Aileron failures can cause serious control problems. Causes of aileron failure include:
- Improper connection of the aileron control circuit during assembly.
- Aileron control lock that was not removed before flight.
- Separation of the aileron gap seal tape.
- Interference of a foreign object with free and full travel of the control stick or aileron circuit.
- Seat belt or shoulder harness in the back seat that was used to secure the control stick and not removed prior to flight.
- Structural failure and/or aileron flutter.
These failures can sometimes be counteracted successfully, partly because there are two ailerons. If one aileron is disconnected or locked by an external control lock, the degree of motion still available in the other aileron may exert some influence on bank angle control. Use whatever degree of aileron is available to maintain control of the glider. The glider may be less difficult to control at medium to high airspeeds than at low airspeeds.
If the ailerons are not functioning adequately and roll control is compromised, the secondary effect of the rudder can be used to make gentle adjustments in the bank angle so long as a safe margin above stall speed is maintained. The primary effect of the rudder is to yaw the glider. The secondary effect of the rudder is subtler and takes longer to assert itself. In wings-level flight, if left rudder is applied, the nose yaws to the left. If the pressure is held, the wings begin a gentle bank to the left. If right rudder pressure is held and applied, the glider yaws to the right, then begins to bank to the right. This secondary banking effect by the rudder is useful if the pilot must resort to using the rudder to bank the glider wings. The secondary effect of the rudder works best when the wings are level or held in a very shallow bank, and is enhanced at medium to high airspeeds. Try to keep all banks very shallow. If the bank angle becomes excessive, it is difficult or impossible to recover to wings-level flight using the rudder alone. If the bank is becoming too steep, use any aileron influence available, as well as all available rudder to bring the wings back to level. If a parachute is available and the glider becomes uncontrollable at low airspeed, the best chance to escape serious injury may be to bail out of the glider from a safe altitude.
Rudder failure is extremely rare because removing and installing the vertical fin/rudder combination is not part of the normal sequence of rigging and de-rigging the glider (as it is for the horizontal stabilizer/elevator and for the wing/aileron combinations). Poor directional control is so obvious to the pilot from the very beginning of the launch that, if rudder malfunction is suspected, the launch can be aborted early.
Rudder malfunctions are most likely to occur after failure to remove the rudder control lock prior to flight or when an unsecured object in the cockpit interferes with the free and full travel of the rudder pedals. Preflight preparation must include removal of all flight control locks and safe stowage of all items on board. The pretakeoff checklist includes checking all primary flight controls for correct, full travel prior to launch.
Although rudder failure is quite rare, the consequences are serious. If a control lock causes the problem, it is possible to control the glider airspeed and bank attitude, but directional control is compromised due to limited rudder movement. In the air, some degree of directional control can be obtained by using the adverse yaw effect of the ailerons to yaw the glider. During rollout from an aborted launch or during landing rollout, directional control can sometimes be obtained by deliberately grounding the wingtip toward the direction of desired yaw. Putting the wingtip on the ground for a fraction of a second causes a slight yaw in that direction; holding the wingtip firmly on the ground usually causes a vigorous yaw or ground loop in the direction of the grounded wingtip.
Careless stowage of cockpit equipment can result in rudder pedal interference at any time during a flight. During flight, if an object is interfering with or jamming the rudder pedals, attempt to remove it. If removal is not possible, attempt to deform, crush, or dislodge the object by applying force on the rudder pedals. It also may be possible to dislodge the object by varying the load factor, but ensure that dislodging the object does not result in its lodging in a worse place where it could jam the elevator or aileron controls. If the object cannot be retrieved and stowed, a precautionary landing may be required.
Commonly misplaced objects that can cause flight control interference include:
- Water bottles,
- Electronic computers,
- Containers of food and similar items,
- Clothing, and
Control these items by proper planning and good cockpit discipline.
Secondary Flight Controls Systems
Secondary flight control systems include the elevator trim system, wing flaps, and spoilers/dive brakes. Problems with any of these systems can be just as serious as problems with primary controls.
Elevator Trim Malfunctions
Compensating for a malfunctioning elevator trim system is usually as simple as applying pressure on the control stick to maintain the desired pitch attitude, then bringing the flight to safe conclusion. Inspect and repair the trim system prior to the next flight.
Spoiler/Dive Brake Malfunctions
Spoiler/dive brake system failures can arise from rigging errors or omissions, environmental factors, and mechanical failures. Interruptions or distractions during glider assembly can result in failure to properly connect control rods to one or both spoilers/dive brakes. Proper use of a comprehensive checklist reduces the likelihood of assembly errors. If neither of these spoilers/dive brakes is connected, then one or both of the spoilers/dive brakes may deploy at any time and retraction becomes impossible. This is a very hazardous situation for several reasons. One reason is that the spoilers/ dive brakes are likely to deploy during the launch or the climb, causing a launch emergency and a possible tow failure incident. Another reason is that the spoilers/dive brakes might deploy asymmetrically: one spoiler/dive brake retracted and the other spoiler/dive brake extended, resulting in yaw and roll tendencies that do not arise when the spoilers/dive brakes deploy symmetrically. A pilot expecting a smooth ,symmetrical deployment would be faced with a control issue that compromises flight safety. Finally, it is not possible to correct the situation by retracting the spoiler/dive brake(s) because the failure to connect the controls properly usually means that pilot control of the spoiler/dive brake has been lost.
If asymmetrical spoiler/dive brake extension occurs and the extended spoiler/dive brake cannot be retracted, several choices must be made. Roll and yaw tendencies due to asymmetry must be overcome or eliminated. One way to solve this problem is to deploy the other spoiler/dive brake to restore the symmetry. The advantages include immediate relief from yaw and roll tendencies and protection against stalling with one spoiler/dive brake extended and the other retracted, which could result in a spin. The disadvantage of deploying the other spoiler/dive brake is that the glide ratio is reduced. If the spoiler/dive brake asymmetry arises during launch or climb, the best choice is to abort the launch, extend the other spoiler/dive brakes to relieve the asymmetry, and make a precautionary or emergency landing.
Environmental factors include low temperature or icing during long, high altitude flights, which may occur during a mountain wave flight. Low temperature causes contraction of all glider components. If the contraction is uneven, the spoilers/dive brakes may bind and be difficult or impossible to deploy. Icing can also interfere with operation of the spoilers/ dive brakes. High temperature, on the other hand, causes all glider components to expand. If the expansion is uneven, the spoilers/dive brakes may bind in the closed position. This is most likely to occur while the glider is parked on the ground in direct summer sunlight. The heating can be very intense, particularly for a glider with wings painted a color other than reflective white.
Mechanical failures can cause asymmetrical spoiler/dive brake extension. For example, the spoiler/dive brake extend normally during the prelanding checklist but only one spoiler/dive brake retracts on command. The other spoiler/dive brake remains extended, due perhaps to a broken weld in the spoiler/dive brake actuator mechanism, a defective control connector, or other mechanical failure. The glider yaws and banks toward the wing with the extended spoiler/dive brake. Aileron and rudder are required to counteract these tendencies. To eliminate any possibility of entering a stall/spin, maintain a safe margin above stall airspeed. If the decision to deploy the other spoiler/ dive brake is made to relieve the asymmetry, controlling the glider becomes much easier but gliding range is reduced due to the additional drag of the second spoiler/dive brake. This may be a significant concern if the terrain is not ideal for landing the glider. Nevertheless, it is better to make a controlled landing, even in less than ideal terrain, than it is to stall or spin.