[Comments or questions]
Copyright © 1996-2001 jsd
11 Slips, Skids, and Snap Rolls
You should learn from the mistakes of others, because
you'll never have enough time to make all those mistakes yourself.
— Ben Franklin
11.1 A Lesson on Snap RollsOne fine spring day I was instructing
a student who had about 5 hours experience. This was her first
lesson in slow flight, but she was doing really
well: she was maintaining the assigned altitude, the assigned
heading, and the assigned airspeed (a couple of knots above the
stalling speed). She was also doing a good job of keeping the
inclinometer ball in the center, which required
considerable pressure on the right rudder pedal because of the high power and low airspeed. I
was really enjoying the flight, but suddenly I developed a feeling
that there was something wrong. Gradually it dawned on me what the
problem was. The problem was that the airplane was
Here's what had happened: her right foot had gotten
tired, so she just removed it from the pedal — all at once. This
produced a sudden yaw to the left. Naturally the left wing dropped,
so she applied full right aileron. The nose was dropping, too,
so she pulled back sharply on the yoke. The next thing anybody
knew, we were upside down.
I took the controls and rolled the plane right-side-up.
We lost about 500 feet of altitude during the maneuver. The
student asked ``What was THAT?'' and I said ``That
was a pretty nice snap roll''.
This is indeed the recipe for a snap roll: starting from a speed
slightly above the stall, apply a sudden yaw with the rudder, apply
opposite aileron, and pull back on the yoke. SNAP! — One wing
stalls and the plane rolls over. In our case, we didn't roll exactly
180 degrees — ``only'' about 135
degrees — but that's upside down enough for most people. It took a
fraction of a second.
In due course the student completed her training
and got her license. She's even still speaking to me. There are
a number of points to be learned from this adventure:
Let me reiterate: Piloting an airplane at low speeds
requires using the rudder pedals. If you don't know how to do
this correctly, you have no business trying to land, take off,
or anything else.
Although the airplane we were flying (a Cessna
152) has a reputation for being a docile aircraft, you should
remember that even pussycats can bite! It is all too easy to
picture the same thing happening just after takeoff, with insufficient
altitude available for a recovery. This is the classic stall/spin
accident that figures so prominently in the accident statistics.
Don't get complacent — it could happen to you.
- This is why we practice slow flight and stalls
a few thousand feet above the ground, and make sure there are
no other aircraft nearby.
- This is why we make sure that fire extinguishers,
chocks, tow bars, etc. are secure, not floating around in the
back of the plane. I don't want to be picking them out of my
ear during the snap-roll recovery.
- This is why we insist on maintaining coordinated
flight (except for intentional slips, which are a special case).
Keep the ball in its house. Don't apply right aileron without
also applying right rudder. Don't apply left aileron without
applying left rudder.
11.2 Intentional SlipsA slip is performed by lowering one wing with the ailerons, and
then applying opposite rudder. This is called ``top rudder'' because
you are pressing the rudder pedal on the same side as the raised wing.
Because the rudder is deflected, the air will be
flowing somewhat across the fuselage. This creates much
more drag than the usual streamlined flow
along the fuselage.
This is often useful for dissipating energy, e.g. for making
the aircraft descend more rapidly on approach. This is always
done at an airspeed well above the stall.
The crosswise flow not only creates drag (a rearward
force) but also creates a sideways force that tends to change
the direction of flight, as discussed in section 8.10.
A slip, therefore, can be viewed as a boat turn in one direction
(because of the crosswise flow), possibly combined with an ordinary
turn in the other direction (because of the bank angle).
If you match the rudder deflection to the bank angle just right, no
net turn results. This is called a nonturning slip.
Nonturning slips are used during crosswind landings, as discussed in
section 12.9. The upwind wing is lowered using the
ailerons and the opposite rudder is used to prevent the aircraft from
turning. The idea is that the bank determines the direction the
airplane is going, while
the rudder determines the direction the airplane is
The idea is to make sure, despite the crosswind, that the direction of
flight and the axis of the airplane are both aligned with the
The definition of a slip ``to the left'' versus a slip ``to the
right'' is a bit arbitrary and hard to remember. The following table
may help. In a slip to the left:
``left'' slip, matching statement
the airplane is moving toward a point that is somewhere to
the left of the nose
the nose is pointing to the right of the direction of flight
the air is hitting the left side of the fuselage
you are applying right rudder
the inclinometer ball is displaced to the left
the slip string is displaced to the right
if this is a nonturning slip, you are banked to the left
if you are not banked, you are making a boat turn to the right
Because of the potential for confusion, I try to avoid the term
``slip to the left'' entirely. Instead, I might say ``let's make a
boat turn to the right'' or ``let's lower the left wing and perform a
Some folks try to define the terms side slip and forward
slip, but it is not worth making such a distinction. In
figure 11.1, depending on your intentions, you could be making
a side slip to runway 11 or a forward slip to runway 9. But you could
change your intentions at any point before the flare. During the
approach, you are performing a nonturning slip toward the runway
intersection, and that's all that need be said. Aerodynamically
speaking, there is no meaningful difference between forward slip and
side slip. Both are synonymous with nonturning slip.
