The absence of the propeller slipstream, loss of effectiveness of the various flight control surfaces at lower airspeeds, and designed-in aerodynamic corrections complicates the task of flight control coordination in comparison to powered flight for the inexperienced pilot. These principles should be thoroughly explained by the flight instructor so that the beginner pilot may be aware of the necessary differences in coordination.
Three elements in gliding turns that tend to force the nose down and increase glide speed are:
- Decrease in lift due to the direction of the lifting force
- Excessive rudder inputs as a result of reduced flight control pressures
- The normal stability and inherent characteristics of the airplane to nose-down with the power off
These three factors make it necessary to use more back pressure on the elevator than is required for a straight glide or a level turn; and therefore, have a greater effect on control coordination. In rolling in or out of a gliding turn, the rudder is required to compensate for yawing tendencies; however, the required rudder pedal pressures are reduced as result of the reduced forces acting on the control surfaces. Because the rudder forces are reduced, the pilot may apply excessive rudder pedal pressures based on their experience with powered flight and overcontrol the aircraft causing slips and skids rather than coordinated flight. This may result in a much greater deflection of the rudder resulting in potentially hazardous flight control conditions.
Some examples of this hazard:
- A low-level gliding steep turn during an engine failure emergency. If the rudder is excessively deflected in the direction of the bank while the pilot is increasing elevator back pressure in an attempt to retain altitude, the situation can rapidly turn into an unrecoverable spin.
- During a power-off landing approach. The pilot depresses the rudder pedal with excessive pressure that leads to increased lift on the outside wing, banking the airplane in the direction of the rudder deflection. The pilot may improperly apply the opposite aileron to prevent the bank from increasing while applying elevator back pressure. If allowed to progress, this situation may result in a fully developed cross-control condition. A stall in this situation almost certainly results in a rapid and unrecoverable spin.
Level-off from a glide is really two different maneuvers depending on the type of glide:
- In the event of a complete power failure, the best glide speed should be held until necessary to reconfigure for the landing, with planning for a steeper approach than usual when partial power is used for the approach to landing. A 10 percent lead (100 feet if the decent rate is 1,000 feet per minute) factor should be sufficient. That is what is given in the Instrument flying Handbook, so that should be the general rule of thumb for all publications.
- In the case of a quicker descent or simulated power failure training, power should be applied as the 10% lead value appears on the altimeter to allow a slow but positive power application to maintain or increase airspeed while raising the nose to stop the descent. Retrim as necessary.
The level-off from a glide must be started before reaching the desired altitude because of the airplane’s downward inertia. The amount of lead depends on the rate of descent and what airspeed is desired upon completion of the level off. For example, assume the aircraft is in a 500 fpm rate of descent, and the desired final airspeed is higher than the glide speed. The altitude lead should begin at approximately 100 feet above the target altitude and at the lead point, power should be increased to the appropriate level flight cruise power setting when the desired final airspeed is higher than the glide speed. At the lead point, power should be increased to the appropriate level flight cruise power setting. The airplane’s nose tends to rise as airspeed and power increases and the pilot must smoothly control the pitch attitude so that the level-off is completed at the desired altitude and airspeed. When recovery is being made from a gliding turn, the back pressure on the elevator control, which was applied during the turn, must be decreased or the airplane’s nose will pitch up excessively high resulting in a rapid loss of airspeed. This error requires considerable attention and conscious control adjustment before the normal glide can be resumed.
Common errors in the performance of descents and descending turns are:
- Failure to adequately clear for aircraft traffic in the turn direction or descent.
- Inadequate elevator back pressure during glide entry resulting in an overly steep glide.
- Failure to slow the airplane to approximate glide speed prior to lowering pitch attitude.
- Attempting to establish/maintain a normal glide solely by reference to flight instruments.
- Inability to sense changes in airspeed through sound and feel.
- Inability to stabilize the glide (chasing the airspeed indicator).
- Attempting to “stretch” the glide by applying back-elevator pressure.
- Skidding or slipping during gliding turns due to inadequate appreciation of the difference in rudder forces as compared to turns with power.
- Failure to lower pitch attitude during gliding turn entry resulting in a decrease in airspeed.
- Excessive rudder pressure during recovery from gliding turns.
- Inadequate pitch control during recovery from straight glide.
- Cross-controlling during gliding turns near the ground.
- Failure to maintain constant bank angle during gliding turns.