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Stupid Question... About Landing Gear

Fri Oct 22, 2004 4:27 am


I have a less intelligent question to ask, but, i was wondering, if the aircraft is on the ground and u try to to put the gear up, (say a 737), can you move the lever up? or is it locked in place? or does it go up and it's like an overide thing where it won't let u bring the gear up...

I would imagine the gear motor wouldn't try to raise it up while the weight of the aircraft is on it...

... again, as I said ... stupid question

replies are welcomed and if you laugh and call me a dumbass, that's ok. It's not the first time I've heard it  Smile


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RE: Stupid Question... About Landing Gear

Fri Oct 22, 2004 4:40 am

Nothing stupid about it!
You can move the lever up. If there's hydraulic pressure on the system that retracts the landing gear then they may retract depending on weight on wheels and whether or not the safety pins are installed. In a 737 just sitting on the ground the gear probably won't retract but there are many situations where at least the nose strut will try.
Generally speaking and this is really general, if there's no hydraulic pressure on the system then the gear doesn't do anything. (And then there's Douglas planes, DC-8 in particular, that'll make you look really stupid really quickly if you don't pay attention. No hydraulics needed.)
Many aircraft have a lever lock that will prevent inadvertant movement of the gear handle in situations where it shouldn't be. In this case there's normally an override button or mechanism that will allow you to reposition the handle for maintenance or other purposes.
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RE: Stupid Question... About Landing Gear

Fri Oct 22, 2004 5:13 am

No A&P, but I believe it's called a "Squat Switch"

which basically tells the plane, "you're on the ground, don't do anything stupid"

so if you throw the lever, the plane knows enough to not try and retract the gear.

this dives into the whole bad idea about taxi to the runway with gear lever UP. bump here, bump there, unload the gear and ............
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RE: Stupid Question... About Landing Gear

Fri Oct 22, 2004 6:16 am

There are pins that are put into the down lock mechanism to prevent inadvertant retraction of the gear during periods that the airplane is parked for an extended period of time. If they are not in place then bad things can happen when the gear handle is placed in the up position.

The down lock mechanism has an overcenter linkage with holes in it where the gear pin goes. If the mechanism cannot override the overcenter linkage, then the gear cannot not be retracted.

Usually (not always) the weight of the airplane will keep the mains from folding (one exception is the body gear on the 747, which will and has retracted). The nose gear is another item and if you have hydraulics and you put the gear handle to the up position, the nose gear is very likely to do what it is commanded to do.
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RE: Stupid Question... About Landing Gear

Fri Oct 22, 2004 6:19 am


Not true for air transport catergory aircraft. The air/ground system (squat switch) only deals with the gate that blocks the gear handle's movement (see Avionikers post). I believe smaller aircraft with electrically retractable gear work with the squat switch as a safety.

On a large commercial airliner, if you put the handle up the system wants to retract. Give it the pressure and things (normally the nose gear) will move. In fact on a B747, the body gear will also attempt to retract. Saw it happen one day outside Hangar 17 at JFK back in '87.

Avioniker is also correct about the DC8. I recall a mechanic was killed when working in the nose wheel well of the aircraft. He reached up and pulled a the cable which actuates the unlock. Since the gear is not perpendicular to the airframe, it promptly started to retract and crushed the mechanic. I'm remembering somewhere in Arizona.
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RE: Stupid Question... About Landing Gear

Fri Oct 22, 2004 7:04 am

On the 737, for example, if the squat switch malfunctioned or was somehow bypassed, placing the landing gear handle in the up position with hydraulic pressure available will put that pressure on the RETRACT side of the extend/retract cylinder. What happens next is probably nothing, except maybe the nose gear will retract.

Mentally view the landing gear from behind. It pivots around a trunnion up inside the wing, more or less centered between the inboard and outboard wheels. The radius from the center of that trunnion to the plane of the taxiway or hangar floor is less at the center of the gear and slightly greater out at the tread of each wheel. To retract the mains it would have to lift the entire airplane, while dragging the tires sideways across dry concrete. If you are lucky it will not do this.

It's tempting to assume that it has plenty of power to do this, but remember that airliners are not overbuilt because that carries a weight penalty. The gear retract mechanism including the geometry that determines leverage and the amount of the 3000PSI or so hydraulic pressure available to the task is decided upon by the engineers on the design team to do one basic task - lift the unencumbered landing gear into the well with no resistance other than gravity.

I'd bet you a thousand bucks the mains would not retract. I would not bet my job on it though!

The nose gear is different. It retracts forward, not laterally. Therefore the wheels can easily roll to overcome the initial resistance.

Years ago I was standing next to a (Beautiful black) P-38 while they cranked the engine over. Somehow a line got hooked up wrong on the landing gear valves and the little bit of pressure from the engine cranking over pulled the downlock and started the mains up. They retract forward and they went over-center allowing the plane to fall on its tail. It bent both tailbooms across the top - junked them. IIRC the props then hit the ground. It was a sad sight.

So, without getting too techincal here - on the ground, gear handle DOWN is a good thing.

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RE: Stupid Question... About Landing Gear

Fri Oct 22, 2004 1:43 pm

Thank you very much, everyone. You learn something new every day.  Smile
I should really brush up on my technical terminology though.


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