: Side Slip Indistinguishable From Forward Slip
In some aircraft the airspeed indicator is grossly perturbed by a
slip, as mentioned in section 2.12.7.
In a typical Cessna 152/172/182, depending on the amount of slip, the
airspeed can easily be off by 20%, which means the energy is off by
40%. This is enough to cause real trouble. In some less-common
aircraft, you can send the airspeed needle below zero.
Suppose the static port is on the left side of
the fuselage, and
suppose you are in a slip to the left (the kind that requires pressing
on the right rudder pedal). In this situation, the left (upwind) side
of the fuselage is a high-pressure point. This high pressure cancels
some of the dynamic pressure in the Pitot tube, so the airspeed
indicator will lose airspeed.
Now suppose you are in a slip to the right. The static port is now on
the downwind side. This will not be a low-pressure point. In
fact, it could have almost as much high pressure as the other side,
because of pressure recovery, as indicated in figure 4.9. More likely, there will be will be relatively little
pressure recovery, as illustrated figure 4.10, and the static
port will measure something rather close to the real static pressure.
Therefore: In a slip toward the side with the static port, expect the
airspeed indicator to lose a lot of airspeed. In a slip toward the
other side, expect a smaller loss.
Better yet, ignore the airspeed indicator and determine the angle of
attack by looking out the window. Observe
the pitch attitude relative to the direction of
flight. Don't forget that because of drag caused by the slip, the
new direction of flight will be angled more downward. Find a new
landmark that remains a fixed angle below the horizon.
The term skid denotes a particular type of slip that occurs
when the airplane is in a bank and the uncoordinated airflow is coming
from the side with the raised wing. Typically this happens because
you have tried to speed up a turn using ``bottom rudder'', that is, pressing the
rudder pedal on the same side as the lowered wing.
I will use the term proper slip to denote a slip that is
not a skid.
If you have plenty of airspeed, the aerodynamics of a skid is the same
as the aerodynamics of a proper slip. In both cases there is air
flowing crosswise over the fuselage. However, you should form the
habit of not skidding the airplane, for the following reason.
If the aircraft stalls, any slight crosswise flow
will cause one wing to stall before the other. In particular,
having the rudder deflected to the right means the aircraft will
suddenly roll to the right. If the aircraft is in a 45 degree
bank to the right and rolls another 45 degrees in the same direction
(because you were applying right rudder pressure), it will reach
the knife-edge attitude (wings vertical). If on the other hand
you were holding top rudder (still
holding right rudder but banking to the left this time), a sudden
roll of 45 degrees would leave you with wings level (which is
a big improvement over wings vertical).
If the wings are level, you can make a proper slip to the left or to
the right; a skid is impossible by definition.
* Bottom Rudder: Right vs. WrongIt is appallingly easy to set up a situation that
leads to an unintentional skid. Suppose you are ready to
make a left turn
from base to final. You start the turn improperly, by
applying a little left rudder. The crosswise
airflow pattern acting on the dihedral of the wings will cause
the airplane to bank to the left and make a relatively normal
turn in the desired direction. You absent-mindedly maintain the
left rudder pressure, so the bank continues to steepen. You decide
to apply right aileron to prevent further steepening of the turn.
That's all you need: you are in a skidding left turn, holding
left rudder and right aileron, at low altitude. If you stall,
you'll never be heard from again. Seriously, folks, this could
happen to you.
Never apply more bottom rudder
than is needed to center the inclinometer ball.
There are only a few cases where bottom rudder is appropriate, for example:
Note that in all these cases you apply only enough
bottom rudder to maintain coordinated flight. Do not skid!
If you are already turning to the left and use
left aileron deflection to steepen the turn, you will need left
rudder deflection in proportion to the aileron deflection, because
of adverse yaw and yaw-axis inertia, as
discussed in section 8.8.
