vastly
less damage if you land and slide off the end of the runway at
low speed, rather than making an unsuccessful attempt to climb
out on one engine.
In many light twins, the climb performance is OK with the landing
gear retracted but very
poor with it extended. Therefore a common
rule is the following: when there is no more useful runway ahead,
retract the gear. If an engine fails before that point, you know you
are committed to landing; if it fails after that point, you know you
are committed to climbing.
17.3.1 Procedure: Low AltitudeOnce you are airborne and assured of single-engine
climb performance, the following checklist applies to our generic
airplane at low altitudes: three things, five things, four things.
Now let's spell each item out in more detail, for
the case where your initial speed is above VMC:
Three things: airspeed, ball and needle.
- Five things: mixtures, propellers, throttles,
- Four things: identify, verify, feather, secure.
Here are the same items again, for the where you
have a fair bit of initial altitude, but your initial speed is
Three things: airspeed, ball, and needle. That
is, pitch to maintain best-climb speed. Then the easiest thing
to do is apply enough rudder pressure to center the inclinometer
on the ball''. This involves more rudder pressure than you
need to establish zero slip, but at this stage of the game you
are in a hurry and centering the ball is a rough-and-ready approximation.
With the ball centered, nonturning flight will require a slight
bank toward the working engine. (Wings-level non-turning flight
is really overdoing it. It involves a slip toward the dead engine,
which puts an unnecessary burden on the rudder and degrades climb
- Five things: mixture controls forward (rich), propeller
controls forward (fine pitch), throttles forward (maximum allowable
power), landing gear retracted, flaps retracted.
- At this point you might want to check airspeed,
ball, and needle again. The airplane has probably decelerated
quite a bit, so you may need to make a pitch adjustment and retrim.
- Four things: identify, verify, feather, secure.
Let's suppose the right engine has failed.
Identify: raise your right knee (dead foot, dead
engine) and say aloud, ``the right engine has failed''.
- Verify: retard the right throttle. There should
be no change in the situation. If you retard the wrong throttle
you will notice immediately; push it forward again, go back to
step 1 (airspeed, ball, and needle), and try again.
- Feather: grab the correct propeller control,
pull it back a little ways and listen to make sure you've got
the right one, then pull it all the way into the feather position.
(If this is a simulated emergency, just pull it back half an inch
or so and tell your instructor that you are simulating the feather.)
- Secure: when the engine has stopped spinning,
shut off its mixture control, its fuel supply, its boost pumps,
its alternator, its magnetos, et cetera. Close its cowl flaps
and open the cowl flaps on the working engine.
- Finally, check airspeed, ball, and needle again.
Make sure you are trimmed for best-climb speed. Establish zero
slip by applying somewhat less rudder pressure than would be necessary
to center the ball. In zero-slip flight the ball will be
off-center by one-third to
one-half of its diameter. Use the rudder trim to hold this arrangement.
In this condition, nonturning flight will require banking a few
degrees toward the good engine: ``raise the dead''.
In the case where your initial altitude and initial
airspeed are both rather low, it may not be possible to regain
VMC by diving. In this case the procedure is
rather simple: you have to close the throttles and make an immediate
Three things: airspeed, ball, and needle. You
urgently need to dive to regain VMC. As always,
``step on the ball'' to get the airflow approximately
coordinated. You may be unable to establish zero slip, even with
full rudder deflection, in which case you should apply full rudder
and let the airplane establish whatever slip is necessary to oppose
the asymmetric thrust. To establish nonturning flight, the wings
should be almost horizontal, with the dead engine raised slightly.
- If you are in danger of losing directional control,
you will have to reduce power on the good engine. Don't worry about
which is which; retard both throttles. Skip the mixture
and propeller controls for now. Retract the gear and flaps.
- Five things: After you have gotten back to VMC,
advance both throttles, both propeller controls, and both mixture
controls. Confirm gear retracted and flaps retracted.
- Continue accelerating to best-climb speed. This
will probably require cashing in additional altitude.
