Editors Note: The December 2000 issue featured a discussion of the planning and decision factors necessary for a safe takeoff. Here, well look at gauging and using the proper technique.
Takeoff is the moment of truth when the pilot is called upon to apply his or her aeronautical knowledge and exhibit the right flying skills. The skill level of a pilot depends on recency of experience, practice and the performance standards of the pilot or instructor. In short, knowing what to do is not adequate if you are unable to perform properly.
Determining your aircrafts takeoff performance is the intellectual side of the start to your flight. The other side is deciding what type of takeoff is to be made and the technique involved. This is the psychomotor side of the equation.
We covered the intellectual side in depth in the December 2000 issue, so now lets examine the different types of takeoff, the associated techniques and some of the variables involved. The techniques outlined here are those commonly practiced. The procedures for the airplane you fly may be different, of course, so look in the manufacturers documentation for any differences that may apply.
Proper knowledge of wind strength, wind direction and crosswind technique are crucial in keeping the airplane on the runway until its flying. Short field and soft field techniques are important if the runway requires special handling. And, of course, all pilots should know how to safely abort a takeoff when things arent working right.
Friend or Foe
Wind direction and velocity relative to the runway orientation and aircraft heading affect aircraft controllability and takeoff distances. The greater the angle at which the wind approaches the side of the airplane, the greater the surface area exposed to its energy.
This in turn results in a tendency for the airplane to point itself into the wind, a condition called weather-vaning. The magnitude of the weather-vaning tendency is determined by the wind velocity, the angle the wind approaches the aircrafts longitudinal axis and the total aircraft surface area being acted upon. Additionally, the headwind or tailwind component affects the airplanes takeoff ground run and total distance to clear a 50 ft. obstacle.
A headwind component decreases the takeoff ground roll and total distance by reducing the airplanes ground speed. The wing is dumb, so it reacts the same whether it is moving forward through the air or the air is passing rearward over it. This combined energy of the forward moving wing and the rearward moving air results in a ground speed at liftoff of less than the indicated airspeed.
The airplane accelerates to its same liftoff airspeed in the same amount of time; but, because of the slower ground speed, will not travel as far before lift off. In the air the effect continues, so the total distance to clear a 50 ft. obstacle is less than in calm air.
Similarly, a tailwind component increases the takeoff ground roll and total distance to clear a 50 ft. obstacle because the airplane accelerates to its liftoff airspeed in the same amount of time, but the ground speed is greater than the airspeed. The airplane will have traveled farther to lift off than it would in calm air. This explains the longer ground roll and total distance to clear a 50 ft. obstacle than it would experience on a calm day.
Why Crosswinds Cross You Up
Crosswinds, of course, contain a crosswind component and either a headwind or tailwind component in all cases where the wind is not exactly 90 degrees off the course.
While virtually all pilots are familiar with the need to hold the controls for wind correction during the roll, the fact that control deflection should lessen as the airplane accelerates leads many to believe that the faster airspeed reduces the crosswinds effect.
Not so. In fact, just the opposite occurs. The crosswinds effect increases as airspeed goes up, even though the wind correction angle goes down.
As the takeoff progresses and the airspeed increases, so does the pushing effect of the crosswind component. As the airplane accelerates it becomes more sensitive to a crosswind.
This is because it is exposed to the crosswind effect for a greater distance per unit of time. If the pilot senses a harder push of the crosswind with an increase in airspeed, it is in reality an accumulating effect of the wind acting on the airplane over a greater distance.
Should a crosswind go uncorrected, the lateral displacement from a straight line in proportion to the distance traveled during a given period of time will be greater at a greater airspeed. This also explains why the airplane is easier to control as it slows down after a crosswind landing. The crosswind effect decreases with a decrease in airspeed, the opposite of that experienced during takeoff.
The effect of a 90-degree crosswind is harder to accept. It produces an effect similar to a headwind due to the fact that it is composed of one component. A greater portion of the airplanes energy (forward speed) is absorbed by a 90-degree crosswind than a headwind of a lesser angle of the same velocity.
This results in the thrust component being divided into two components: forward and lateral. The forward component of thrust is reduced in proportion to the quantity of lateral thrust required to offset the 90-degree crosswind.
