Landing Gear Off?

[Easy226]

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

[Onan the Clumsy]

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.

[Easy226]

Thanks a lot for the replies so far. I look forward to an in depth answer!
Dan

[Notso Fantastic]

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.

[eng123]

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.

[woderick]

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.

[eng123]

NSF,we must have been writing simultaneously!

[18greens]

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.

[777AV8R]

On my 777...hydraulically powered up and automatically cut off from hydraulics after locked up....and...yup...freefall down!

[Easy226]

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

[pigboat]

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. :eek:

[Max Angle]

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.

[Easy226]

So i take it that there is no mechanism to stop you from accidently moving the lever to the 'down' position?
Dan

[Kestrel_909]

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.

[QNH1013]

You pull the lever slightly before you lift it up or down to raise or lower the landing gear.

[Easy226]

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

[QNH1013]

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.

[Flight Detent]

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

[eng123]

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.

[Cornish Jack]

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. ;)

[Jet II]

Flight Detent

On 777 and 757 in 'Normal' operation, hydraulic system pressure is not supplied to the retract actuator during extension - purely weight of the gear and airstream assistance.

Have a look at the Maintenance Manual Ch 32-32-00

[Just More Crap]

As I understand, the 767-400 can be congiured depending on the customer, so presumably a customer could choose the "UP,OFF, DOWN" if they already operated -200's/-300's, or the "UP,DOWN" if they operated 777's. Feel free to correct me. If I am correct, what would happened if you operated the 767-300's and the 777's, such as Delta?

[PAXboy]

Silly question. Noting that the gear will respond to the lever with no interlock of IAS ... if in level flight would it disrupt things much? Other than the folks at front possibly having forms to fill in when they get home?!

If the gear is extended accidentally (highly unlikly) can you just reverse the lever, or must you allow the gear to complete the extend cycle before retracting again?

thanks.

[Lump Jockey]

Pretty sure the 764 has just the up and down settings.

[Max Angle]

The reason for the difference is that on 777 and 767-400 the gear is electrically signalled rather than by a big cable attached to the back of the lever. No need to have an off position as the electronics take care of it and no need for huge great lever, a little tiny one (or just a button for that matter) will do just nicely.

[stanley]

on the A300 you can not select down by accident as you need to pull the lever as well as going down.
yes you off load the hyds to save wear and problems,

[A4]

On the A320/1, the hydraulics to the gear are inhibited above 250 knots. So even if the lever is selected down nothing will (should :uhoh: ) happen.

******************

Yes there would be forms to fill in but that would probably be to do with trying to locate the gear doors which would more than likely detach if the gear extended in the cruise. Of course the other big worry there is what damage they would do as they departed (and landed).

******************

I've always understood gear extention to be freefall, but damped but the fluid in the retraction cylinders. The only pressure applied is to the overcentre lock to lock the gear in place.

A4

[Tight Slot]

Jet II re 757's

Boeing Operations Manual (Landing gear - system description)

"The gear are hydraulically powered to the down and locked position. The downlocks are powered to the locked position, all hydraulically actuated gear doors close, and the main gear trucks hydraulically tilt to the flight position."

Just a quote....

[Jet II]

Tight Slot

Thanks for the correction - having read properly this time the Maint Manual, only the nose gear free falls - main gears are powered down

MM 32-34-00 page 1

Hydraulic
pressure is supplied to the retract actuator only during landing
gear retraction.

:ok:

[Notso Fantastic]

A4- all very well disengaging the gear from operating above a certain speed, but many will remember the 727 that 'departed' at altitude and fell into a freefall dive. Apart from extending the speedbrakes, the Captain extended the gear (at enormous speed) to do anything to increase drag. Sure enough, the gear doors were torn off and bits of the aeroplane bent, but they survived, and I suspect maybe owe it to the gear. Airbuses are obviously great aeroplanes, but I wonder sometimes at an aeroplane that knows better than a pilot what to allow!
And I didn't even get around to the Habsheim incident!
http://www.chez.com/crashdehabsheim/ (video du crash)

[Lee]

The landing gear lever in the 747-400 has 3 positions. UP, OFF and DOWN.

When the landing gear lever is in the OFF position, the landing gear hydraulic system depressurizes.

[Pegasus912]

Paxboy

Interesting discussion a while back on accidentally selecting DOWN, when aiming for OFF!:

http://www.pprune.org/forums/showthread.php?s=&threadid=9281

[swish266]

It`s not a "nudge" it is a "good" pull on all 757/767. N d pull is d safety measure in flt. Some guys though still manage to go through the "off" pos when under stress at the sim.
N to close the thread B767/757 have a performance chart in the QRH "flight with Landing Gear Down". Means dat if u could not pick it up, u can at least fly it with d gear down, meaning unrestricted flight with hyd press at 3000 psi. As long as u dont run out of fuel like d poor sods from LTU at Vienna in 1999999999...
Enough guys? :cool:

[Bomber Harris]

Cornish Jack, I think you hit the nail on the head. The main reason is because there is no need to have it pressurised. Like all systems on this generation of aircrat...they are complicated. The aircraft is reauired to have an alternate extension system for certification purposes. The cheapest and most reliable alternate gear extension system is a freefall system. In order to have a freefall system you must have uplocks which can be released by a handle (or switch for the more modern boeings). So...if you have an uplock why bother to pressurise the wntire retraction system in the cruise. If it springs a leak you could dump your whole hydraulic sytsem fluid overboard in order to keep the gear in the up position...when there are uplocks to do the job. It simply makes no sense to pressurise the gear retraction system for a variety of reasons when you have a mechanical uplock system.

[Centaurus]

Kestrel. And if the gear did go down after inadvertent going through the OFF position into the down (even just a little bit down), and the IAS was above 235 knots when it happened, you cannot raise the gear again until you get below max raising the gear speed of 235 Knots. Happens in the sim a lot due pilots too quickly selecting gear off from up and not being careful of feeling for the slot (as the actress said to the bishop).