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13 Takeoff

Takeoff is optional.
Landing (sooner or later) is mandatory.

The most important part of taking off is making the decision to do so. Discussion of decisionmaking (section 13.8) will be postponed until after we have discussed normal takeoffs — not because it gets lower priority, but just because it's hard to appreciate an abnormal situation unless you understand the normal situation.

13.1 Simplest Takeoff

This section presents a ``case study'' of a takeoff in which the pilot has to do remarkably little work. (In subsequent sections we will describe ways in which you can get better results by doing a little more work.)

This procedure applies when you have a well-paved runway with plenty of length and no obstructions to worry about. As shown in figure 13.1 and table 13.1, part way down the runway you rotate so that the pitch attitude is about 7.5 degrees. You then just hold that pitch attitude. Period.

Figure 13.1: Simplified Takeoff

Angle of Attack

Angle of Climb

Pitch Attitude

Incidence
Airspeed

Initial roll 4.5^° 0^° 0.0^° 4.5^° small, incr.

After rotation 12.0^° 0^° 7.5^° 4.5^° increasing

At liftoff 12.0^° 0^° 7.5^° 4.5^° 6% below V_Y

Initial climb decr. incr. 7.5^° 4.5^° increasing

Steady climb 7.0^° 5^° 7.5^° 4.5^° 10% above V_Y

Table 13.1: Simplified Takeoff

Remarkably, at the moment of liftoff, the pilot doesn't have to do anything. The plane lifts off when it is ready, that is, when it has enough airspeed to support its weight at a 12 degree angle of attack. This will occur a few knots below V_Y, assuming V_Y corresponds to a 8.5 degree angle of attack (which is pretty typical; see also section 2.4). To construct the last phase of the scenario (asymptotic climb), I made some additional assumptions, namely that your engine is just powerful enough to provide a climb gradient of 5^° at a speed 10% above V_Y. In particular, I imagine climbing out with airspeed = 83 knots and vertical speed = 735 feet per minute, in an airplane where V_Y is 75 knots. These are certainly believable numbers.

Note that before liftoff, most of the engine power is going into increasing your kinetic energy; a little is needed to overcome drag, and none is going into potential energy. Then, in the initial climb, we have a funny situation where we are climbing and accelerating at the same time. Finally, in the asymptotic climb phase, most of the power is going into potential energy; some is needed to overcome drag and none is going to increase airspeed.

The technique just described is smooth, simple, and elegant, but it has drawbacks. It does not give optimal climb performance (see section 13.3), it can cause problems if there is a gusty wind (section 13.2) or a crosswind (section 13.5), and it can cause problems if climb performance is impaired for any reason (section 13.8.1 and section 2.9).

13.2 Normal Takeoff

Imagine that you are using the simplified technique of the previous section, that is, rotating early and letting the airplane ``fly itself off'' whenever it is ready. Then imagine that just after liftoff, a gust of wind comes along and robs you of a few knots of airspeed. This will cause the airplane to settle back onto the runway. This is not elegant. To get around this, use a different procedure: do not rotate until the airplane has a few knots more than the liftoff airspeed. This means that liftoff will occur right then, while you are rotating. It also means that by the time you are airborne, you can stay airborne even if you lose a few knots.

Here is a second refinement: Most runways are not perfectly smooth. If the nosewheel hits a bump at 50 knots, it is likely to knock the nose of the airplane into the air, which has several disadvantages: (1) It will cause your passengers to be bounced around more than is necessary. (2) It could cause a premature liftoff. (3) It causes unnecessary wear and tear (and possibly outright damage) to the airframe. Therefore, you can semi-rotate. That is, fairly early in the takeoff roll you can rotate to a pitch attitude of 3 degrees or so. This is enough to get the nosewheel slightly off the ground, but not so much that the airplane will lift off (at any reasonable speed), and not so much that the nose will obstruct your vision (in most airplanes). This semi-rotation involves a pretty tiny pitch attitude compared to, say, proper landing attitude. When the airspeed reaches V_X or thereabouts, you should lift off, by rotating another few degrees.

Finally, here is a third refinement: You know that the airplane will climb more steeply at V_Y than it will at a higher airspeed. Therefore, during the initial climb, you should watch for the point where the airplane reaches V_Y. At that point, you should make a pitch adjustment: increase the pitch attitude a small amount (another 2.5 degrees, according to the numbers in our scenario) and trim to maintain V_Y.