- Four things: identify, verify, feather, secure
— the same as before.
Reading about these things is good, but not sufficient. You really
should to up with an instructor and practice these things. Practice
until the right actions become routine. Review it at least once every
17.3.2 Procedure: Higher AltitudeFinally, here is the procedure for the case where
you have a reasonable airspeed and a reasonable altitude, say
1000 feet AGL or more. You should not be in any big hurry
to feather the offending engine. If the problem is minor, restarting
will be a lot easier if the engine is not feathered. The checklist
Take a systematic approach to debugging. Start somewhere
on the panel and then check everything you come to, systematically.
Three things: airspeed, ball, and needle.
- Five things: mixtures, propellers, throttles,
- Four things: identify, verify, debug, think.
It doesn't hurt to be logical, but remember that
in an actual emergency, you will be much less logical than you
normally are. Unless it is obvious what the problem is, check
everything, in order. Don't just check the things that come to
mind. Systematic habits are more likely to stay with you.
After you've checked everything once, then try applying
logic. What was the last thing you fiddled with before the failure?
Did you just shut off the fuel boost pumps? Maybe you should
switch them back on; look at the fuel pressure... or did you
miss the boost pumps and turn off the magnetos instead? Did you
just switch from the inboard to the outboard tanks? Maybe you
should switch back, or switch to crossfeed.
Remember that you may be unable to climb or even
maintain altitude on one engine. See section 17.2.2
for a discussion of this.
17.3.3 Airspeed ManagementThe airspeed that gives the best single-engine rate of climb is
referred to as VYSE. The value of VYSE for standard conditions (max
weight, sea level, etc.) is marked on the airspeed indicator by
a blue radial line, and is commonly called blueline airspeed.
If an engine fails, you should (except in certain
special situations) maintain a speed at or above VYSE.
Maintain thine airspeed lest the ground arise and smite thee.
One exception to the foregoing rule: If you need altitude to avoid an
obstacle, you'll be better off at VXSE
(best angle of climb) as opposed to VYSE (best rate of
climb). In typical trainers, the single-engine performance is so
anemic that VXSE will be only slightly slower than VYSE, for
reasons illustrated in figure 7.8. Indeed, if you are above the
single-engine absolute ceiling, the climb
rate is negative and VXSE is slightly faster than VYSE.
Another exception: The optimal airspeed on final approach is typically
less than VYSE. You're not climbing, so you don't need to worry
about climb performance (unless you need to go
around, as discussed in section 17.3.4).
Yet another exception: Suppose
your airplane has enough single-engine climb performance that the
zero-climb speed VZSE is significantly slower than VYSE. (See
figure 7.7.) Further suppose you lose an engine at night at low
altitude over a dark forest, at a very low airspeed. You don't want
to dive all the way to VYSE, because that could take you into the
trees. A much more modest dive will produce a speed above VZSE.
Thereafter you can accelerate in level flight, or climb at constant
airspeed. In this scenario you don't need best rate of climb as
long as you have some rate of climb.
Another relevant airspeed is the minimum control
airspeed, VMC. As discussed in section 17.2.7,
you could get into big trouble if the airspeed gets too much below
VMC. At any speed above VMC
you should apply full power on the good engine and accelerate
to best-climb speed. Don't be shy about diving to get to best-climb
speed; remember the airplane might not be able to climb or accelerate
at all at lower speeds.
