Landing Gear Off?
Thread Starter
Landing Gear Off?
Hi everyone, i have noticed that on most Boeing aircraft (737, 757, 767) have an 'OFF' position as well as 'UP'. Is it something to do with the hydraulics or something else?
Also, on aircraft such as the Airbus and 777 (i think), there is only two positions. Why do newer aircraft now have an 'OFF' position?
Many Thanks
Dan
Also, on aircraft such as the Airbus and 777 (i think), there is only two positions. Why do newer aircraft now have an 'OFF' position?
Many Thanks
Dan
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I thought it was to deliberately disengage the circuit. In the up or down position, the system would run until the limits were reached. If it were unable to recognise these limits, the system would keep running which might eventually cause problems. Having an OFF position removes this potential problem.
I look forward to seeing replies from people who actually DO know the answer though.
I look forward to seeing replies from people who actually DO know the answer though.
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When the landing gear is retracted, uplocks hold it in place. Therefore there is no need to pressurise the hydraulic lines to hold it in place- it takes energy to power the system to 3000psi for hours on end unnecessarily. The gear freefalls with a down selection and the hydraulics become pressurised to close the doors again and the downlocks are powered to the locked position.
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On a 737 at least,once the gear is powered up,it 'hangs' on the uplock by a geometric lock.As such,there is no more need to continually supply hydraulic pressure to keep the gear up.By selecting the gear to 'off',it isolates the gear from the hyd supply.Only when the gear is selected 'down' is further hyd pressure required,in order to operate the 'uplock' actuator which will then break the geometric lock,so allowing the gear to freefall until it locks in the down position.
Don't know about the 777 etc.
Don't know about the 777 etc.
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It's a primative Boeing thing, depressurize Landing Gear associated Hydraulics in flight, certainly 70,72,73,74 (classic), 75. I haven't reached the dizzy heights of 76 or 77 yet but suspect that the more modern ? have taken a leaf from the bus book and do it automatically. That would leave out the 76 then as that is really a 75 on steroids ! no idea about the 744 either but I expect it's similar to the 742.
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Was this not mentioned in Fate is the Hunter.
They took off and could not switch off the hydraulic pump that powered the undercarriage. Eventually it overheated and caught fire. Perhaps this is why there is an off position.
There then followed an intersting landing below minimums into one of the greenland airports.
They took off and could not switch off the hydraulic pump that powered the undercarriage. Eventually it overheated and caught fire. Perhaps this is why there is an off position.
There then followed an intersting landing below minimums into one of the greenland airports.
Thread Starter
So, if you left the lever in the 'UP' position throughout the flight (instead of 'OFF'), would it cause any problems?
Woderick, i'm pretty sure the 767 has an 'OFF' position too.
Dan
Woderick, i'm pretty sure the 767 has an 'OFF' position too.
Dan
18greens yep, baically that's what happened. The airport was at Stephenville NF, (CYJT) been there a few times. Once did the same split arse approach as Gann did, in severe clear weather with a G1, just for the fun of it. It would be downright impressive IFR with a possible onboard fire. 
Can be quite easy to select the gear down again when moving it to the off postion if the mechanism is a bit worn. I used to not accelerate past Vle until the lever was in the off position.
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Easy226,
Yes, the 767 does have an off position. The 300 does, so I guess the 200 does and would also think the -400 does.
As for a safety mechanism, I don't think there is any. I would say it is a lever that needs a little bit of a nudge to move it, and even if you did put it down accidentaly, sure the lights would come up and in some airliners, be a bit of noise.
Yes, the 767 does have an off position. The 300 does, so I guess the 200 does and would also think the -400 does.
As for a safety mechanism, I don't think there is any. I would say it is a lever that needs a little bit of a nudge to move it, and even if you did put it down accidentaly, sure the lights would come up and in some airliners, be a bit of noise.
Thread Starter
Thanks for the replies everyone. Just wondering, if you try to lower the landing gear when the plane is travelling quicker than the maximum gear extension speed, will the mechanism allow the gear to extend?
Kestrel, from the picutres of the 767-400, there is only two positions. I take it they must have changed the whole landing gear system for the -400 series?
Dan
Kestrel, from the picutres of the 767-400, there is only two positions. I take it they must have changed the whole landing gear system for the -400 series?
Dan
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Yes, the gear will still go down as commanded whatever the speed. Lets hope this never happens to anyone but if you were in an unusual attitude high speed dive you would drop the gear to slow and try and stabilise the aircraft. This is already an emergency procedure.
The newer Boeing aircraft have done away with the 'OFF' position as it will automatically isolate the landing gear hydraulic actuators and lines when the gear is in it's up and locked position.
The newer Boeing aircraft have done away with the 'OFF' position as it will automatically isolate the landing gear hydraulic actuators and lines when the gear is in it's up and locked position.
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Yes, you are quite right,
In later Boeings, the 'gear system is automatically depressurized when all the legs are up and all the doors are closed, all we are doing in the Classics is the same thing manually - therefore there is no 'OFF' posn. in the auto systems.
But I certainly don't agree with the people whom are suggesting that any normal gear extension cycle is 'freefall', none do, as the extending gear is regulated in speed of extension by the rate of displacement of the hydraulic fluid, together with the system/s pressure applied to both the uplocks initially, then the extend/retract actuators.
The only time the gear 'freefalls', is when the emergency extension is selected, and then only the uplocks are released, electrically, and the gear falls into place under it's own weight plus airstream assistance, again controlled in it's rate by the displacing of the hydraulic fluid on the 'UP' side of the actuator.
And with no down system pressure to assist, especially in the last part of the extension, the emergency extension always takes a little longer, and the doors stay open. (except the B737, which has no doors!)
Cheers
In later Boeings, the 'gear system is automatically depressurized when all the legs are up and all the doors are closed, all we are doing in the Classics is the same thing manually - therefore there is no 'OFF' posn. in the auto systems.
But I certainly don't agree with the people whom are suggesting that any normal gear extension cycle is 'freefall', none do, as the extending gear is regulated in speed of extension by the rate of displacement of the hydraulic fluid, together with the system/s pressure applied to both the uplocks initially, then the extend/retract actuators.
The only time the gear 'freefalls', is when the emergency extension is selected, and then only the uplocks are released, electrically, and the gear falls into place under it's own weight plus airstream assistance, again controlled in it's rate by the displacing of the hydraulic fluid on the 'UP' side of the actuator.
And with no down system pressure to assist, especially in the last part of the extension, the emergency extension always takes a little longer, and the doors stay open. (except the B737, which has no doors!)
Cheers
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Flight Detent,yes you are correct in saying the gear doesn't free fall during normal ext.The fluid is also ported to the hyd actuator.The rate is controlled by a restrictor in the return line.
Just to be picky,on 737 there is no electrical operation of the uplock actuators during manual extend.It is purely through the mechanical cables.The only electrical input is on 737NG,when as the door that covers the em gear release cables is opened,a switch will cause the ports in the landing gear selector valve to go to return,to enable fluid movement.
Just to be picky,on 737 there is no electrical operation of the uplock actuators during manual extend.It is purely through the mechanical cables.The only electrical input is on 737NG,when as the door that covers the em gear release cables is opened,a switch will cause the ports in the landing gear selector valve to go to return,to enable fluid movement.
One reason for the 'OFF' position, not yet mentioned, is the reduction in the likelihood of system leaks. Operating in a low density environment for most of the flight, any system weaknesses would be liable to generate a leak if left pressurised. It doesn't need to be pressurised (other than the tilt mechanism) so turn it off and avoid the problem. 