The whole scenario is depicted in figure 13.2 and table 13.2

Figure 13.2: Normal Takeoff

Angle of Attack

Angle of Climb

Pitch Attitude

Incidence
Airspeed

Initial roll 4.5^° 0^° 0.0^° 4.5^° small, incr.

After semi-rotation
7.5^° 0^° 3.0^° 4.5^° increasing

Just after rotation & liftoff 12^° 0^° 7.5^° 4.5^° just above V_X

Initial climb decr. incr. 7.5^° 4.5^° increasing

Steady climb 8.5^° 6^° 10.0^° 4.5^° V_Y

Table 13.2: Normal Takeoff

The last phase of this scenario assumes your engine can sustain a 6 degree climb gradient at V_Y. In particular, I imagine 800 feet per minute at 75 knots.

In the figure, the dotted-line flight path and the uncolored airplane show the results you would have obtained using the simplified procedure described in the previous section. Remember that by climbing out at V_Y you gain more altitude (per unit time) than you would at any other airspeed.

* Flaps for Normal Takeoff

Extending the flaps for takeoff will improve your ability to see over the nose. This is because it increases the incidence; therefore the airplane will fly at a lower pitch attitude (for any given angle of attack). If the Pilot's Operating Handbook recommends flaps for a short-field or soft-field takeoff, there's no law against using them even when the field is long and smooth.

* Perceiving the Airspeed

Choosing an attitude and letting the airplane ``fly itself off'' as described in the previous section has the advantage that you don't need to look at the airspeed indicator, meaning you can devote all your attention to outside references. However, this can get you into trouble if you choose the wrong attitude (see section 2.9). Airspeed, not attitude, is your best information about angle of attack (section 2.12).

At the opposite extreme, certainly it is not a good idea to devote all of your attention to the airspeed indicator. Fortunately, you can use your eyes (to perceive your speed relative to ground references), your ears (to perceive the sound of the engine and the sound of the wind on the airframe), and your fingertips (to perceive the forces on the yoke). This means you can get qualitative information about airspeed while keeping most of your attention focused outside. Every so often, though, you should glance at the airspeed indicator to supplement the qualitative information with quantitative information.

13.3 Obstructed-Field Takeoff

This section describes the procedure to use when you have a well-paved runway with an obstruction relatively nearby in the departure area.¹

Plan the takeoff carefully. Take into account density altitude, runway slope, headwind or lack thereof, et cetera. Make sure you know the value of V_X under these conditions, and choose a suitable rotation speed V_R as discussed below.

Use the proper flap settings, as specified in the Pilot's Operating Handbook. Here's a useful cross-check: on most light aircraft, when you extend the flaps for an obstructed-field takeoff, you will observe that the angle of the flap matches the angle of a fully-deflected aileron.

Start at the beginning of the runway. If the taxiway leads you onto the runway some distance from the beginning, you will have to back-taxi on the runway, back to the very beginning.

Open the throttle smoothly, but not so slowly that you use up significant amounts of runway before the engine reaches full power. Some people advocate using the brakes to hold the aircraft stationary until the engine comes up to full power, but this is rarely necessary; if you open the throttle properly the airplane will move only a few feet while you're doing so.²

As shown in figure 13.3 and table 13.3, you should choose a rotation speed V_R at or near V_X — that is, quite a bit higher than what you would use for a soft-field takeoff (section 13.4) or even a normal takeoff. The idea is to use the wheels to support the weight of the airplane until you have built up a lot of energy. It's OK to semi-rotate a little bit, to take some load off the nosewheel, but you don't want the wings to be producing significant lift until you're ready to climb away. Then rotate smoothly to the ``climb-out'' pitch attitude, whereupon the airplane will lift off immediately. Climb away at V_X. Trim for V_X. After you have cleared the obstruction, you can accelerate to V_Y. Finally, after you have reached a comfortable altitude, you can accelerate to ``cruise climb'' speed and trim again.

Figure 13.3: Obstructed-Field Takeoff

Angle of Attack

Angle of Climb

Pitch Attitude

Incidence
Airspeed

Initial roll 4.5^° 0^° 0.0^° 4.5^° small, incr.

climb 13.0^° 7^° 15.5^° 4.5^° V_X

Table 13.3: Obstructed Field Takeoff

In the last phase of the example scenario, I imagine a climb rate of 780 fpm at 63 knots, which gives a climb gradient of 7 degrees.