- In a steady bank to the right during a low-airspeed,
high-power climb, you may need some right rudder deflection to
compensate for engine torque effects (mainly the effect of the
helical propwash hitting the vertical
fin and rudder, as discussed in section 8.4).
- In a long-winged airplane in a steep turn at
low airspeed, the long-tail slip effect
will require you to hold bottom rudder to maintain coordination,
as discussed in section 8.9.
- In a multi-engine airplane with
one engine inoperative, you need to bank the airplane toward the side
with the working engine, and deflect the rudder toward the same side,
as discussed in section 17.2.5.
11.4 Anticipate Correct Rudder UsageAs discussed in chapter 8, there are
four or five things that can cause the airplane to yaw. Your
job is to use the rudders to eliminate the unwanted yaw, so that
the airplane is always pointing the way it is going.
The objective is to anticipate how much rudder
is required in various circumstances, so you aren't constantly
correcting for errors.
The hardest thing to deal with is yaw-axis inertia.
The rule is: rolling to the left requires left rudder; rolling
to the right requires right rudder. The amount of rudder pressure
should be proportional to the rate of roll. Adverse yaw complicates
the situation, and requires rudder deflection whenever the roll
rate does not match the aileron deflection. To a fair approximation
the two effects can be covered by the rule: ``rudder deflection
proportional to aileron deflection''.
Note that (unlike yaw-axis inertia) adverse yaw occurs
even if you aren't turning. Suppose that a wind gust causes the
left wing to drop. You immediately use right aileron to raise
the wing. Right rudder is required. Don't get the idea that
rudder is only required when you intend to turn.
Another tricky case arises when you make your first
left turn after takeoff. You are holding a large amount of right
rudder pressure because of the helical propwash, and you need
to apply left aileron. Rather than using actual left rudder pressure,
it probably suffices to use a reduction in right rudder pressure.
This is harder to learn than it sounds. You may find it more
convenient to maintain whatever right-rudder pressure is
required to compensate for the helical propwash, and to make left
turns by applying countervailing force on the left rudder pedal.
If you have a rudder trim control, by all means use
it to compensate for the helical propwash effect.
11.5 Perceiving Slip, Perceiving Coordination
11.5.1 Looking Out the SideTo learn good coordination, first practice looking out the side. When
you roll into a turn, you should see the wing go up or down like a
flyswatter. If it slices down-and-forward, or up-and-backward, you
are not using enough rudder.
You can control the airplane quite nicely while looking
out the side. You can judge pitch attitude by the angle the wing
chord makes with the lateral horizon. You can judge bank angle
by the height of the wingtip above or below the horizon. And,
as just mentioned, you can judge coordination by watching for
forward or backward slicing motions when you roll into or out
of a turn.
It is really important to be able to do this. Just
for starters, there is no way you can do a decent job of scanning
for traffic if you can't control the airplane precisely while
looking out the side.
I even have my students do fancy things like stalls
(and stall recoveries) while looking out the side.
Don't be a ``gauge junkie'' — the sort of
pilot who can't even fly a rectangular traffic pattern except
by reference to the directional gyro. When making a 90 degree
turn, identify a landmark 90 degrees from your original heading
and turn toward it. No gauges are required.
11.5.2 Looking Out the FrontThe next step is to learn how to perceive correct coordination while
looking out the front. This requires having a precise visual
reference. There are several ways to arrange this.
Start by getting the airplane trimmed for straight and
level flight at a reasonable airspeed, headed toward a definite point.
In the figure, the plane is headed toward a point a couple of degrees
to the right of the mountains.
You can now use your finger as a reference, as shown in figure 11.2. Rest your hand on the top of the instrument panel
and align your finger with the straight-ahead point on the horizon.
Another option is to use a mark on the windshield, as shown by the red
wedge in the figure. It really helps to have a mark that falls very
close to the line from your dominant eye to the aim point on the
horizon, so if you can't find a scratch or a bug-corpse in just the
right point, you should make a mark. You can use a grease
pencil, a washable marking pen, a bit of tape, or whatever.
A single reference of this sort only works if your head is in the
right position — wherever it was when you established the reference.
Since you need to move around to look for traffic, be careful to move
back into position before using the reference.
On the other hand, if you use both a finger and a mark on the
windshield, you can easily detect if your head is out of position.
