Landing Gear Handle Off/Neutral Position

[kontiki99]

Anybody know what requirements drive the implementation of a landing gear lever OFF position in large aircraft?

This is a curiosity question, we've been dealing with some inflight gear issues this week; the question keeps coming up. It's been on every large aircraft I've supported, I'm not sure I've ever seen a good explanation why.

Thanks,



In theory, there is no difference between theory and practice however, experience suggests that in practice, there is!

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm affiliated with.

 

[VEBill]

Not my area, so standby for those that actually know...

I found this: "Hydraulic pressure is removed from the landing gear system with the LANDING GEAR lever in the OFF position"

 

[Worldtraveller]

Not quite sure I understand your question. Control system requirements are defined in the regulations:
§25.671 General.
§25.672 Stability augmentation and automatic and power-operated systems.
§25.675 Stops.
§25.677 Trim systems.
§25.679 Control system gust locks.
§25.681 Limit load static tests.
§25.683 Operation tests.
§25.685 Control system details.
§25.689 Cable systems.
§25.693 Joints.
§25.697 Lift and drag devices, controls.
§25.699 Lift and drag device indicator.
§25.701 Flap and slat interconnection.
§25.703 Takeoff warning system.

The specific shape of the controls are also defined (to a certain extent) so they are universal to all aircraft. Your answer is probably in the FARs somewhere.

 

[rb1957]

i think the question was "why have an OFF position ?" (implicit, are LG either "down and locked" or "up and locked" ?)

and i like the answer, that in this position the hydraulics are de-energised.

another day in paradise, or is paradise one day closer ?

 

[debodine]

VE1BLL is correct. Hydraulic pressure is applied during the UP cycle to stow the landing gear and during the DOWN cycle to deploy the landing gear. Hydraulic pressure is removed in the OFF position.

I am not a hydraulic design engineer so now I am stepping past acquired systems knowledge into opinion. The landing gear as we all know receives some of the roughest treatment of any article/system on an aircraft. From a system safety standpoint, assume during takeoff a small leak develops in the landing gear system (also assume it is not a catastrophic blowout). Because the landing gear hydraulic system is isolated and hydraulic pressure is removed for the duration of the flight, only minor residual fluid leaks until the landing gear is deployed for landing. The hydraulic system should still retain enough fluid to safely operate the aircraft control surfaces until landing is complete. If the pressure had remained in place during the entire flight, enough leaking may have occurred to render the hydraulic system useless for controlling the aircraft.

Again, this is strictly my opinion. Someone who has actually participated in the safety analysis during an aircraft design program may be willing to correct me if I am in error.

 

[bashdarov]

good information thanks


Graduated from Soran University with First Class Degree with Honours in Computer Science.

 

[kontiki99]

debodine, I have heard the same sorts of things. Every once in a while I try to verify things like that are true, which is why I posted the question. Your comments sense and is all that anyone seems to come up with. As I type, I remember I had a book on aircraft landing gear, I haven't seen it for years. Maybe it's still around.


In theory, there is no difference between theory and practice however, experience suggests that in practice, there is!

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm affiliated with.

 

[PHovnanian]

A better understanding of what the 'off' position is used for might be obtained from the QRH (quick reference handbook) for various aircraft models. Check which conditiions the 'off' position is called for and that might give some idea about why it is provided.

Also, check the FARs. There are requirements that provisions must be made for the flight crew to remove 'power' from aircraft systems. In this case, that would be hydraulic power.