In turn, the divided thrust component results in a ground speed lower than the airspeed, as in the case of a headwind. Acceleration is practically the same as experienced in calm air – wind has very little effect on acceleration – so the airplane lifts-off at the same time as in calm air but, due to its slower ground speed, travels a shorter distance.
In practice though, a gust factor figure is added to the normal lift-off airspeed for controllability.
The gust factor is half the difference between the sustained wind speed and the strongest gust at the moment. For example, if the winds are reported as 25 knots, gusting to 40 knots, add half of the difference between 40 and 25, or 8 knots, to your normal lift-off airspeed.
Most people consider a gust factor of 15 knots to be the maximum, because once gusts top 30 knots runway lengths and aircraft controllability on the ground become problems.
In addition, consider your airplanes maximum demonstrated crosswind component, adjusted to your skill level. As a practical (but not legally binding) limitation, the max demonstrated crosswind component is the highest crosswind used by test pilots during certification. Beyond that and youre on your own.
Crosswind Takeoff Technique
During the ground roll and immediately following liftoff, the flight controls play two distinct roles. During the ground roll they assist nosewheel steering in maintaining directional control, then once airborne they become the primary means of directional control.
When preparing for takeoff into a stiff crosswind, the ideal is to taxi into position, apply full aileron into the wind, apply maximum braking, and then apply maximum power.
The reason for applying aileron into the wind is to assure the wing remains down. With control neutral there is the possibility of the crosswind acting beneath the upwind wing and raising it, exposing more surface area to the crosswind and compounding directional control problems or even flipping the airplane.
With single-engine airplanes the application of power prior to brake release directs the high-energy propstream over the empennage, improving immediate rudder effectiveness.
On a twin-engine airplane, a slightly higher power setting on the upwind engine at brake release will aid in offsetting the weather-vaning effect of the crosswind. Once attaining the airspeed at which the rudder becomes effective, the power is then matched on both engines and the takeoff continued as it would in a single-engine airplane.
In a crosswind takeoff, rotation to liftoff is made at an airspeed slightly higher than with a normal takeoff. This is to assure that there is adequate control effectiveness to provide an immediate response to any sudden gust that may affect directional control of the airplane.
Take, for example, an airplane whose manufacturer recommends a rotation speed of 50 knots and a liftoff speed of 65 knots for a normal takeoff.
During a crosswind takeoff in this airplane, a good strategy is to delay rotation until approaching an airspeed of 65 knots and then firmly, but smoothly, rotate to a normal climb attitude. This will result in a positive liftoff with good control effectiveness.
Some airplanes may want to fly sooner than the higher rotation speed, particularly with the trim in the normal takeoff position, so the pilot must apply a bit of forward pressure. That ensures the pilots ability to avoid liftoff at an airspeed where there would be marginal aircraft control. After liftoff make a slight turn into the wind to offset wind drift and maintain a straight ground path, and the climb is stabilized.
Short Stuff
When confronted with a short runway or a field where obstructions at the departure end may come into play, the pilot usually wants to ensure the airplane is flying before it reaches the end of the runway and climbs out before it encounters obstacles. The same techniques apply up to the point of liftoff, after which the technique may vary depending on the circumstances.
In a short field situation the goal is to produce the greatest gain in altitude per distance traveled. The airplane should be off and climbing in the shortest distance possible, with airspeed then adjusted to Vx, the best angle of climb speed.
Since distance is a concern, make sure you have as much runway as possible at the start of your roll. Position the airplane as close to the beginning of the runway as possible through normal taxiing procedures.
Some pilots position the airplane beyond the end of the runway to get a running start, but if you think thats necessary – given the possibility of hitting a hole, light, rock or other obstruction – you should probably reconsider the takeoff in the first place.
With brakes fully applied, smoothly apply takeoff power, check pressures and temperatures to assure full power is being developed, place control surfaces in neutral positions to minimize drag unless a crosswind is present and then fully release the brakes.
As the airplane accelerates, note that airspeed indicator movement and indications are normal (airspeed alive). When your elevator becomes effective begin to apply back pressure such that you pitch up to approximately the Vx attitude. Just how much depends on the airplane, but in a typical single or light twin the initial pitch attitude is 5 degrees to 8 degrees nose up.
Monitor your position relative to the hypothetical end of ground roll point you selected on the runway and your indicated airspeed.
As the airplane accelerates the controls become more sensitive and you will probably have to relax elevator pressure a bit to stop the nose from rising. As the airplane lifts off do whatever is necessary with the primary flight controls to maintain a constant pitch attitude at about Vx.