This also helps rule out the image from your non-dominant eye
(although the easiest thing is to close that eye if it is confusing
Figure 11.3 shows how the situation should look after rolling
smoothly into a turn to the right with 30 degrees of bank.
You should use the visual reference as your primary indicator
of pitch attitude and heading. Throughout the roll-in, turn,
and roll-out, the rate of turn (i.e. the rate of heading change)
should be proportional to the amount of bank. As the bank increases,
the rate of turn should increase.
The rate of turn should be proportional to the amount of bank.
It is common mistake to think that the airplane should simply pivot
around the roll axis and then start turning. If you look
closely, you will see in figure 11.3 that the sight line has
already moved to the right a little. This represents the amount of
turn that occurred during the roll-in. (If you roll in more slowly,
this amount will increase.) Remember: The rate of turn should be
proportional to the amount of bank. If the sight mark initially
stands still (or backtracks!) and only later starts turning in the proper direction, it
means you aren't applying enough rudder to compensate for yaw-axis
inertia (and adverse yaw).
During the roll-out, the same rule still applies: the rate of turn
should be proportional to the amount of bank. As the bank goes away,
the rate of turn should go away. (Of course, the proportionality
factor always depends on airspeed, but at each airspeed there is a
definite proportionality between bank and rate of turn.) If you
neglect to compensate for yaw-axis inertia, the nose will
toward the continuation of the turn).
Summary: Don't let the nose backtrack on roll-in. Don't let the
nose overshoot on roll-out. The rate of turn should be proportional
to the amount of bank. The yaw-axis inertia and adverse yaw lead
to the rule: rudder deflection should be proportional to aileron
You can see in figure 11.3 why we went to the trouble of
putting a mark on the windshield, rather than using, say a bolt on the
cowling at the location marked by the cross in the figure (near the
end of your thumb). Such an off-axis reference will not exhibit a
rate of turn proportional to the amount of bank. As you can see, the
problem is that the cross necessarily rotates a little ways to the
outside of the properly-coordinated turn. If you tried to prevent
this reference from swinging to the outside of the turn, you would be
applying too much rudder during the roll-in. The amount of the error
would depend on the angle between the bogus sight line and the actual
roll axis — which depends on the shape of the airplane and the
height of the pilot.
Once you have learned to make really good turns using the roll-axis
sight mark, you should gradually learn to do without it. Make
a point of imagining where the mark would be relative to
other visual references such as the cowling, the window-frame,
Later (after making a few hundred coordinated turns) you should
be able to do it with your eyes closed, just by knowing how the
controls should feel.
By the way: as you may have noticed, the sight line in figure 11.3 is slightly above the horizon. This is because
you need to pitch up a little bit to deal with the load factor
in the turn.
Notes: (1) If you make a mark on the windshield, use a bright color,
since black is too hard to distinguish from traffic. (2) The best
time to make the mark is before takeoff. Taxi into position at the
end of a long taxiway, and make a mark that lines up with the horizon
at the far end. Even if the pre-flight mark is not perfect, it will
facilitate making a better mark later.
11.5.3 Using the Inclinometer BallThe inclinometer ball will
remain centered throughout the roll-in, turn, and roll-out if
everything is done correctly. There are two drawbacks: (1) The
response of the ball to coordination errors is sluggish and complex,
so you have to be quite an expert to get useful feedback from
it. In particular, I see lots of pilots who apply approximately
the right amount of rudder, but apply it too late or too early.
Diagnosing such errors using just the ball is not worth the trouble.
(2) In general, anything that can be done by outside references
should be done by outside references.
The inclinometer ball definitely is helpful for providing information
about a long-term slip — in particular, for telling you how much
rudder trim to dial in during a high-power / low-speed climb.
Especially in an unfamiliar airplane, it can be hard to tell whether
one wing is down a little bit, without referring to the ball.
11.5.4 Using the Seat of Your PantsThe phrase ``flying by the seat of your pants'' has become
such a common cliché that people forget its real meaning:
you can use the sense of touch in your rear end to determine whether
or not your control usage is properly coordinated.1
The idea of using your rear end as an inclinometer might sound trivial
or obvious; after all, even non-pilots can notice immediately if they
sit down on a park bench that is inclined. But the non-pilots are
probably cheating, using their sense of sight (referring to the horizon) and their sense of which way
is up (based on the acceleration-detecting organs in the inner ear).
