Figure 11-10.  Typical landing gear switch with three light indicator. Operational Procedures  Preflight Emergency Gear Extension Systems 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. 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. 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. 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. 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  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  11-12 LANDING GEAR LIMIT (IAS) OPERATING EXTEND 270 .8M RETRACR 235K EXTENDED 320 .82K FLAPS LIMIT (IAS) UP OFF DN L A N D I N G G E A R RIGHT GEAR RIGHT GEAR LEFT GEAR LEFT GEAR NOSE GEAR NOSE GEAR LANDING GEAR LIMIT (IAS) OPERATING  EXTEND 270—.8M RETRACT 235K EXTENDED 320—.82K FLAPS LIMIT (IAS) Landing gear indicator (top) illuminated (red) Landing gear lever Landing gear limit speed placard  Override trigger Landing gear indicator  (bottom) illuminated  (green)—related gear  down and locked