When an aircraft enters a descent, it changes its flightpath from level to an inclined plane. It is important that the pilot know the power settings and pitch attitudes that produce the following conditions of descent.
- Partial power descent—the normal method of losing altitude is to descend with partial power. This is often termed “cruise” or “en route” descent. The airspeed and power setting recommended by the aircraft manufacturer for prolonged descent should be used. The target descent rate should be 400–500 fpm.
- Steep approach—the normal maneuver used to descend at a steep angle. This is typically used to descend for landing if higher than expected upon approaching the runway. The throttle is set to idle and the airspeed is increased so the excessive drag allows the WSC aircraft to descend at the steepest angle. The control bar is pulled in to achieve this steep approach—the further the bar is pulled in, the steeper the descent rate. Each WSC aircraft is different, but pulling the control bar to the chest may be necessary to achieve the required angle.
- Descent at minimum safe airspeed—a nose-high descent. This should only be used for unusual situations such as clearing high obstacles for a short runway in an emergency situation. The only advantage is a steeper than normal descent angle. This is similar to the best angle of climb speed and should only be used with caution because stalling near the ground could have catastrophic consequences for the pilot, passenger, and people/property on the ground.
- Glide—a basic maneuver in which the aircraft loses altitude in a controlled descent with little or no engine power; forward motion is maintained by gravity pulling the aircraft along an inclined path, and the descent rate is controlled by the pilot balancing the forces of gravity and lift. [Figure 6-15]
Although glides are directly related to the practice of poweroff accuracy landings, 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 gives full attention to details other than the mechanics of performing the maneuver. Since glides are usually performed relatively close to the ground, accuracy of their execution, the formation of proper technique, and habits are of special importance.
The glide ratio of a WSC aircraft is the distance the aircraft, with power off, travels forward in relation to the altitude it loses. For instance, if it travels 5,000 feet forward while descending 1,000 feet, its glide ratio is said to be 5 to 1.
The glide ratio is affected by all four fundamental forces that act on an aircraft (weight, lift, drag, and thrust). If all factors affecting the aircraft are constant, the glide ratio is constant.
Although the effect of wind is not covered in this section, it is a very prominent force acting on the gliding distance of the aircraft in relationship to its movement over the ground. With a tailwind, the aircraft glides farther because of the higher groundspeed. Conversely, with a headwind the aircraft does not glide as far because of the slower groundspeed.
Variations in weight for an aircraft with a rigid wing do not affect the glide angle provided the pilot uses the correct airspeed. Since it is the lift over drag (LD) ratio that determines the distance the aircraft can glide, weight does not affect the distance. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft glides. The heavier the aircraft, the higher the airspeed must be to obtain the same glide ratio. For example, if two aircraft having the same LD ratio but different weights start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed arrives at the same touchdown point in a shorter time. Both aircraft cover the same distance, only the lighter aircraft takes a longer time.
However, the WSC aircraft has different characteristics because it has a flexible airframe. As more weight is added to the WSC wing, it flexes more creating more twist in the wing decreasing aerodynamic efficiency, as discussed in chapter 2. For example, a pilot is accustomed to a glide ratio of 5 to 1 flying solo; a passenger is added, and this glide ratio may decrease to 4 to 1. This decrease in glide ratio for added weight is true for all descent speeds. The amount of decrease in glide ratio varies significantly between manufactures and models because each wing flexes differently. The more flexible the wing is, the greater the decrease in glide ratio. Pilots should become familiar with glide ratios for their aircraft at all speeds and all weights.
Although the propeller thrust of the aircraft 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 at which the aircraft travels the greatest forward distance for a given loss of altitude in still air. This best glide speed corresponds to an angle of attack resulting in the least drag on the aircraft and giving the best lift-to-drag ratio (LDMAX). [Figure 6-16]
Any change in the gliding airspeed results in a proportionate change in glide ratio. Any speed, other than the best glide speed, results in more drag. Therefore, as the glide airspeed is reduced or increased from the optimum or best glide speed, the glide ratio is also changed. When descending at a speed below the best glide speed, induced drag increases. When descending at a speed above best glide speed, parasite drag increases. In either case, the rate of descent increases and the glide ratio decreases.