In the figure, the dotted-line flight path and the uncolored airplane show the results you would have obtained following the normal-takeoff procedure, that is, accelerating while climbing and then climbing at V_Y. Note that using by using obstructed-field procedure, you have not climbed as high, but you have better obstacle clearance because you have not flown nearly so far horizontally.

It may seem paradoxical that you get better obstacle clearance by staying on the runway longer, but it's true. The rationale is as follows: You want to pass over the obstruction at a reasonable altitude with a reasonable airspeed. This requires a certain amount of energy. To maximize energy you want to minimize drag throughout the maneuver. Keeping the airplane on the runway until reaching a high speed is rough on the airplane, but supporting its weight with the wheels usually involves less drag than supporting its weight with the wings. To say it another way: rolling resistance is less than induced drag, unless the field is quite soft or bumpy.

Once airborne, you want to climb at V_X for reasons discussed in section 7.5.3.

The idea of choosing V_R to be equal to V_X is only an approximation. There are exceptions:

For example, if you are facing a 20-foot-high billboard that is the only obstruction in the area, it is theoretically logical to zoom over at a speed several knots below V_X, then dive back down on the other side.³ Short-term altitude gain (as given by the law of the roller coaster) is more important than long-term rate of climb (as given by the power curve).
On the other side of the coin, if the elevation of your departure airport is near the absolute ceiling of your airplane (so that you will have very little rate of climb once airborne) and if the runway is long and well-paved but obstructed, then it makes sense to say on the runway (or at least in ground effect) until the speed is well above V_X.

Still, for typical circumstances, choosing V_R at or near V_X is a reasonable guideline.

* Skimming versus Wheelbarrowing or Flap-Popping

The procedure outlined above (staying on the runway at high speed, with the flaps extended) may not be possible in your airplane. Depending on the incidence of the wings, the airplane may fly itself off well before you reach the desired rotation speed.

Usually the best way to deal with this situation is to let the airplane come off the ground, and then fly along in ground effect, rather like a soft-field takeoff.

Another possible procedure (which is usually not recommended) is to keep the flaps retracted until you are ready to leave the runway. Less flaps means less incidence. A big disadvantage is that ``popping'' the flaps like this increases your workload at a time when there are lots of other things you should be attending to. Another disadvantage is that you run the risk of extending the flaps past the takeoff position to the landing position, creating lots of drag, which is really not what you want in this situation. If your POH calls for this procedure, go ahead, but be careful. Make sure you have some sort of detent to block inadvertent over-extension.

An ever worse situation arises if you try to keep the plane on the ground by pushing forward on the yoke. This is called wheelbarrowing. What happens is that while you are holding the nose wheel down, the main wheels come off the ground. You are counteracting the incidence with a negative pitch attitude. The steering becomes dangerously unstable. There is also a risk of the propeller striking the ground.

13.4 Soft-Field Takeoff

Sometimes you want to get the airplane airborne at the lowest possible airspeed, using the shortest possible takeoff roll. For example, gooey mud on the runway will cause tremendous amounts of friction on the wheels. The sooner you become airborne, the sooner you are free of that friction and the better you will be able to accelerate. Additional reasons for using soft-field procedure will be given below.

The procedure is as follows: Extend the flaps as recommended by the manufacturer; in the absence of a specific recommendation, extend the flaps so that they just match a fully down-deflected aileron. The idea is to get the most coefficient of lift without undue drag.

At the beginning of the takeoff roll, pull the yoke fully backward. Early in the takeoff roll, the nose will rise, as indicated in figure 13.4. Allow it to rise to the pitch attitude that corresponds to the stalling angle of attack, or slightly less. This is typically about 15 degrees nose up.

Figure 13.4: Soft-Field Takeoff

To maintain this pitch attitude as the aircraft accelerates, you will have to gradually let the yoke move forward. You will become airborne at a very low airspeed — roughly the stalling speed.⁴ If you were to maintain the liftoff attitude, a typical airplane will accelerate poorly while climbing poorly, but that's not what we want. (A lower-powered airplane might get into a situation where it can neither accelerate nor climb.) Instead, gradually lower the nose, so that you fly parallel to the ground, remaining one foot above the ground. As the aircraft accelerates in ground effect, the required angle of attack will decrease, so you will see the pitch attitude get lower and lower.

There are two ways of completing the maneuver.

