A glide is a basic flight maneuver in which an airplane loses altitude in a controlled descent with little or no engine power. Forward motion is maintained as gravity contributes to the airplane’s movement along a descending flight path. The pilot controls the glide by managing pitch attitude, airspeed, and the forces acting on the airplane.
When leveling off from a partial-power descent with a descent rate of 1,000 feet per minute, the pilot should begin the level-off approximately 100 feet above the desired altitude. This 10-percent lead allows time to raise the nose smoothly, adjust power as required, and maintain the desired airspeed and altitude.
Although glides are directly related to the practice of power-off accuracy landings, they also have an important operational purpose during normal landing approaches and forced landings following engine failure. Therefore, glides must be performed accurately and with well-established techniques. During their execution, the pilot may need to give full attention to tasks other than the mechanics of the maneuver.
Since glides are often performed relatively close to the ground, accurate execution and the development of proper techniques and habits are especially important.
The glide ratio of an airplane is the horizontal distance traveled in relation to the altitude lost. For example, if an airplane travels 10,000 feet forward while descending 1,000 feet, its glide ratio is 10:1.
The best glide airspeed is used to maximize the horizontal distance flown for a given loss of altitude in still air. This airspeed is particularly important when a pilot is attempting to reach a suitable landing area following an engine failure. The best glide airspeed occurs at the highest lift-to-drag ratio (L/D). [Figure 1]
When gliding above or below the best glide airspeed, the glide ratio decreases because of the increased drag relative to lift. As the airspeed increases or decreases from the best glide airspeed, the horizontal distance traveled for a given loss of altitude is reduced. [Figure 2]
Variations in weight do not significantly affect the glide angle when the airplane is flown at the appropriate airspeed for its weight. Since the lift-to-drag ratio determines the glide angle, airplanes with the same L/D ratio can achieve approximately the same glide distance from the same altitude in still air.
However, a heavier airplane must glide at a higher airspeed to achieve the same glide ratio. For example, if two airplanes with the same L/D ratio but different weights begin gliding from the same altitude, the heavier airplane, flown at its appropriate best glide airspeed, reaches the same touchdown point in less time. Both airplanes cover approximately the same horizontal distance, but the lighter airplane takes longer.
Since the highest glide ratio occurs at the maximum L/D, consideration should be given to drag-producing components of the airplane, such as flaps, landing gear, and cowl flaps. When drag increases, the pilot must adjust pitch attitude to maintain the desired airspeed. The resulting glide path becomes steeper, reducing the distance traveled.
To maximize glide distance, unnecessary drag should be minimized. The configuration of the landing gear, flaps, and other aircraft components must follow the applicable AFM/POH procedures.
Wind affects the horizontal distance traveled during a glide. With a tailwind, the airplane covers more ground because of the increased groundspeed. Conversely, with a headwind, the airplane covers less ground because of the reduced groundspeed. This is important when managing an engine-related emergency and planning a subsequent forced landing.
During powered flight, the airplane’s design and the pilot’s control inputs compensate for propeller effects, including P-factor and propeller slipstream. During a glide, these effects are reduced or changed because engine power and propeller slipstream decrease, but the airplane’s built-in aerodynamic characteristics remain.
During glides, slight left rudder pressure may be required in some airplanes to maintain coordinated flight. The required control inputs depend on the airplane’s design and operating conditions. In addition, the pilot may need to use greater control deflection because airflow over the control surfaces is reduced at lower airspeeds.
Minimum sink speed is used to maximize the time an airplane remains airborne by producing the lowest rate of altitude loss. It occurs at a lower airspeed than the best glide speed and generally results in less horizontal distance traveled over the same period of descent.
Minimum sink speed is useful in situations where remaining airborne for a longer time is more important than maximizing glide distance. An example is an emergency situation in which a pilot is attempting to remain airborne while preparing for a ditching. Minimum sink speed is not always published as a specific airspeed, so the applicable aircraft information and training guidance should be consulted.
In an emergency, such as an engine failure, attempting to stretch a glide back to the runway by applying excessive elevator back pressure can cause the airplane to land short. It may also lead to a loss of control if the airplane stalls.
This leads to a cardinal rule of airplane flying: The pilot should not attempt to stretch a glide by applying back-elevator pressure and allowing the airspeed to decrease below the recommended best glide speed. The purpose of pitch control during a glide is to maintain the appropriate glide airspeed, normally the best glide speed when maximum still-air glide distance is required. This may require forward or aft flight control pressure as necessary.
To enter a glide, the pilot should reduce the throttle to the setting specified by the applicable procedure and, if equipped, adjust the propeller control as recommended by the manufacturer. The pilot should initially use elevator back pressure to maintain altitude while the airspeed decreases toward the recommended best glide speed.
In many airplanes, reducing power decreases propeller slipstream over the horizontal stabilizer. This can reduce the tail-down force and cause the airplane’s nose to lower. To maintain the desired pitch attitude during the power change, the pilot should anticipate this tendency and apply appropriate elevator pressure.
This point is particularly important in faster airplanes because they may not lose airspeed readily. Even a slight downward movement of the nose can cause the airspeed to increase.
Once the airspeed has decreased to the recommended best glide speed, the pilot should establish the pitch attitude required to maintain that speed. This should be done primarily by reference to the natural horizon, with a cross-check of the flight instruments.
