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This question came up after reading the following comments on the answer to this question: Raising the flaps right after touchdown. Good or bad?

Philippe Leybaert Jan 20 at 23:00

The retractable landing gear is an excellent point. It makes perfect sense. I've read a few stories of pilots retracting the gear after landing instead of raising the flaps.

voretaq7♦ Jan 20 at 23:02

The retractable landing gear issue is particularly notable because early in the landing roll you may not have enough weight on the wheels to activate the squat switch that prevents you from retracting your gear while you're using them to hold the plane up. Your perfect landing could turn into a nice, smooth gear-up incident if you bump the wrong switch.

Questions

  • How does the squat switch work (which prevent gear up on the ground)?
  • Are there any reports on issues of gear up on ground? (I only know one)?
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    $\begingroup$ A more formal name is "weight-on-wheel switch". It can actually be anything from a strain gauge on the landing gear leg to a contact switch which is activated by the oleo strut position. $\endgroup$
    – Peter Kämpf
    Nov 3, 2014 at 16:23
  • $\begingroup$ @PeterKämpf: Another name sometimes seen is "ground-shift mechanism". $\endgroup$
    – Vikki
    Feb 8, 2021 at 2:51

1 Answer 1

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The squat switch is essentially a sensor that senses if the weight of the aircraft is resting on the gear. This can be done by load cell or displacement sensor on the suspension arm.

Like any other component these sensors can fail though engineers prefer them to fail in an always on (weight on the gear) state; that way the pilot just needs to deal with gear failing to retract after takeoff and needing to land again to check it out rather than dropping the plane on the ground and causing more damage.

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    $\begingroup$ As one visual example, here's the squat switch from a King Air. When the plane is resting on the gear the strut is compressed, and the scissor link is bent as you see it here. When weight comes off the wheels the strut extends and the metal piece attached to the fork presses the switch, signaling the rest of the system that there is no weight on that wheel. $\endgroup$
    – voretaq7
    Nov 3, 2014 at 20:05

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