If the field is unobstructed, remain in ground effect until the pitch attitude (and angle of attack) have decreased to their normal takeoff values, as discussed in section 13.2. Then climb while accelerating to V_Y just as in the normal takeoff .
If, however, there are obstructions, it is better to remain in ground effect until the speed approaches V_X, then raise the nose and climb out while maintaining V_X as in the obstructed-field takeoff (section 13.3).

You may be surprised at how well soft-field procedure works. Just after liftoff, the airspeed is extremely low. In ordinary conditions of flight, your airplane might well have a negative rate of climb at that airspeed — yet in this case it not only maintains altitude, but accelerates. The special ingredient in this case is ground effect: a wing produces very little induced drag while it is in ground effect (that is, roughly, within one wingspan or less of the ground) for reasons discussed in section 3.12.4.

Just after liftoff using this procedure,

there is no rolling friction because the wheels are not touching the ground;
there is very little induced drag because you are in ground effect; and
there is very little parasite drag because you are moving slowly; and
no power is being used for climb because you are moving horizontally.

The engine is producing full power, so if none of it goes into drag and none of it goes into climb, the airplane will accelerate like crazy.

There are many situations where this procedure is useful.

If the runway is covered with mud, tall grass, sand, or snow, there can be troublesome amounts of friction against the wheels. Soft-field procedure allows you to transfer the airplane's weight from the wheels to the wings as early as possible, decreasing friction and improving acceleration.
If the runway is rough and bumpy, the problem is not so much friction, but rather damage from hitting a bump at high speed. The sooner you lift off, the less harm to the airplane. Remember, the force involved in hitting a bump goes like the square of the groundspeed.
Suppose the runway is perfectly smooth and firm, but very short — and suppose it is surrounded by open fields with lots of bumps but no serious obstructions. You can become airborne over the runway, and then accelerate in ground effect over the fields.
Suppose you are attempting an ordinary takeoff from an ordinary field, but due to a gust (or perhaps even a lapse in pilot technique) you become airborne at a too-low airspeed. The best strategy is to accelerate in ground effect; you don't want to re-contact the runway (especially if there is a crosswind) and you don't want to try climbing at the too-low airspeed.

In all cases you must be careful to remain in ground effect until you have accelerated to a proper climb speed. If you try to climb at the liftoff speed you will have a big problem: in many cases, you will be unable to climb out of ground effect. That is, as soon as you climb to a height where ground effect is no longer significant, the induced drag will become so large that you will be unable to climb or accelerate.

* Brief the Passengers

If you have passengers aboard who haven't seen a soft-field takeoff before, give them the courtesy of an explanation. Otherwise, they may find the procedure extremely disturbing.⁵ Just tell them you will lift off at a low airspeed and they fly horizontally for a few moments while you accelerate to the optimal climb speed. Tell them that (a) this is standard procedure for getting best performance, and (b) it minimizes jolts to the passengers.

* Maneuver by Reference to the Edge Line

Whereas in a normal takeoff you can guide the airplane by looking out the front, in a soft-field takeoff the nose will block your view during most of the maneuver. Therefore you must use the edge of the runway as your reference. Practice this skill during taxi. You will need this skill for landings and for soft-field takeoffs, but those aren't the best times to be learning it.

13.5 Crosswind Technique

There is not a ``crosswind procedure'' that you would use instead of normal procedure, soft-field procedure, or obstructed-field procedure. Rather, you use crosswind technique in conjunction with such procedures.

A crosswind takeoff is not as tricky as a crosswind landing, but it does call for some special care. Consider the following scenario: You are trying to take off in gusty conditions using the (over)simplified techniques of section 13.1. You've already rotated, and are accelerating toward liftoff speed with the wings level. As the speed increases, the wings produce more and more lift, lightening the load on the main wheels. The wind is still blowing against the side of the fuselage as strongly as ever. The ability of the wheels to provide a sideways force to resist the wind is proportional to the downward load on the wheels.⁶ If you keep the wings level, there will necessarily come a point — prior to liftoff — where the wind overpowers the wheels and blows the airplane to the side, scraping the tires across the runway.

So, here are the correct techniques for handling a crosswind takeoff.

Regarding rudder usage: To counteract the airplane's weathervaning tendency (section 8.11), you must press on the downwind pedal to keep the plane going straight. Before rotation, both the rudder and the nosewheel contribute useful steering. After rotation, weathervaning continues, but the rudder has to do 100% of the steering. Therefore you can plan on applying a little additional pedal deflection following rotation.