In the airplane, as you roll into a turn, the situation is much more
challenging. First of all, you want the load vector (gravity plus
centrifugal force) to be directed straight down into your seat
(perpendicular to the wings, not to the horizon) — so visual
reference to the horizon doesn't tell you what you need to know about
inclination. Secondly, the organs of your inner ear are sensitive not
only to the load vector but also to the rate of roll — so they don't
tell you what you need to know, either.
This is a good illustration of why learning to fly
an airplane is hard: the airplane is inclined (relative to the
horizon) but it is not inclined (relative to the load vector).
One sense (sight) conflicts with two others (inner ear and seat
Because the sense of sight is so dominant, the visual
references discussed in section 11.5.1 and
section 11.5.2 are the typically the easiest way to
proper coordination. But you should also pay attention to what
the seat of your pants is telling you. If you are being sloshed
side to side as you roll into a turn, there is something wrong.
It may help to close your eyes while the instructor makes a series
of coordinated and uncoordinated turns.
While we are on the subject of the sense of touch:
as you get experience with a particular airplane, you will learn
how much force is required on the rudder to go with a certain
amount of force on the ailerons (depending on airspeed, of course).
Once you've got the feel of the controls, you should be able
to make a decent turn without much thought or effort.
Flying by the seat of your pants may sound like a
throwback to the days when airmail was carried in fabric-covered
biplanes, but it is a useful technique even in modern instrument
flying. Proper coordination is still important, and modern airplanes
still suffer from yaw-axis inertia and adverse yaw, especially
at approach speeds. As you maneuver to stay on the localizer,
you don't want to be looking at the inclinometer ball — you've
got too many other things competing for your visual attention.
11.5.5 Intentional SlipsThe previous sections concentrated on how to maintain coordinated
flight. Sometimes, though, you want to perform a slip. You might
want to get rid of some energy, or to align the airplane for a
crosswind landing. The procedure is straightforward.
The difference between heading A and heading B is the slip angle.
Make a note of the pitch attitude and direction of flight,
since in some aircraft the airspeed indicator is perturbed by a slip,
as discussed in section 11.2. You will have to
maintain angle of attack by looking at the angles themselves.
- Make a note of which way the airplane is going. For a
crosswind landing, maneuver so that the motion is aligned with the
- Make a note of which way the airplane is pointing. Call
this heading A.
- Using the rudder, yaw
the nose to a new
direction. Call this heading B. For a crosswind landing, choose
this to be aligned with the runway.
- Bank the airplane as required to keep it going the same
direction as before.
11.5.6 Slip Angle versus Bank AngleDo not confuse slip angle with bank angle. In fact, they are
perpendicular. That is, slip involves a rotation around the yaw axis,
while bank involves a rotation around the roll axis, as defined in
Although in a non-turning slip you can perhaps judge the amount of
slip by the amount of bank, in general perceiving the bank angle is a
rather poor substitute for perceiving the slip angle.
If you don't use the rudder, then
Using the rudder and ailerons, you can perform a wings-level boat
turn, which involves a slip angle with zero bank. See section 8.10.
A roll, i.e. a change in bank, can be a major cause of
slip, because of adverse yaw and yaw-axis inertia, as discussed in
- In contrast, a steady bank produces a
relatively minor amount of slip, via the long-tail slip effect, as
discussed in section 8.9.
In a twin with an engine out, you can have a turn with no bank and no
slip, or a slip with no bank and no turn, or (preferably) a bank with
no turn and no slip. See section 17.2.
Causes of slip include:
Causes of turn include:
- aileron deflection and roll rate,
- asymmetric thrust, and
- tight turns (via the long-tail slip effect)
- bank, and
- slip (via the boat-turn effect)
* Coordination ExercisesSection 16.7 discusses some
good coordination exercises.
· A proper slip results from applying more top rudder
(or less bottom rudder) than required for coordinated flight.
· A skid results from applying more bottom rudder
(or less top rudder) than required for coordinated flight.
A skid is more dangerous than a proper slip, because it is more likely
to flip you upside down if anything goes wrong. Therefore, never
apply excess bottom rudder (no exceptions). To say it another way,
never try to speed up a turn with the rudder (no exceptions). Never
try to roll out of a turn without applying coordinated rudder (no
exceptions). Right aileron deflection requires right rudder
deflection; left aileron deflection requires left rudder deflection.
- In common usage, the phrase is a metaphor for any
situation where the practitioner has such a good feel for the
situation that quantitative information is superfluous — or where
the practitioner is forced to rely on imprecise indications because
quantitative information is unavailable.
[Comments or questions]
Copyright © 1996-2001 jsd