Landing Gear | Safety Devices

Most airplanes with a retractable landing gear have a gear warning horn that will sound when the airplane is configured for landing and the landing gear is not down and locked Normally, the horn is linked to the throttle or flap position, and/or the airspeed indicator so that when the airplane is below a certain airspeed,configuration, or power setting with the gear retracted, the warning horn will sound Accidental retraction of a landing gear may be prevented by such devices as mechanical down-locks, safety switches, and ground locks Mechanical down-locks are built-in components of a gear retraction system and are operated automatically by the gear retraction system To prevent accidental operation of the down-locks, and inadvertent landing gear retraction while the airplane is on the ground, electrically operated safety switches are installed A landing gear safety switch, sometimes referred to as a squat switch, is usually mounted in a bracket on one of the main gear shock struts, When the strut is compressed by the weight of the airplane, the switch opens the electrical circuit to the motor or mechanism that powers retraction In this way, if the landing gear switch in the cockpit is placed in the RETRACT position when weight is on the gear, the gear will remain extended, and the warning horn may sound as an alert to the unsafe condition Once the weight is off the gear, however, such as on takeoff, the safety switch will release and the gear will retract Many airplanes are equipped with additional safety devices to prevent collapse of the gear when the airplane is on the ground These devices are called ground locks One common type is a pin installed in aligned holes drilled in two or more units of the landing gear support structure Another type is a spring-loaded clip designed to fit around and hold two or more units of the support structure together All types of ground locks usually have red streamers permanently attached to them to readily indicate whether or not they are installed.

Emergency Gear Extension Systems:

The emergency extension system lowers the landing gear if the main power system fails Some airplanes have an emergency release handle in the cockpit, which is connected through a mechanical linkage to the gear up-locks When the handle is operated, it releases the up-locks and allows the gears to free fall, or extend under their own weight  Due to the wind stream, limitations may apply to the speed at which the mechanism may be employed to ensure the gear lock when extended On other airplanes, release of the up-lock is accomplished using compressed gas, which is directed to up-lock release cylinders  In some airplanes, design configurations make emergency extension of the landing gear by gravity and air loads alone impossible or impractical and so provisions are included for forceful gear extension in an emergency Some installations are designed so that either hydraulic fluid or compressed gas provides the necessary pressure, while others use a manual system such as a hand crank for emergency gear extension  Hydraulic pressure for emergency operation of the landing gear may be provided by an auxiliary hand pump, an accumulator, or an electrically powered hydraulic pump depending on the design of the airplane.

Omid Mohammadi

Maintenance Engineer Of I.R.I

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Landing Gear | Safety Devices

by Omid Mohammadi time to read: 2 min
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