Every year dozens of pilots land airplanes with the landing gear still retracted. A few more fail to even leave the ground before the gear comes up, no doubt ruining what otherwise would have been a good day of flying. To try and ensure this would never happen, engineers devised a brilliant, but basic, solutions called a squat switch.
Most light aircraft landing gear are electrically actuated, hydraulically driven systems. That means when the gear lever is selected up or down, the electrical system tells the hydraulic system to get to work. When the airplane is on the ground, a circuit in the electrical system is open, making it impossible for the gear to come up, regardless of the handle position. When weight starts to come off the gear, it extends and closes the circuit, allowing the gear to operate as the pilot chooses.
Squat switches can be in either one of the main gears, or in the nose, although clearly the mains are a more desirable position.
Top Gun moments
Squat switches do not guard against stupidity and “Watch this” moments. Many airplanes have taken off, only to settle back on the runway after their owners thought they would be cool and allow the switch to retract the gear the second the wheels left the ground. Try explaining that to the FAA and the insurance company.
Avionics integration
Since the squat switch is a convenient indicator of more or less when the wheels are off the ground, avionics manufacturers have tied various capabilities into the system. Transponders that automatically go between standby mode on the ground and altitude mode in the air often operate via the squat switch, as do some traffic systems.
Related Articles