When the airspeed has stabilized, the airplane should be trimmed to relieve sustained flight control pressure. Accurate control of best glide airspeed is necessary to achieve the intended glide performance.
A stabilized, power-off descent at the best glide speed is often referred to as a normal glide. The beginning pilot should learn the airplane’s pitch attitude and airspeed in relation to the natural horizon. The pilot should also note the sounds of airflow over the airplane’s structure, the forces felt through the flight controls, and the overall feel of the airplane.
Initially, the learner may be unable to recognize slight variations in airspeed and bank angle by sight or by the pressure required on the flight controls. The instructor should explain that an increase in sound level can indicate increasing airspeed, while a decrease in sound level can indicate decreasing airspeed.
When a change in sound level is noticed, the learner should cross-check visual references and flight control pressures. The learner should consciously use all three airspeed cues—sound, visual references, and control pressure—until sufficient experience is gained. The pilot should then remain alert to variations in attitude, feel, and sound.
After a solid understanding of the normal glide is attained, the learner should be instructed in the differences between normal and abnormal glides. Abnormal glides are those performed at airspeeds other than the best glide speed.
Glide airspeeds that are too slow or too fast may prevent the airplane from reaching the intended landing spot. They may also contribute to flat approaches, hard touchdowns, floating, overruns, stalls, and accidents.
Gliding Turns
The absence or reduction of propeller slipstream and P-factor, the reduced effectiveness of flight control surfaces at lower airspeeds, and the airplane’s inherent aerodynamic characteristics can make coordination more difficult during gliding turns than during powered flight. These principles should be thoroughly explained to the learner by the flight instructor.
Three factors in gliding turns tend to lower the nose and increase glide speed:
- A decrease in the vertical component of lift as the airplane banks.
- Excessive rudder inputs resulting from the reduced control pressures associated with lower airflow over the control surfaces.
- The airplane’s inherent stability and aerodynamic characteristics, which may contribute to a nose-down tendency when power is reduced.
These factors may require additional elevator back pressure compared with a straight glide or a level turn. They also affect control coordination.
The rudder is used to compensate for yawing tendencies when entering or leaving a gliding turn. However, the required rudder pedal pressure may be reduced because of the lower aerodynamic forces acting on the control surfaces. A learner accustomed to powered flight may apply excessive rudder pressure during a glide.
This overcontrol can cause slips or skids and create potentially hazardous flight conditions.
Examples of these hazards include:
- A low-level, steep gliding turn during an engine-failure emergency: If the rudder is excessively deflected in the direction of the bank while the pilot increases elevator back pressure in an attempt to maintain altitude, the airplane may enter a stall and spin. At low altitude, recovery may not be possible.
- A power-off landing approach: Excessive rudder pressure may produce a skid or slip and affect the airplane’s bank. The pilot may incorrectly apply opposite aileron to prevent the bank from increasing while also applying elevator back pressure. If allowed to progress, this may result in a cross-controlled condition. A stall in this condition can lead to a rapid spin, with little or no altitude available for recovery.
Level-Off from a Glide
Leveling off from a glide involves different procedures depending on the type of glide and the situation.
- Actual power failure: The pilot should maintain the recommended best glide speed until it is necessary to configure the airplane for landing. The pilot should plan the approach carefully, considering the available altitude, wind, landing area, and aircraft-specific procedures. A 10-percent lead—approximately 100 feet when the descent rate is 1,000 feet per minute—may be used as a training guideline when planning a level-off, but the appropriate procedure depends on the situation.
- Simulated power failure during training: Power should be applied as the 10-percent lead point appears on the altimeter, according to the exercise being performed. This allows a smooth, positive application of power while the pilot raises the nose as required to stop the descent and retrims the airplane.
The level-off from a practice glide should begin before reaching the desired altitude because of the airplane’s downward inertia. The amount of lead depends on the rate of descent and the desired airspeed at the completion of the level-off.
For example, assume the airplane is descending at 500 feet per minute and the desired final airspeed is higher than the glide speed. The altitude lead should begin at approximately 50 feet above the target altitude when using the 10-percent training guideline. At the lead point, power should be increased to the appropriate level-flight cruise setting.
As airspeed and power increase, the airplane’s nose may tend to rise. The pilot should control pitch smoothly so that the level-off is completed at the desired altitude and airspeed.
When recovering from a gliding turn to a normal glide, the elevator back pressure used during the turn should be reduced as necessary. Otherwise, the airplane may pitch up and lose airspeed. This requires careful attention and deliberate control adjustments to re-establish the normal glide airspeed.
Common Errors
Common errors in the performance of glides and descending turns include:
- Failure to adequately clear for other aircraft in the direction of the turn or descent.
- Inadequate elevator back pressure during glide entry, resulting in an excessively steep glide.
- Failure to slow the airplane to approximately the recommended glide speed before lowering the pitch attitude.
- Attempting to establish or 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, resulting in continual chasing of the airspeed indicator.
- Attempting to stretch the glide by applying excessive back-elevator pressure.
- Skidding or slipping during gliding turns and failing to recognize the difference in rudder forces between powered and power-off flight.
- Failure to lower the pitch attitude appropriately during entry into a gliding turn, resulting in a decrease in airspeed.
- Excessive rudder pressure during recovery from gliding turns.
- Inadequate pitch control during recovery from a straight glide.
- Cross-controlling during gliding turns near the ground.
- Failure to maintain a constant bank angle during gliding turns.