Regarding aileron usage, there are two options:

The less-common method is more or less the reverse of a crosswind landing. During the takeoff roll, deflect the ailerons into the wind, to place more weight on the upwind wheel. The ailerons create force in proportion to airspeed squared, so at the beginning of the takeoff roll you will need full aileron deflection. As the airspeed builds up, gradually reduce the deflection. Rotate normally, maintaining appropriate aileron deflection, so that the downwind wing comes up while the upwind wing remains down. Keep the upwind wheel firmly planted, so that it can provide friction to resist the wind. Now the airplane is in a bank, trundling down the runway on one wheel; the sideways lift of the wings serves to counteract the force of the wind on the fuselage. As the load on the remaining wheel decreases to zero, the airplane will lift straight up.

Since the ailerons are deflected one way and the rudder another, you are commanding a slip. Indeed, the moment before liftoff you are (as desired) in a nonturning slip. The moment after liftoff you are still in this slip, but it is no longer what you need. You should level the wings and swing the nose to windward (to get the fuselage aligned with the airflow).
The more-common method is as follows: You deflect the ailerons into the wind, but not as much as in the previous method. The idea is not to transfer all the weight to the upwind wheel, but merely to equalize the weight, counteracting the wind's tendency to flip the airplane over onto the downwind side. To keep the wind from pushing you sideways, you keep weight on both wheels, delaying rotation until you have almost 100% of flying speed (rather like the obstructed-field takeoff procedure, section 13.3). You then rotate and fly away. This method is not optimal for soft or bumpy runways (because it involves driving along the runway at high speed).

Again, immediately after liftoff you must make a heading change to establish a crosswind correction angle, so that the fuselage is aligned with the airflow.

Note that in both cases, the heading change that occurs right after liftoff is not a normal, coordinated turn. The motion of the center of mass is already aligned with the runway, so you do not want to change the direction of motion, just the heading. Use the rudder, not the ailerons.

After you have lifted off, you must take care not to settle back onto the runway. Since the airplane's heading is no longer aligned with the runway, re-landing would cause a severe sideways force on the landing gear.

As you climb out, you should expect that the crosswind will be stronger at altitude than it was near the ground. To compensate, make the appropriate heading changes.

13.6 Multi-Engine Takeoff

In a multi-engine airplane, an engine failure shortly after takeoff is a very critical situation. It places considerable demands on the pilot. Make sure you know what to do; brief yourself in detail before the takeoff. Engine failures and related procedures are discussed in section 17.2.

Early in the takeoff roll, verify that both engines are developing the same amount of power. If the aircraft is trying to pull to one side, you've got a problem. Also, check the engine gauges to make sure (a) you've got the normal RPM on both engines, (b) you've got the normal manifold pressure on both engines, and (c) you've got the normal fuel flow on both engines. The instruments that measure these three quantities are usually a single gauge with two needles, so if you notice that the needles are split you've got a problem.

If anything funny happens while there is runway remaining ahead of you, close both throttles immediately and stop straight ahead. Even if you are airborne, close the throttles and re-land if there is sufficient runway length. 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 run off the end of the runway at low speed, rather than making an unsuccessful attempt to climb out on one engine.

You really don't want to be airborne at a speed below V_MC, i.e. at a speed where you can't maintain directional control on one engine. In many aircraft, you should aim for a lift-off speed of V_MC plus 5 knots. To make sure you do not lift off too soon, you can delay rotation until reaching V_MC. You can semi-rotate earlier if you want; just make sure you don't rotate to a pitch attitude that will cause liftoff below the desired airspeed. After liftoff, climb while accelerating to V_Y (which ought to be greater than or equal to V_YSE).

In many twins, V_MC is essentially equal to the stalling speed. In others, however, it is considerably higher, which makes soft-field takeoffs problematic. You don't want to lift off at ``the lowest possible airspeed'' (like you would in a single) since if you lost an engine at that speed you'd have a big problem: uncontrollable yaw. It would be a lot safer to lift off at V_MC or higher, even if this means staying away from soft, bumpy fields.

13.7 Other Elements of the Takeoff

At a tower airport, you will need to get approvals and clearances before taxiing or taking off.

During the takeoff roll and climb-out, you will need to apply right rudder to compensate for the helical propwash, as discussed in section 8.4.