Coordinate aileron and rudder application to establish a wind correction angle to accommodate any crosswind and maintain the intended ground track. Use the shallowest bank possible while doing this because bank reduces both angle and rate-of-climb.
Typically, takeoff flaps are raised slowly as soon as possible after takeoff to reduce drag, though some aircraft may have other requirements. Likewise, whether the landing gear on a retractable gear airplane should be retracted before or after passing an obstacle depends on the manufacturers recommendation. On some airplanes the drag of gear doors opening and closing and landing gear movement affect the angle of climb performance. With these airplanes the landing gear should be left down until the obstacle is cleared.
Until theres enough air below you, however, make sure you do whatever is needed with the primary flight controls to maintain the proper pitch attitude. When all obstacles are cleared, accelerate to the best-rate-of-climb speed.
From there on out, its like a normal initial climb. At pattern altitude if VFR or circling minimums if IFR, configure the airplane for climb and complete the climb checklist.
If youre faced with a short runway but no obstacles, the initial short field technique is followed until after liftoff. At that point the airplane can be accelerated to Vy and cleaned up for a normal climb.
Soft Field Takeoff
The object of the soft field takeoff is to get the airplane off the ground as soon as possible and then allow it to accelerate to a selected climb speed when clear of the surface. Soft surfaces such as tall grass (especially when wet), mud, slush and standing water hinder acceleration. This retarding force is the result of an exponential relationship between the surface condition, acceleration and distance.
For example, if you experience a retarding force that reduces acceleration by 10 percent, it will increase your ground roll distance by 21 percent. However, the retarding force is not a constant and becomes greater over a given surface as the airplane accelerates.
As the aircraft accelerates the surface condition (water, mud, gravel, etc.) builds up a ridge in front of the tire. For example: If the tire rotates in a clockwise direction it sets up a relative motion with the surface condition causing a clockwise rotation of the material in front of it, building a ridge.
The height of the ridge is proportional to its inability to pass under or get out of the way of the tire. The higher the ridge and more dense the surface condition, the greater the resistance to the aircrafts movement.
There are, of course, many conditions under which this resistance is so great the airplane cannot accelerate to flying speed. Thus, mud and frozen slush offer more resistance than wet grass or water. The tire reacts quite differently to a liquid surface than it does to a dense semi-solid surface.
When a condition is more liquid than solid, the liquid pressure beneath the tire may override the ridge and ride on top. When this occurs the coefficient of friction is drastically reduced. That aids acceleration but it helps reduce directional control.
This condition is called dynamic hydroplaning. Combined with airspeeds below those necessary for flight control effectiveness, it can result in an out-of-control situation. This is especially hazardous if you need to abort the takeoff for some reason.
Good to Go?
If the soft runway is adequate for takeoff, the right technique can help ensure you end up in the air instead of in the trees. Get the airplane airborne and accelerating to a selected climb speed as soon as possible.
When ready for takeoff, taxi onto the runway and keep the airplane moving while continuously adding more and more power; timed so that as the airplane is aligned with the center of the runway maximum power has been applied. Apply back pressure to the elevator so the nose tire doesnt dig in.
As the airplane accelerates and control surfaces become more effective, lift the nose only high enough for the nose tire to clear the surface condition. If the airplane is too nose-high it will have excessive induced drag that will increase the time and distance to liftoff. High induced drag combined with a high ground resistance condition might not allow the airplane to attain flying speed.
The objective of the nose-high attitude and increased angle of attack is to transfer the weight of the airplane from the wheels to the wing. When the point is reached where total lift equals total weight, the airplane is flying.
Knowing the characteristics of your airplane at liftoff is very important to avoid surprises and undesirable behavior. Some airplanes have a tendency to pitch up when lifting off. Once airborne, the ideal situation is to accelerate in ground effect until Vx is attained before attempting to climb out of ground effect.
Because of the airflow change around the wing when leaving ground effect, lift is reduced and induced drag increased. Below Vx the loss of lift could be such that the airplane could settle back onto the runway.
A word of caution on accelerating while near the surface. Some pilots have been taught to lower the nose to accelerate to Vx or Vy. Although the objective is correct, the procedure is incorrect. When you lower the nose you are reducing the angle of attack and the lift coefficient, causing the airplane to settle.