At speeds below VMC, you will be forced to use less than full
power on the good engine, to keep the yaw from getting out of hand
while you accelerate to VMC. Losing an engine at an airspeed below
VMC is a really nasty situation which (for obvious reasons) most
people don't practice during training. To recover, you have to
partially close the throttle on the good engine, which takes a lot of
willpower. You don't have much time to think. Then you have to dive,
cashing in quite a lot of altitude to get the needed airspeed. The
usual procedure calls for accelerating to VMC plus a few knots, to
give yourself a little margin, before returning the good engine to
17.3.4 Engine-Out Go-AroundsThe first thing to be said about engine-out go-arounds
is that you should make every possible effort to make sure that
you do not ever need to perform one. The most common reason for
a go-around is that you are about to land long and run off the
end of the runway. Therefore, if at all possible, fly to somewhere
that has a really long runway before attempting any engine-out
The second thing to be said is that for typical airplanes there is a
certain height above the ground — often a surprisingly great height
— below which an engine-out go-around is simply not possible. The
reason for this is simple: the typical approach speed is quite slow
— not only below VYSE (best-climb speed) but near or even below
VZSE (zero-climb speed, as defined in figure 7.7). If you try to
climb out at low airspeed, the rate of climb may well be negative. In
order to accelerate from approach speed to any reasonable climb speed,
you will need to cash in quite a lot of altitude. You will also
consume time (and altitude) while you retract the landing gear, et
cetera. In a Seneca, the decision to go around must be made above 400
feet AGL; below that altitude, you are going to touch down. If
the runway is obstructed, land on the taxiway, or the infield, or
whatever. If you have enough runway to touch down but not stop,
consider doing a touch and go (which works better if you leave the
gear down). Also consider landing anyway, with the expectation of
going off the end at low speed; this is vastly preferable to hitting
obstructions at high speed during an unsuccessful go-around.
17.3.5 Low-Speed Engine-Out DemonstrationsThere are several key ideas I want my students to
know about low-speed engine-out performance, including:
The rest of the discussion assumes you need the maximum achievable
power from the good engine.
- If you are ever in a position where you can descend to a safe
landing without using high power on the good engine, by all
means do so. This is not a game where you get extra points for
climbing when you don't have to.
The aircraft manufacturer is supposed to specify
a minimum safe speed for intentional engine cuts, denoted VSSE,
which is typically quite a bit higher than VMC.
- Starting from moderate speeds, as you slow down
you will need more and more rudder to maintain coordinated flight.
This is the coordinated regime. The amount of bank needed to
maintain nonturning flight is basically constant.
- There comes a point where you run out of rudder
authority and cannot maintain coordinated flight. As the speed
decreases further, the slip angle automatically increases, and
more boat turn gets added to the pseudo boat turn. This is the
uncoordinated regime. The bank angle must increase as airspeed
decreases if you want to maintain nonturning flight.
- You can maintain control down to VMC (i.e. FAR 23.149 redline)
in the takeoff configuration.30
- If you persist in engine-out flight down to sufficiently
low airspeed, at some point the wings and/or rudder will stall
and you will be very sorry.
- If you are below VMC, you should
reduce power on the good engine, dive to regain VMC,
and then re-open the throttle on the good engine.
- If you are below VYSE, you should dive to
regain VYSE, obstructions permitting.
- To clear distant obstacles, you want to dive to achieve
VXSE as soon as possible. To clear nearby obstacles, you don't
want to dive below their altitude, obviously. For a combination of
obstacles, you face some tricky tradeoffs. The best solution is to
make sure you never get into a low-altitude low-airspeed situation.
To demonstrate these key ideas, you should start
in the takeoff configuration at a speed at or above VSSE.
Then cut one engine, and gradually decelerate. This will demonstrate
idea #1 immediately. If there is a chance you will reach VMC
before you have a chance to demonstrate idea #2, it
is a good idea to artificially
limit the available rudder deflection, perhaps by blocking the
pedal with the toe of your other shoe. We do not wish to demonstrate
idea #4. After demonstrating flight slightly above VMC
(idea #3), return to VYSE (idea #6) and then
resume normal flight.
To demonstrate a portion of idea #5, we use a separate
maneuver. Starting with both engines at idle, perform a power-off
stall. Recover to VMC, then using only one
engine, recover to VYSE.
The FAA commercial pilot multi-engine practical test standard
(``PTS'') contains a ``task'' called ``ENGINE INOPERATIVE —
DIRECTIONAL CONTROL DEMONSTRATION''. The requirements are a bit
confusing. For one thing, the PTS speaks of banking ``for best
performance and controllability'' but doesn't say how to trade off
performance versus controllability. Best climb performance typically
requires less bank than best ultra-low-speed controllability.