In an aircraft with retractable landing gear, you have to decide when to retract them. It is not a good procedure to retract them the instant you become airborne. The reason is that sometimes things go wrong in the first seconds after liftoff, and you don't want to foreclose the option of re-landing on the remaining runway. Therefore the usual procedure is to retract the gear when it is no longer possible to re-land on the departure runway. You should say aloud the checklist item: ``No more useful runway; gear coming up''.

On a really, really long runway, it's OK to reduce drag by getting the gear up somewhat before you've flown all the way down the runway. However: (1) it's usually not worth the trouble, and (2) make sure that you're high enough that, in the event you do want to land immediately, you have time to re-extend the gear.

Very early in the climb, pick a landmark somewhere a few miles along your intended flight path, so you can maintain direction of flight primarily by outside references. The upwind leg of the traffic pattern is supposed to be an extension of the runway centerline. Similarly, note the pitch angle relative the horizon, so you can maintain the proper angle of attack and detect any windshear. You can cross-check direction, pitch angle, and angle of attack using the directional gyro, horizon gyro, and airspeed indicator, but you don't want to spend more than a tenth of your time looking at gauges. You need to be looking outside to check for traffic.

When ATC gives you a takeoff clearance, it means you are supposed to have the runway to yourself. This applies to the runway itself, not to the airspace, so as soon as you are airborne, you are 100% responsible for seeing and avoiding other traffic. Even on the runway, it pays to keep your eyes open; there's always a chance that ATC has made a mistake, and an even bigger chance that some other pilot has pulled onto the runway without a clearance.

Upon reaching a comfortable altitude, say 500 feet AGL, there are a number of things that might need doing: If your aircraft has cowl flaps, check them. On a normal takeoff they will already be open, but on a go-around you will have to open them. This is also be a good time to throttle back to normal climb power, which is less than takeoff power on most aircraft with controllable-pitch propellers. This is also a good time to retract any remaining flaps. Finally, this might be a good time to accelerate from V_Y to a nice cruise-climb speed.

You should not mess with the cowl flaps or other items until you are several hundred feet up. Turbulence might cause a pitch or bank excursion while your attention is distracted, or you might bump the yoke. At low altitude, basic aircraft control should get your undivided attention.

In some aircraft, the fuel-boost pumps should be turned off at 1000 AGL; in other aircraft they stay on throughout the initial climb. Other aircraft don't use boost pumps at all.

13.8 Decisionmaking

The most important thing a pilot can do to promote aviation safety is to know when to leave the airplane tied down. Don't pressure yourself — or let others pressure you — into making a questionable flight.

I advise all my passengers explicitly:

A flight can be delayed or diverted for many reasons, including weather, mechanical trouble, pilot fatigue, et cetera. If you feel they have to go or return at a particular time, you should make alternate arrangements.

Different takeoff situations call for different takeoff techniques. You have to ask yourself:

Should I be making this flight at all?
Is the runway long and unobstructed, or is it short and obstructed?
Is the runway firm and smooth, or is it soft and bumpy?
Is there a significant crosswind?

Use a takeoff checklist that is appropriate to the particular aircraft you are flying (not a generic substitute). Some airplanes require the fuel boost pump on for takeoff, while others require it off. A C-152 requires 10 degrees of flaps for short-field takeoff, while a C-172 requires zero.

13.8.1 Monitoring Takeoff Performance (wrong)

I sometimes hear statements such as these:

Statement #1 (wrong): ``On any runway, if you have attained 70% of your takeoff speed before you have used up 50% of the runway, then you will have 100% of your takeoff speed by the end of the runway.''

People even claim to ``prove'' statement #1, using physics plus a number of hare-brained assumptions, including:

Assuming friction is negligible. In fact, friction is much more important in the second half of takeoff roll.⁷
Assuming the engine puts out constant thrust. Although constant thrust might be a fair approximation for jets or rockets, for piston engines (especially ones with constant-speed props) constant power is a better approximation. Therefore we expect considerably less thrust in the second half of the takeoff roll.
Assuming zero wind. This might be true sometimes, but it's certainly not safe to assume this in general. With a strong enough headwind, you can attain 70% of flying speed with no engine power at all.

The following modified version is also wrong, and even more dangerous:

Statement #2 (wrong): ``On any runway, if you have attained 70% of your takeoff speed before you have used up 50% of the runway, then the takeoff will be successful''.

A little thought shows this cannot possibly be correct in general. It cannot even be repaired by changing the percentages. As shown in figure 13.5, consider a very, very long runway and a density altitude slightly above the airplane's absolute ceiling. You will able to reach 100% of flying speed before you have used up even 10% of the runway. You will be able to take off and climb a few feet, but you will never be able to climb out of ground effect, no matter how long the runway. Therefore: forget any X% — Y% rule you may have heard.

