Descents Descents

A descent, or glide, is a basic maneuver in which the airplane is losing altitude in a controlled descent with little or no engine power; forward motion is maintained by gravity pulling the airplane along an inclined path, and the descent rate is controlled by the pilot balancing the forces of gravity and lift.

Although power off descents (glides) are directly related to the practice of power off accuracy landings, as will be seen in later discussions, they have a specific operational purpose in normal landing approaches, and forced landings after engine failure. Therefore, it is necessary that they be performed more subconsciously than other maneuvers because most of the time during their execution, the pilot will be giving full attention to details other than the mechanics of performing the maneuvers. Since glides are usually performed relatively close to the ground, accuracy of their execution and the formation of proper technique and habits are of special importance.

Because the application of the controls is somewhat different in power off descents than during power on descents, gliding maneuvers require the perfection of a technique somewhat different from that required for ordinary power on maneuvers. This control difference is caused primarily by two factors - the absence of the usual propeller slipstream, and the difference in the relative effectiveness of the various control surfaces at slow speeds.

The glide ratio of an airplane is the distance the airplane will, with power off, travel forward in relation to the altitude it loses. For instance, if an airplane travels 10,000 feet forward while descending 1,000 feet, its glide ratio is said to be 10 to 1. Technically, it is practically impossible to know exactly what the glide distance of the airplane will be, because so many things affect it; however, it is very important that the pilot has a fair idea of how far the airplane will glide under certain conditions.

The glide ratio of the airplane is affected by all four fundamental forces that act on the airplane (weight, lift, drag, and thrust). If all factors affecting the airplane are constant, the glide ratio will be constant. Therefore, in order to judge the gliding distance of the airplane, the pilot must keep all of these forces constant. Although the effect of wind will not be covered in this section, it is a very prominent force action on the gliding distance of the airplane in relation to its movement over the ground. It is sufficient to say here that the stronger the headwind, the less the gliding distance.

In light general aviation type airplanes, the weight of the airplane can be considered to remain constant for any given loading condition, since the fuel burn off has insignificant effect. However, the heavier the airplane the higher the airspeed must be to obtain the same glide ratio.

The lift of the airplane will remain constant for any one airspeed, since the angle of attack of the wing remains the same regardless of variations of the flightpath from the horizontal. Therefore, if a constant airspeed is maintained, the lift will be constant.

Under different conditions of flight, the drag factors may be varied through the operation of the landing gear and/or flaps. When the landing gear is lowered or the flaps are lowered, the drag is greater and the airspeed will decrease unless the pitch attitude is lowered. As the pitch attitude is lowered, the glidepath steepens and reduces the distance traveled. With the power off, a windmilling propeller also creates considerable drag, thereby retarding the airplane's forward movement.

Although the propeller thrust of the airplane is normally dependent on the power output of the engine, the throttle is in the closed position during a glide, so the thrust is constant. Since power is not used during a glide or power off approach, the pitch attitude must be adjusted as necessary to maintain a constant airspeed.

The best speed for the glide is one which will give a minimum rate of descent and provide a safe margin of flying speed above a stall. Changes in the gliding airspeed will result in proportionate changes in glide ratio, for as the airspeed is reduced or increased from the optimum glide speed, the glide ratio is also changed. When below the optimum speed, the faster altitude will be lost. For this reason the pilot should never try to stretch a glide by reducing the airspeed below the airplane's recommended glide speed.

The most efficient gliding speed will vary with the gross weight of the airplane, the configuration of landing gear and flaps, and the windmilling of the propeller. Within certain limits, the wind component would also affect the speed required for the most efficient glide angle. In airplanes for which the manufacturer does not provide the optimum glide speed, and with which the pilot is not familiar, it can be determined by experimentation. This can be accomplished by establishing a power off glide and noting the airspeed and vertical speed, and then gradually reducing the airspeed until the vertical speed reaches its minimum and starts to increase. The airspeed at that moment is the best glide speed in still air. When the correct pitch attitude and airspeed have been determined, the pilot should commit to memory the position of reference points in relation to the horizon, and the tone of the sound made by the air passing over the airplane structure.

 As in straight and level flight, turns, and climbs, the pilot should perform descents by reference to both flight instruments and outside visual references (Fig. 6-10). To enter the glide, the pilot should close the throttle and advance the propeller control (if so equipped) to low pitch (high RPM). A constant altitude should be held with back pressure on the elevator control until the airspeed decreases to the recommended glide speed, then the pitch attitude should be allowed to decrease to maintain that gliding speed. When the speed has stabilized, the airplane should be retrimmed for "hands off" flight.  When the approximate gliding pitch attitude is established, the airspeed indicator should be checked. If the airspeed is higher than the recommended speed, the pitch attitude is too low, and if the airspeed is less than recommended, the pitch attitude is too high; therefore, the pitch attitude should be
readjusted accordingly. After the adjustment has been made, it is important to retrim the airplane so that it will maintain this attitude without the need to hold pressure on the elevator control.

When the proper glide has been established, flaps may be used, but then the pitch attitude will have to be changed accordingly to maintain the desired glide speed. Again the pitch attitude should be adjusted first, then the airspeed checked after it has had time to decrease. It is best to always establish the proper flight attitudes by checking the visual reference first, then use the flight instruments as a secondary check. It is good practice to always retrim the airplane after each pitch adjustment.

In order to maintain the most efficient glide in a turn, more altitude must be sacrificed than in a straight glide since this is the only way speed can be maintained without power. Turning in a glide decreases the glide performance of the airplane to an even greater extent than does a normal turn with power.

Skidding to the outside in a gliding turn, particularly when close to the ground, is even worse than skidding in a climbing turn. While the results are the same, the proximity of the ground makes the former more likely to end disastrously.

 The level off from a glide must be started before reaching the desired altitude because of the airplane's downward inertia (Fig. 6-11). The amount of lead depends upon the rate of descent and the pilot's control technique. With too little lead, there will be a tendency to descend below the selected altitude. For example, assuming a 500 FPM rate of descent, the altitude must be led by 100 to 150 feet to level off at
an airspeed higher than the glide speed. At the lead point, power should be added to the appropriate level flight cruise setting so the desired airspeed will be attained at the desired altitude. Since the nose will tend to rise as the airspeed increases, the pilot should smoothly control the pitch attitude to attain the level flight attitude so that the level off is completed at the desired altitude.

When recovery is being made from a gliding turn, the pressure on the elevator control which was applied during the turn must be decreased or the nose will tend to rise too rapidly, making it difficult to attain the desired cruise speed and altitude. This error will require considerable attention and conscious control adjustment.