Among many examiners, the traditions concerning this
task are as follows:
Before the checkride, you should discuss these unwritten
rules with your examiner, to make sure you are both singing the
Start at a safe altitude and safe airspeed.
The PTS calls for VYSE plus ten knots.
- The PTS calls for flaps set for takeoff. However,
there are some planes (such as a Seneca) where the certified takeoff
checklist calls for zero flaps, and where redline is essentially
equal to the stalling speed. In such planes you can lower the
stalling speed by extending the flaps, which will make the demonstration
safer and easier. Most examiners are happy to permit this. Call
it ``short field'' takeoff configuration if you like.
In other planes (such as an Apache) where the stalling speed
is already well below redline, don't bother extending the flaps.
- Reduce power on one engine to idle. Do not actually
stop or feather the engine.
- Depress the rudder to establish zero slip. This
gives best performance.
- Bank to establish nonturning flight. This will
be a very shallow bank.
- Block the rudder motion to ensure that you run
out of rudder before the airspeed gets close to the edge of the
envelope. Stay away from redline and wing-stall speed, whichever
is higher. The PTS calls for staying 20 knots above the wing-stall
- Gradually decelerate.
- After you run out of rudder deflection, the unwritten
rule is that you should not increase the bank. That means the
airplane will start to turn. The turn is your signal that it
is time to begin the recovery phase. You are not being
asked to demonstrate key idea #2.
The airspeed limit is needed to ensure safety. The
artificial limits on rudder deflection and bank are needed so
that you can demonstrate a nice gentle boat turn, by pretending
to run out of control authority; otherwise the airplane would
be controllable at all safe airspeeds and there would be nothing
Note that in everyday (non-checkride) flight, if
you run out of rudder authority at a speed above redline (and
if you are sure you want to be flying so slowly) you would just
smoothly enter the uncoordinated regime and increase the bank.
You should not do demonstrations the way FAR
23.149 seems to suggest:
Full-blown FAR 23 VMC determinations
should be left to professional test pilots. For that matter,
not even test pilots dare to experiment with loss of control at
low altitude. They are not crazy; they experiment at a series
of safe altitudes and then extrapolate.
Do not suddenly shut down one engine at a low
airspeed. Shut it down at or above VSSE and
then decelerate. (Alternatively, I suppose it would be safe to
fly below VSSE and gradually reduce power
on one engine, but I can't think of a reason why you would want
- Do not explore low-speed performance at low altitude.
The practical test standard calls for a minimum of 3000 feet
single-engine aircraft is required (by FAR 23.49) to
have a stalling speed of 61 knots or less, and in certain models it is
quite a bit less. This is important for safety in case of a forced
landing. In contrast, a twin with sufficiently good engine-out
performance is exempt from this restriction. The theory is that a
twin that can climb on one engine should never need to make an
- We won't discuss aircraft that have
centerline thrust, e.g. the Cessna 337 Mixmaster.
- This means total
lift, including the contributions of the wings, horizontal tail, et
cetera. The center of lift will be located quite close to the center
of mass, for reasons discussed in section 6.1.3.
effects are reduced if the working engine is developing less power
(because of small engine size, high altitude, and/or reduced throttle
setting), the airspeed is high, and the rudder is large. Conversely,
in a twin with a small rudder, a large engine, full power, and
a low airspeed, a sudden failure will definitely get your attention.
- If you forget to
roll the wings level before using the rudder to stop the heading
change, you could easily find yourself stepping on the wrong rudder.
For instance, if you are in a turn to the right and the left
engine fails, you might be tempted to stop the turn by stepping
on the left (wrong!) rudder.
- To know how much bank and/or
how much inclinometer ball deflection corresponds to zero slip,
you can (a) recall from your training flights what configuration
corresponds to best performance, (b) recall from flights with
a slip string what configuration corresponds to zero slip, or
(c) let the inclinometer ball go off-center by half its width,
which is usually ``close enough'' to the right answer.