Figure 13.5: Takeoff Failure despite plenty of airspeed and runway

13.8.2 Monitoring Takeoff Performance (right)

Suppose that you are on your takeoff roll, and several subtle things have gone wrong: (a) you have underestimated the density altitude; (b) for various reasons (see below) the engine is only producing 80% as much power as it should, even at this altitude; (c) the parking brake is partially stuck so the brakes are dragging; (d) you didn't notice a shift in the wind, so you now have a few knots of tailwind; (e) you didn't notice that the runway has a slight up-slope; and (f) your mother-in-law has stowed away in the back seat, so the airplane is 15% heavier than you planned for. You may not be able to complete the takeoff safely. The question is, can you somehow notice the performance deficit in time to abort the takeoff?

If you are familiar with the airplane, you should know how the engine is supposed to sound; if it sounds rough, have it checked. Similarly, you may know what engine RPM to expect early in the takeoff roll; if you get less, abort the takeoff and investigate.

Unfortunately, if you are not intimately familiar with the airplane, it can be very difficult to notice a performance deficit until it is too late. Careful planning and checking is required, as we shall see.

Using the Pilot's Operating Handbook (POH), calculate the takeoff ground roll distance that is expected for your takeoff conditions. Also calculate the landing ground roll distance for the same conditions. Choose a runway that is at least as long as the two distances combined, plus a suitable margin for error. Observe and note well what part of the runway should be consumed by the takeoff roll.⁸ Then commence your takeoff. If you are not airborne by the predicted point, close the throttle and apply the brakes immediately. Taxi back to the hangar and figure out what's wrong.

Do not attempt to use ``extra'' runway length to salvage the takeoff if there is a significant performance deficit. If you've got a deficit, you should figure out why, and the takeoff roll is no place to be doing complicated figuring.

Now let's consider the annoying situation where the available runway is just a little shorter than the aforementioned ``takeoff plus landing'' ground roll distance. The POH tells you that a takeoff should be possible, if everything goes right, but it does not tell you how to make a timely determination that you've got a problem. In such a situation, there are three possibilities. One is to change the situation; that is, you can offload some fuel, toss out some payload, wait for cooler air, and wait for more headwind — so that you can attempt a takeoff using the procedure described in the previous paragraph. The second possibility is to figure out how much runway your airplane should consume reaching various speeds less than flying speed, so that you can have earlier opportunities to abort the takeoff. This is a job for a test pilot; the typical POH does not provide such information, and takeoff performance is notoriously hard to predict accurately. Please do not try this; playing ``amateur test pilot'' is like playing Russian roulette. The third possibility, if you have any remaining doubts about your airplane's performance, is to stay home.

13.8.3 Causes of Diminished Power

There are dozens of things that could go wrong with an aircraft engine.

One of the exhaust valves could be burned or stuck, so it won't fully close.
One of lobes on the camshaft could be worn, so a valve won't fully open.
The magneto timing could be not quite right.
There could be a bird's nest in the air intake.
et cetera.

Such problems are not particularly rare; I have personally experienced the first four items in this list.

If some such thing goes wrong, the engine will usually not stop cold. It will continue to run, producing a fairly large percentage of its normal power. In flight, this resilience is clearly an advantage.

During takeoff, this resilience is a two-edged sword. Because the engine continues to develop lots of power, you might not notice the degradation. You might be tempted to take off with such an engine. This could lead to big trouble, especially on an obstructed-field takeoff.

13.8.4 Plan & Practice Rejected Takeoffs

There are many types of problems that you may not notice until you have begun your takeoff roll. Early in the takeoff roll, scan the airspeed, engine RPM, manifold pressure, and fuel flow to make sure you're getting reasonable readings.⁹

You should always plan your takeoff. This includes planning for a rejected takeoff, for reasons discussed in section 13.8.2.

Be sure you practice this. The first few times the rejected-takeoff situation arises, your expectation of a normal takeoff will be so strong that it is difficult to accept the situation and close the throttle. A rejected takeoff is psychologically at least as difficult as a go-around. Actually, most single-engine pilots find it more difficult than a go-around, if only because it isn't given as much emphasis during training.

