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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 the main gear shock struts. [Figure 11-12]  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 flightdeck is placed in the RETRACT position when weight is on the gear, the gear remains 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 releases and the gear retracts. 

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 flightdeck, 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 gear to free fall or extend under their own weight. [Figure 11-13] 

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. In these airplanes, 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. [Figure 11-14] 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. 

## Operational Procedures Preflight 

Because of their complexity, retractable landing gear demands a close inspection prior to every flight. The inspection should begin inside the flightdeck. First, make certain that the landing gear selector switch is in the GEAR DOWN position. Then, turn on the battery master switch and ensure that the landing gear position indicators show that the gear is DOWN and locked. 

External inspection of the landing gear consists of checking individual system components. [Figure 11-14]  The landing gear, wheel well, and adjacent areas should be clean and free 

