A secondary stall occurs after a recovery from a preceding stall. It is caused by attempting to hasten the completion of a stall recovery with abrupt control input before the glider has regained sufficient flying speed and the critical AOA is again exceeded. When this stall occurs, the back-elevator pressure should again be released as in a normal stall recovery. When sufficient airspeed has been regained, the glider can then be returned to wings-level, straight flight.
Although the stalls already discussed normally occur at a specific airspeed, the pilot must thoroughly understand that all stalls result solely from attempts to fly at excessively high angles of attack. During flight, the AOA of a glider wing is determined by a number of factors, the most important of which are airspeed, gross weight of the glider, and load factors imposed by maneuvering.
At gross weight, the glider consistently stalls at the same indicated airspeed if no acceleration is involved. However, the glider stalls at a higher indicated airspeed when excessive maneuvering loads are imposed by steep turns, pull-ups, or other abrupt changes in its flightpath. Stalls entered from such flight situations are called “accelerated maneuver stalls, a term that has no reference to the airspeeds involved. Stalls that result from abrupt maneuvers tend to be more rapid or severe than the unaccelerated or steady state stall. Accelerated stalls occur at higher-than-normal airspeeds and may be unexpected by pilots. These accelerated stalls result when the AOA exceeds the angle necessary to stall the airflow over the wing. The relative wind angle increases as the loads on the wings require more lift to change direction, either vertically or horizontally, and inertia pushes the wings into the airmass resulting in an increased AOA. Depending on the wing configuration and quality of coordination, one wing may stall prior to the other wing resulting in a wingover entry into a spiral or spin. If the wings have a slight or pronounced sweep, one wing can easily develop more lift than the other wing almost instantaneously resulting in a wingover before the pilot can react. This is the common killer scenario of a pilot turning too tightly in the traffic pattern and crashing upside down.
Accelerated maneuver stalls should not be performed in any glider in which this maneuver is prohibited by the GFM/ POH. If they are permitted, they should be performed with a bank of approximately 45° and never at a speed greater than the glider manufacturer’s recommended airspeeds or the design maneuvering speed specified for the glider. The design maneuvering speed is the maximum speed at which the glider can be stalled or the application of full aerodynamic control will not exceed the glider’s limit load factor. At or below this speed, the glider is designed so that it stalls before the limit load factor can be exceeded. The objective of demonstrating accelerated stalls is not to develop competency in setting up the stall, but rather to learn how they may occur and to develop the ability to recognize a prestall situation immediately and then take the proper recovery action. It is important that recovery is made at the first indication of a stall, or immediately after the stall has fully developed; a prolonged stall condition should never be allowed.
A glider stalls during a coordinated turn as it does from straight flight, except the pitching and rolling actions tend to be more sudden. If the glider is slipping toward the inside of the turn at the time the stall occurs, it tends to roll rapidly toward the outside of the turn as the nose pitches down because the outside wing stalls before the inside wing. If the glider is skidding toward the outside of the turn, it has a tendency to roll to the inside of the turn because the inside wing stalls first. If the coordination of the turn at the time of the stall is accurate, the glider’s nose pitches away from the pilot just as it does in a straight flight stall, since both wings stall simultaneously. The configuration of the wings has a strong influence on exactly how a glider reacts to different airflows. The safe approach is to fly the specific glider into these situations at higher altitudes to determine how that glider reacts. The glider pilot should commit those newly discovered prestall conditions and indications to memory to avoid those conditions at lower altitudes where recovery is improbable or impossible.
Glider pilots enter an accelerated stall demonstration by establishing the desired flight attitude and then, with smooth actions, firmly and progressively increasing the AOA until a stall occurs. Because of the rapidly changing flight attitude, sudden stall entry, and possible loss of altitude, it is extremely vital that the area be clear of other aircraft. Entry altitudes should be adequate for safe recovery.
Actual accelerated stalls occur most frequently during turns in the traffic pattern close to the ground while maneuvering the glider for the approach. The demonstration of an accelerated stall is accomplished by exerting excessive back elevator pressure. It usually occurs during improperly executed steep turns, stall and spin recoveries, and pullouts from steep dives. The objectives are to determine the stall characteristics of the glider and develop the ability to instinctively recover at the onset of a stall at other-than-normal stall speed or flight attitudes. An accelerated stall, although usually demonstrated in steep turns, may actually be encountered any time excessive back-elevator pressure is applied and/or the AOA is increased too rapidly.
