B737 NG Manual Gear Extension

[Cakov]

Can anyone explain the reasoning behind the proximity switch in the manual gear extension access door on the B737NG?

If the access door is left in the open position, it is not possible to retract the landing gear.

Yours bemusedly,

Cakov

[DDG]

Cakov,

WHEN YOU OPEN THE ACCESS DOOR TO THE MANUAL EXTENSION CONTROL MECHANISM,AN ACCESS DOOR POSITION SWITCH SENDS A SIGNAL TO THE MANUAL EXTEND SOLENOID IN THE LANDING GEAR SELECTOR VALVE.THIS MOVES THE BYPASS VALVE IN THE LANDING GEAR SELECTOR VALVE TO CONNECT ALL HYDRAULIC COMPONENTS IN THE NORMAL EXTENSION AND RETRACTION SYSTEM TO THE HYDRAULIC SYSTEM RETURN(to prevent a hydraulic lock).
This will ensure that when you pull the release handle(manually release`s the uplock for the particular landing gear) the landing gear will fall under its own weight to the down position,the down lock springs will then ensure that the side stay locks in the down position.


If the manual extension access door is left open,the landing gear selector valve will be in the bypass position.If you select the gear up via the landing gear selector handle in the flight deck, hydraulic up pressure will be blocked at the selector and will not reach any of the retraction components.Hence the gear will stay down and locked.
Regards DDG.

[TR4A]

If the access door is left in the open position, it is not possible to retract the landing gear

We had a few that would not retract. The access door was not completely closed.