This leads to a cardinal rule of aircraft flying that a student pilot must understand and appreciate: the pilot must never attempt to “stretch” a glide by applying nose up pressure and reducing the airspeed below the aircraft’s recommended best glide speed. Attempts to stretch a glide invariably result in an increase in the rate and angle of descent and may precipitate an inadvertent stall.
To enter a glide, the pilot should close the throttle and obtain the best glide speed. 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; if the airspeed is less than recommended, the pitch attitude is too high. Therefore, the pitch attitude should be readjusted accordingly by referencing the horizon. After the adjustment has been made, the aircraft should be retrimmed (if equipped) so that it maintains this attitude without the need to hold pitch pressure on the control bar. The principles of attitude flying require that the proper flight attitude be established using outside visual references first, then using the flight instruments as a secondary check. It is a good practice to always retrim the aircraft after each pitch adjustment.
A stabilized power-off descent at the best glide speed is often referred to as a normal glide. The flight instructor should demonstrate a normal glide, and direct the student pilot to memorize the aircraft’s angle and speed by visually checking the:
- Aircraft’s attitude with reference to the horizon.
- Noting the pitch of the sound made by the air.
- Pressure on the controls, and the feel of the aircraft.
Due to lack of experience, the beginning student may be unable to recognize slight variations of speed and angle of bank immediately by vision or by the pressure required on the controls. The student pilot must use all three elements consciously until they become habits, and must be alert when attention is diverted from the attitude of the aircraft. A student must be responsive to any warning given by a variation in the feel of the aircraft or controls or by a change in the pitch of the sound.
After a good comprehension of the normal glide is attained, the student pilot should be instructed of the differences in the results of normal and abnormal glides. Abnormal glides are those conducted at speeds other than the normal best glide speed. Pilots who do not acquire an understanding and appreciation of these differences experience difficulties with accuracy landings which are comparatively simple if the fundamentals of the glide are thoroughly understood.
Gliding turns have a significant increase in descent rate than straight glides because of the decrease in effective lift due to the direction of the lifting force being at an angle to the pull of gravity. Therefore, it should be clearly understood that the steeper the bank angle, the greater the descent rate.
In gliding turns, the decrease in effective lift due to the direction of the lifting force being at an angle to the pull of gravity make it necessary to use more nose-up pressure than is required for a straight glide. However, as discussed earlier for steeper turns, airspeed must be maintained well above stall speed which increases during turns or the WSC could stall in the turn.
When recovery is being made from a medium or high banked gliding turn, the pitch force which was applied during the turn must be decreased back to trim, which must be coordinated with the rollback to level.
In order to maintain the most efficient or normal 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. Attention to the front tube angle with the horizon and the reference point on the front tube provide visual reference of attitudes while gliding. [Figures 6-17 and 6-18]
Common errors in the performance of descents and descending turns are:
- Failure to adequately clear the area.
- Inability to sense changes in airspeed through sound and feel.
- Failure to maintain constant bank angle during gliding turns.
- Inadequate nose-up control during glide entry resulting in too steep a glide.
- Attempting to establish/maintain a normal glide solely by reference to flight instruments.
- Attempting to “stretch” the glide by applying nose-up pressure.
- Inadequate pitch control during recovery from straight glides.
Pitch and Power
No discussion of climbs and descents would be complete without touching on the question of what controls altitude and what controls airspeed. The pilot must understand the effects of both power and pitch control, working together, during different conditions of flight.
As a general rule, power is used to determine vertical speed and pitch control is used to determine speed. However, there are many variations and combinations to this general statement. Decreasing pitch and diving do provide a quicker descent but is not typically used as a flight technique for long descents. Changes in pitch through moving the control bar forward and backward are used for maintaining level flight in rising and falling air, and pulling back on the control bar is used for a steep approach technique to lose altitude; however, these techniques are used only for short durations and not the primary altitude control for the WSC.
The throttle is the main control used for determining vertical speed. At normal pitch attitudes recommended by the manufacturer and a constant airspeed, the amount of power used determines whether the aircraft climbs, descends, or remains level at that attitude.