Landing gear safety systems are crucial in ensuring the reliable operation of landing gear. As such, there are many different landing gear safety devices, mostly those that prevent the gear from retracting or collapsing while in use. Other devices do things such as communicate the position of each individual landing gear to the pilot. There are four primary landing gear system safety devices. They are the landing gear safety switch, ground locks, landing gear position indicators, and nose wheel centering. This blog will explain each of them, their role, and how they work.
Landing Gear Safety Switch
The landing gear squat switch, found on most aircraft, is a switch positioned to open and close depending on the extension or compression of the main landing gear strut. The squat switch is wired into many of the system operating circuits. One such circuit prevents the gear from being retracted while the aircraft is on the ground. This locking can be achieved in a number of ways. A solenoid that extends a shaft to physically disable the gear position selector is perhaps the most common method. When the landing gear is compressed, the squat safety switch is open and the center shaft of the solenoid protrudes a lock-pin to hold the landing gear in place. After takeoff, the strut extends, retracting the lock-pin and allowing the gear to be raised.
Ground locks are used in landing gear as extra insurance that the landing gear will remain locked in position while the aircraft is on the ground. A ground lock is an external device that is placed in the retraction mechanism to prevent its movement. Ground locks can be as simple as a pin in the holes of gear components that stop the gear from collapsing. Another type of ground lock clamps onto the exposed piston of the gear cylinder, preventing it from retracting. All ground locks are marked with a red streamer so they are easily visible and removed before flight. Ground locks are usually carried in the aircraft and put in place by the flight crew.
Landing Gear Position Indicators
As their name suggests, landing gear position indicators are used to inform the pilot of the landing gear position status. They are located on the instrument panel adjacent to the gear selector handle, though there are many arrangements for gear position indicators. An illuminated green light is the most common denotation for the landing gear being down and locked. Three green lights means the aircraft is ready to land. If all lights are out, it means that the gear is up and locked.
Nose Wheel Centering
As most aircraft have steerable nose wheel gears for taxiing, a way to align the nose gear before retraction is necessary. This is done by centering cams built into the shock strut structure. An upper cam connects into a lower cam recess when the gear is fully extended, thereby aligning the gear for retraction. After landing, when weight returns to the wheels, the shock strut is compressed and the center cams separate. This results in the lower shock strut rotating into the upper strut cylinder. This rotation is controlled to steer the aircraft. Smaller aircraft sometimes incorporate an external roller or guide pin on the strut. As the strut folds into the wheel during retraction, the roller guide pin engages a track mounted to the wheel well structure. This track serves as a guide for the roller or pin such that the nose wheel is straightened as it enters the wheel well.
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