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Asked by: Ben Jenkins
Could someone please advise as to how exactly the squat switch on a retractable undercarriage prevents the system from activating when the weight is on the wheels?
Dave on Feb 04, 2014
The Squat Switch is mounted on the gear by two attach points – one on each side of the two telescoping parts of the gear, allowing the switch to be electrically closed or open, depending on if the gear is compressed or extended (weight on/weight off).
When the aircraft is in the air the telescoping part of the landing gear is fully extended mostly by the pressure of the gas inside it. In this extended position the Squat Switch is held in the electrically closed position, allowing the gear to be operated up and down.
When the aircraft is on the ground the landing gear is compressed by the weight of the aircraft upon it. In this compressed position the Squat Switch is held in the electrically open position, preventing the gear from being operated up or down.
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on Feb 04, 2014
The Squat Switch (also called Weight on Wheels Switch, or WoW) is mounted on the telescoping landing gear by two attach points. Normally the body of the switch is mounted on the upper half of the gear, close to the telescoping point and does not physically move. This way the wiring to the switch is not required to flex and wear out prematurely. The actuating lever of the switch, through a small attaching arm, is mounted to the lower half of the gear, just on the other side of the telescoping point.
When the aircraft is in the air the gear halves are extended (elongated) by the pressure of the gas inside. When the aircraft is on the ground the gear halves are compressed (shortened) by the weight of the aircraft upon them. This travel of the telescoping landing gear halves is such that the actuating arm moves to electrically open or close (weight off/weight on) the switch.
The Squat Switch actually activates/deactivates the Touchdown Relay. The landing gear, certain avionics, and many accessories are all wired through the Touchdown Relay. The idea, obviously, is to prevent inappropriate systems from functioning when not in a safe condition to do so.
Airborne, the switch is open, deactivating the Touchdown Relay, and allowing the gear to be operated up and down. On the ground the switch is closed, activating the Touchdown Relay, and preventing the gear from being operated up or down.
Works both ways – you can’t retract your gear while on the ground, and you can’t perform an enhanced alignment of your inertial nav (at least on a C-130H) while airborne.
John D Collins
on Feb 04, 2014
It depends on the design of the aircraft. In the Bonanza series, a switch is mounted on the strut or struts with the power to the circuit for the retract routed thru it. When the strut is depressed, the switch is open and doesn’t allow current to flow to the motor. When airborne, with the weight off the wheels, the strut extends and the switch closes, thus permitting power to get to the motor. Later Bonanza models changed this design to use a relay controlled by the squat switch that has the same effect, Regardless, this should never be relied on by a pilot and there are untold number of gear up landings because the pilot raised the gear accidentally on the roll out thinking they had grabbed the flap switch. Some hot dog pilots have used the squat switch to raise the gear on takeoff as soon as the struts extended. These often end badly. Switches can be out of adjustment or fail and a pilot can raise the gear while stopped on the taxiway or in the hangar and usually the nose gear will retract before it can be stopped. The squat switch protection is very limited in usefulness for this purpose and I would not depend on it.
on Feb 04, 2014
Don’t know how much detail you are looking for but, as John said, it will depend on the aircraft involved. Have you looked at the POH for yours? I know there’s a pretty decent description for the one in a Cutlass in Section 7 of the POH.
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