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View Full Version : Gear Off position on a 747-400


shon7
25th Aug 2004, 13:16
What is the purpose of the Gear UP and Gear OFF positions on the 747-400? How are these two different?

Along the same lines, why is there no Gear OFF position on a 777?

LukeEGTE
25th Aug 2004, 13:44
Gear UP - Uses the hydrolic power to raise the gear.

Gear OFF - Releases the hydrolic power and alows the gear to hang in it's locks.

Gear OFF is selected a couple of minutes into the climb.

BN2A
25th Aug 2004, 14:07
Gear OFF position also exists on 757 for exactly the reasons stated.

I believe the 777 turns the hydraulic supply off automatically.

:cool:

FunctionedSatis
25th Aug 2004, 20:11
Yes the 777 is automatic. The ELMS removes the gear up signal 10 seconds after the gear retracts.

Stu

woodpecker
25th Aug 2004, 22:06
Try lowering the gear by the normal system on a 757 when a bolt in the linkage "falls out" during retraction with the valves in the "up" position. Nothing happens!!

The emergency free fall may release the up locks but with the gear being powered up still nothing happens.

Even depressurising hydraulics didn't help. Three greens were regularly replaced by reds as the over centre locks were broken by minimal flow/pressure from the depressurised system.

Two hours later, cycling the speed brake on the approach to completely dissipate the "up" hydraulic pressure, and three greens were achieved for long enough.

Stop on the runway.. gear pins inserted... taxy to the stand...go home.. have a beer, a job well done.

Wish I had been there when it was suggested, to Boeings rep at London, by looking at the thread that the bolt had never had a lock-nut installed at manufacture!!

Cornish Jack
26th Aug 2004, 11:45
Worth bearing in mind that, once the gear is up, leaving the lines pressurised is a pretty good recipe for inducing hydraulic leaks - especially in the low atmospheric pressure of the cruise.
The mechanical locks hold it all in place so it makes sense to de-pressurise the system until you need it for 'dangling the Dunlops'.

Blacksheep
27th Aug 2004, 08:30
With 200 Bars on the inside of the pipes, the odd half a bar reduction on the outside at altitude doesn't make much difference to the chance of a leak Jack. Its the risk of temperature cycling between very, very hot and very, very cold in the wheel wells buggering up the seals that concerns us. If a seal goes and the lines are pressurised, out goes the fluid. Plenty more hydraulic lines under permanent use are out in the cold, but the temperature variations are not so great and their seals are at less risk. Its always wise to shut down anything thats not being used, just in case.

ZFT
27th Aug 2004, 09:02
Isnít there also some issue with Magnetic Compass errors if the gear lever solenoid remains enabled?.

I seem to recall reading something, somewhere, sometime ago.

NSEU
27th Aug 2004, 13:40
"Isnít there also some issue with Magnetic Compass errors if the gear lever solenoid remains enabled?."

What type of aircraft, ZFT? As far as I know, most gear lever lock solenoids are energised in flight... and release on touchdown (and if power is lost to the solenoid).

Rgds.
NSEU.

ZFT
28th Aug 2004, 01:52
NSEU,

Definitely Boeing. Maybe itís the opposite of what I recalled, namely with a gear lever solenoid failure you get Mag Compass errors?

NSEU
29th Aug 2004, 07:24
Boeing 767's, 747Classic and 747-400 all require an energized solenoid to unlock the gear lever, ZFT. i.e. In flight, it is energized.

An electrical failure would most likely de-energize the solenoid, re-locking it. Manual override will get the gear up if it is down. I seem to recall, however, it's not necessary to push the override button/plunger if you are moving the lever from UP to OFF or DOWN because of the geometry of the mechanism.

I have no explanation for the compass behaviour.

Rgds.
NSEU>

Blacksheep
1st Sep 2004, 06:29
Standby compass swings are done with the gear lever solenoid in the flight condition. Most pilots and compassswinging people know not to operate the gear lever during a swing, but we leave the landing gear locks fitted just in case ;)]

NSEU
3rd Sep 2004, 07:36
"Standby compass swings are done with the gear lever solenoid in the flight condition. Most pilots and compassswinging people know not to operate the gear lever during a swing, but we leave the landing gear locks fitted just in case "

Not sure I understand, this, Blacksheep...

Moving the lever is not going to activate the solenoid. The air-ground system does this (in response to gear tilt).

Also... usually the lever lock solenoid is activated by one of the main DC busses. If you've got these, chances are you've got main AC busses, too, to drive the instrumentation. Why would you need to calibrate the compass in this configuration?

Perhaps moving the lever to it's normal flight position is simply because you have moved the lever mechanism (metallic, mechanical part) to a different location?

Rgds.
NSEU.

FunctionedSatis
5th Sep 2004, 20:37
From Chapt 34 in the the 747-400 maintenance manual there is no requirement to touch the landing gear leaver or to energize the lock solenoid specifically.

You must energize all lighting, electronic and radio equipment. The solenoid must be deemed not be be of a suficient magnetic field strength to be of concern or the manual would specify.

Stu.