- See section 19.4 for a
discussion of E-down and related concepts.
- The confusion
is understandable, since asymmetric thrust is about the only way you
can maintain an inclination without being in a slip of some kind.
- ... or cruise performance, for that matter — engine-out or
- In fact, an undeflected rudder
produces a less stall-resistant shape, which will probably stall
at a higher airspeed.
- If redline
(as defined in the following section) is down near the bottom
of the green arc, it is a good guess that wing stall is what limits
the airplane's low-speed controllability. Conversely, if redline
is much higher than the bottom of the green arc, you can guess
that rudder stall is what limits the low-speed controllability
(unless the redline is artificially high because of the arbitrary
5 degree bank limit in FAR 23.149).
- A very similar regulation,
FAR 25.149, applies to transport-category aircraft (e.g. airliners).
- The notion of ``critical
engine'' is discussed in section 17.2.12.
- If the airplane weights more than
6000 pounds, FAR 23.66 requires the airplane to be able to climb
with one engine inoperative, at an airspeed ``equal to that
achieved at 50 feet'' after takeoff. Even this does not require
climb at VMC.
- There are of
course other redlines: at the high end of
the airspeed indicator, on the tachometers, etc., but they are not
relevant to the present discussion.
- ... except perhaps in a minor way that is not
relevant here. See section 11.5.6 for a general
discussion of slip angle versus bank angle.
- There are many airplanes that are
quite nicely behaved even under conditions that require more than 5
degrees of bank in order to maintain non-turning engine-out flight.
- That's right: there is no
guarantee that 5 degrees is safe. It is commonly assumed that ``the
manufacturer must have tested a 5 degree bank because that is the
maximum allowed''. But in fact, best control might be achieved at 2
or 3 degrees, and there is no reason to assume that the manufacturer
ever tried more than that. Remember, in non-slipping flight the bank
required is quite modest (and independent of airspeed), and there is
not much that the manufacturer can achieve by slipping that could not
be better achieved by more rudder deflection.
means the takeoff configuration as specified in the Pilot's Operating
Handbook or Airplane Flight Manual. Remember that these documents
are legally part of the airplane. You can't have a certified
VMC without a certified takeoff checklist.
this example, we are assuming that wing stall (not rudder stall)
is what limits low-speed handling.
option is available to us because, for this airplane, the certified
takeoff configuration did not call for flaps to be extended.
there were an unbalanced torque, the airplane would not only turn, it
around the yaw axis.
are talking about rather modest bank angles, perhaps 15 or 20
degrees, that would not get you into trouble in other circumstances.
may be a good idea to check the wing's stalling speed by performing
a stall with both engines at zero thrust. The zero-engine stalling
speed won't be quite the same as the one-engine stalling speed,
but it should be a useful estimate.
have done calculations that indicate that for certain light trainers,
the wings will almost always stall before the rudder, but you
absolutely should not assume that this is true for all airplanes
in all circumstances.
- At the end of this
section, other possibilities will be considered.
- One could imagine
designing an airplane with the landing gear so far aft that they
were behind the lateral center of effort, in which case they would
increase yaw-axis stability.
- The propwash from the left engine is actually
a little hard to see how this situation could arise in the course
of normal flying. However, (a) such a situation is sometimes
created as part of a training exercise, and (b) it could arise
if the pilot mishandles an engine-out situation, squandering the
other configurations, you can maintain control down to redline or
VS, whichever is higher.
- In airplanes where VMC is at
or near VS, VS + 20 may seem like a generous or even excessive
margin of safety. In other airplanes, however,
VS + 20 is not
nearly enough. You need to be careful, since there are plenty
of airplanes where VS + 20 is near (or even below) VMC. A better
criterion might be to stay above VS + 10 and above redline + 10,
whichever is higher.
[Comments or questions]
Copyright © 1997-2001 jsd