At that point, most pilots apply pronounced back pressure to stop the airplane from settling, then correcting in the opposite direction to avoid a pronounced nose-up pitch attitude. These pilot induced phugoids disrupt the climb performance and are to be avoided.
Instead, reduce back pressure only slightly after liftoff, only enough to maintain the liftoff attitude. As the airplane accelerates, continue reducing back pressure gradually to maintain the same attitude. Once Vx is attained, adjust the pitch attitude to maintain Vx until out of ground effect and all obstacles are cleared. Then adjust to a normal climb attitude and airspeed.
Time to Stop
If you reviewed takeoff accidents and asked what maneuver could be practiced to avoid most of them, the aborted takeoff would top the list. This maneuver, like the missed approach or go-around, is one we often talk about, were trained and tested on, but never really thought we would ever have to perform.
In general, the longer its been since the last time you practiced something, the worse youll do at it. When giving flight tests as a Pilot Examiner I never ceased to be surprised at how even the most experienced pilots react to an unannounced aborted takeoff.
The aborted takeoff involves all the basic flying skills the pilot possesses. Because of the element of surprise, the pilot will probably be challenged to exercise every one of them. The proper use and application of takeoff performance chart information, application of ground run information, density altitude and wind effect, surface conditions and the decision to abort and properly execute the procedure to do so, are all neatly packaged in this one maneuver.
Many – perhaps most – pilots expect something dramatic to forewarn them of an impending abort. A rough engine, smoke, open door or incorrect instrument indications may be present, but do not depend on it.
Instead of worrying about abnormal pressures, temperatures or engine smoothness, if the airplane is not accelerating as it should be, it is time to abort. This is why it is so important to be aware of the position of the hypothetical point selected at the end of the ground run distance.
Whatever you do, do not sit there and wait for the problem to cure itself and the airplane to fly. If it is not ready to fly when approaching the point at which it ought to fly, you have a problem. Remember, if you leave the ground with a problem, the problem goes along with you, with the promise that things will only get worse.
The abort can only begin when the pilot compares the actual performance of the airplane with the expected performance and accepts the fact that a deficiency exists. Now is not the time to determine why. It is time to act. Immediately reduce power, apply braking as necessary, maintain directional control and continue with any combination of these actions to assure you will be able to stop within the remaining runway.
Do not apply brakes without completely retarding power; it is a contradiction of purpose and may cause directional problems. Also, maximum braking should not be applied unless absolutely needed.
Abort, taxi back and determine what the problem is while safely on the ground. Do not confuse this procedure with that followed in air transport category airplanes where, once attaining a selected airspeed, you continue the takeoff.
This cant be done in most light twins, except in a few airplanes below realistic operating weights. Poor performance in a single may make takeoff moot, so its better to stay on the runway and out of the weeds.
Only after weighing all the factors affecting the takeoff can a pilot determine what type of takeoff is to be made and the technique involved. The process begins with the proper use and interpretation of takeoff performance charts. Performance is compared to the runway environment and a determination made if the flight can be conducted safely.
At times adjustments in aircraft loading must be made to accommodate existing conditions, so the pilots knowledge of weight and balance procedures plays an important role in takeoff planning.
No discussion of takeoffs would be complete without mentioning the role of accelerate-stop distance. The accelerate-stop distance in a light twin is provided by means of a performance chart. In a light twin the accelerate-stop distance exceeds the all-engines operating distance to attain a height of 50 feet, and determines the runway length requirement.
Since the manufacturer does not provide the accelerate-stop distance in the manual, the rule of thumb in a single-engine airplane is to use the greater of the takeoff distance to climb to 50 feet or the sum of the takeoff and landing ground roll distances as a runway length requirement. This procedure will provide at least some form of protection in the event of an abort.
The takeoff is one of the most critical of all maneuvers. This is when the airplane is at its heaviest, the airspeed is at its slowest, the altitude is at its minimum and the obstacles are the nearest.
The pilot cannot afford to be lacking in proper preflight preparation, applied aeronautical knowledge, or flying skills. Practice and review is the best advice a pilot can be given for avoiding a takeoff accident.
Also With This Article
Click here to view “A Variety of Nuances.”
-by Tom Oneto
Tom Oneto is a 13,000-hour ATP/CFII, Part 121 and corporate pilot and former Designated Pilot Examiner.