Instructors: here's an instructional technique: During preflight, brief the student on the procedures for rejected takeoff. Choose a runway that is plenty long. During the takeoff roll, wait until the airspeed is about half of the liftoff speed. Then slap a suction cup on the airspeed indicator and say, ``simulated airspeed indicator failure''.

If something seemingly minor happens early in the takeoff roll, reject the takeoff. The rationale is that during the takeoff roll you don't have time to make an intelligent decision about what is minor and what is not, so (assuming there is plenty of runway remaining) the safest thing is to stop now and think later. See also section 15.3.

13.9 Summary

Four of the most-common takeoff procedures are related in a fairly logical way, as summarized in table 13.4.

Unobstructed Obstructed

Well-paved Semi-rotate early.
Fully rotate at V_R.
Climb while accelerating to V_Y. Rotate at V_X.
Climb at constant airspeed: V_X.

Soft Hop into ground effect just above V_S.
Accelerate horizontally (1 foot AGL) to V_R.
Climb while accelerating to V_Y. Hop into ground effect just above V_S.
Accelerate horizontally (1 foot AGL) to V_X.
Climb at constant airspeed: V_X.

Table 13.4: Basic Takeoff Procedures

Additionally, in each of the four cases, you must take into account the crosswind if any.

Proper planning is important. A wise ``no-go'' decision could save you a lot of trouble. Make sure you know the proper procedures, including the critical airspeeds. Make sure you know how much runway you will need. If, during the takeoff roll, it looks like you are getting less performance than you should, stop and figure out what's wrong. Practice rejected takeoffs.

Make sure you know what angle of climb you should expect. You need this to check obstacle clearance. This also affects your choice of initial pitch attitude.

When choosing an initial pitch attitude, remember that pitch attitude is not the same as angle of attack. See section 2.9 for information on the right (and wrong) ways to handle cases where the correct pitch attitude differs from what you expected.

Keep the aircraft properly trimmed and fly with a light touch. Don't forget the after-takeoff checklist.

L

1: In your Pilot's Operating Handbook, this is probably called ``short-field takeoff''. However, as we shall see, this is definitely not the right procedure for a short unobstructed field — it actually uses more runway than a normal takeoff. If you have a really short but unobstructed field, consider using soft-field procedure (section 13.4).
2: I like to avoid running the engine at high power when the airplane is not moving at all, since this tends to suck up rocks, damaging the propeller. If you are moving, by the time the rock gets off the ground you will be somewhere else, possibly escaping damage.
3: I wouldn't do this except in an emergency, because it would imply operating without adequate safety margins.
4: ... but you shouldn't be looking at the airspeed indicator. It doesn't provide any useful information at these speeds.
5: Imagine how it looks: The airplane is airborne but not climbing, and you are flying directly toward the bases of the trees at high speed. Just when they're convinced they're about to die, you pop the nose up and climb out.
6: Most types of friction behave this way.
7: Indeed, if friction were negligible, airplanes would fly much faster and would use much less fuel.
8: There are several good ways to do this. (a) Some runways have standard markings every 500 feet. (b) Sometimes the required takeoff distance is a half or a third (or some other convenient fraction) of the total runway length. (c) Sometimes you can pace off the distance between runway lights, and then count lights. (d) Sometimes you just have to pace off the whole distance
9: Other problems you might notice during takeoff include: A door that is not properly latched may pop open as the airspeed builds up. A seatbelt hanging out can cause a very loud, aperiodic banging noise. Neither of these is aerodynamically serious, so don't over-react.

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	Angle of Attack	Angle of Climb	Pitch Attitude	Incidence	Airspeed
Initial roll	4.5^°	0^°	0.0^°	4.5^°	small, incr.
After rotation	12.0^°	0^°	7.5^°	4.5^°	increasing
At liftoff	12.0^°	0^°	7.5^°	4.5^°	6% below V_Y
Initial climb	decr.	incr.	7.5^°	4.5^°	increasing
Steady climb	7.0^°	5^°	7.5^°	4.5^°	10% above V_Y

	Unobstructed	Obstructed
Well-paved	Semi-rotate early. Fully rotate at V_R. Climb while accelerating to V_Y.	Rotate at V_X. Climb at constant airspeed: V_X.
Soft	Hop into ground effect just above V_S. Accelerate horizontally (1 foot AGL) to V_R. Climb while accelerating to V_Y.	Hop into ground effect just above V_S. Accelerate horizontally (1 foot AGL) to V_X. Climb at constant airspeed: V_X.