Squat Switch

[CK]

How exactly does a squat switch prevent a gear from coming up?

http://ntsb.gov/ntsb/brief.asp?ev_id=20080613X00852&key=1

I just read that report. I figure he took off with the gear handle in the up position, but the hydrolic light intrigues me. Does a squat switch only prevent the uplocks from disengaging or would it prevent the entire hydrolic system from engaing? If it only prevents the uplocks from unlocking that would cause the hydolic flow light at low power as the hydrolic system would be powered. I have the Flight Safety book for the 421, but still can't find the answer.


Thanks,

Alex.

 

[Polar742]

I think HOW it works is dependent on the airplane.

On the 3 Part 25 airplanes I've flown, all it does (interms of the gear) is mechanically lock the landing gear lever in the down position. Once the aircraft is airborne the mechanical lock retracts. There is a way to manually override that in very rare circumstances.

As far as Pt 23 aircraft, I don't know if there is a standard, and it might be left up to the manufactures as to what the safety switch does.

 

[c172captain]

On my airplane the landing gear retraction circuit is not connected until the gear has been extended 8inches (takeoff). Once the gear extends the 8inches, the circuit makes contact and the gear can then be retracted. Prior to that (on the ground) no matter what position the gear selector is in nothing will happen because the circuit has not been completed.

Kind of hard to explain without a visual diagram... but the squat switch is on the tip of a compressable piece of steel in the shape of a sideways triangle. When you are on the ground, the triangle is compressed and the switch is tilted away from the circuit almost pointing towards the ground. Once you takeoff the triangle extended which flatens out the squat switch to lay down flat onto the rest of the landing gear circuit, thus making contact and allowing electrial energy to flow to control the landing gear.

 

[Goldmember]

The squat switch itself is nothing more than circuitry. When you are on the ground and the gear is locked down, the squat switch circuit, which is on one of the mains, is separated or "open." When you lift off the ground, the gear falls to full down position, thus closing or "completing" the circuit of the switch and allowing the electric current to activate your hydraulic pump. This is the reason your gear won't come up when you are on the ground and the lever is up...in this case, since you can't activate the pump because the circuit is open, your lever is useless, kind of like turning the power to your tv on with it unplugged.

Did I get that backwards? Open and closed circuits to activate-deactivate? I'm sure someone on here can answer smarter than I. I don't quite see what the squat swithc has to do with the above case, However, you might find this tidbit interesting. On my commercial checkride my examiner was telling me how he was taxiing a King air 90 and the gear just went up on him. Then, during his investigation, he got a call from a buddy of his who said that a king air 90 did the same thing sitting on the ramp with the mechanic. Nobody in it, not on, but connected to external power, boom, gear up, king air flat on it's belly. The problem ended up being a crossed wire problem with the sensor that shows the gear up or down.

 

[Cessnaflyer]

The squat switches on Cessna singles is on the nose gear. Not a great placement with far aft CG because the switch sometimes completes the circuit and up goes the gear.

 

[bLizZuE]

I can't speak for the cessna, but the 'nole has a 'squat' switch on the left main for this.

When the weight of the a/c is on the mains the switch is open. Once the weight comes OFF the mains, the gear fully extends the strut and connects the switch. This allows the electric pump to be activated with the gear lever.

I don't like the term 'squat' switch, because it's really the opposite. When the plane comes OFF the ground it activates, and a lot of people think of it vice versa.