From straight flight at maneuvering speed or less, the glider should be rolled into a steep banked (45° maximum) turn and back-elevator pressure gradually applied. After the bank is established, back-elevator pressure should be smoothly and steadily increased. The resulting apparent centrifugal force pushes the pilot’s body down in the seat, increases the wing loading, and decreases the airspeed. Back-elevator pressure should be firmly increased until a definite stall occurs.
When the glider stalls, recovery should be made promptly by releasing back-elevator pressure. If the turn is uncoordinated, one wing may tend to drop suddenly, causing the glider to roll in that direction. If this occurs, the glider should be returned to wings-level, straight flight with coordinated control pressure.
A glider pilot should recognize when an accelerated stall is imminent and take prompt action to prevent a completely stalled condition. It is imperative that prolonged stalls, excessive airspeed, or loss of altitude, and spins be avoided.
The objective of a crossed-control stall demonstration maneuver is to show the effect of improper control technique and to emphasize the importance of using coordinated control pressures whenever making turns. This type of stall occurs with the controls crossed—aileron pressure applied in one direction and rudder pressure in the opposite direction—and the critical AOA is exceeded. [Figure 7-34]
This is a stall that is most likely to occur during a poorly planned and executed base-to-final approach turn, and often is the result of overshooting the centerline of the runway during that turn. Normally, the proper action to correct for overshooting the runway is to increase the rate of turn by using coordinated aileron and rudder. At the relatively low altitude of a base-to-final approach turn, improperly trained pilots may be apprehensive of steeping the bank to increase the rate of turn.
The addition of rudder pressure on the inside of the turn causes the speed of the outer wing to increase, creating greater lift on that wing. To keep that wing from rising and to maintain a constant angle of bank, opposite aileron pressure is required. The added inside rudder pressure also causes the nose to lower in relation to the horizon. Consequently, additional back-elevator pressure would be required to maintain a constant pitch attitude. The resulting condition is a turn with rudder applied in one direction, aileron in the opposite direction, and excessive back-elevator pressure—a pronounced crossed-control condition.
Since the glider is in a skidding turn during the crossed-control condition, the wing on the outside of the turn increases speed and produces more lift than the inside wing, and the glider starts to increase its bank. The down aileron on the inside of the turn helps drag that wing back, slowing it and decreasing its lift. This further causes the glider to roll. The roll may be so fast that it is possible the bank will be vertical or past vertical before it can be stopped.
For the demonstration of the maneuver, it is important that it be entered at a safe altitude because of the possible extreme nose-down attitude and loss of altitude that may result. Before demonstrating this stall, the pilot should clear the area for other air traffic. While the gliding attitude and airspeed are being established, the glider should be retrimmed. When the glide is stabilized, the glider should be rolled into a medium banked turn to simulate a final approach turn that would overshoot the centerline of the runway. During the turn, excessive rudder pressure should be applied in the direction of the turn but the bank held constant by applying opposite aileron pressure. At the same time, increased back-elevator pressure is required to keep the nose from lowering.
All of these control pressures should be increased until the glider stalls. When the stall occurs, releasing the control pressures and simultaneously decreasing the AOA initiates the recovery. In a crossed-control stall, the glider often stalls with little warning. The nose may pitch down, the inside wing may suddenly drop, and the glider may continue to roll to an inverted position. This is usually the beginning of a spin. It is obvious that close to the ground is no place to allow this to happen.
Recovery must be made before the glider enters an abnormal attitude (vertical spiral or spin); it is a simple matter to return to wings-level, straight flight by coordinated use of the controls. The pilot must be able to recognize when this stall is imminent and must take immediate action to prevent a completely stalled condition. It is imperative that this type of stall not occur during an actual approach to a landing, since recovery may be impossible prior to ground contact due to the low altitude.
Common errors during advanced stalls include:
- Improper pitch and bank control during straight-ahead and turning stalls.
- Rough or uncoordinated control procedures.
- Failure to recognize the first indications of a stall.
- Failure to achieve a stall.
- Poor recognition and recovery procedures.
- Excessive altitude loss or airspeed or encountering a secondary stall during recovery.