Its there to prevent the gear being retracted when the door is open, regardless of gear lever position.
If you were to restore sys A pressure having done a manual extension, the gear wont retract with the gear lever in the up position. They will retract in this situation when the manual gear extension door is closed.
This is only a side effect of this system.
Normally you move the landing gear by selecting hydraulic pressure to the extend or to the retract side of the gear actuator. This will be done by a pushpull cable which connects the landing gear lever with the landing gear selector valve.
When you want to extend the Gear by free fall, it is necessary to connect connect both sides of the actuator with the return line. This prevents a hydraulic lock which may not permit manual extension of the landing gear.
So the landing gear selector valve needs 3 positions:
- UP
- OFF
- DOWN
(Picture of the landing gear selector 737CL)
Therefore on the 737CL you must select the OFF position before you pull the handles.
But this can be a forgotten in a stress situation. (No offense, just human factor)
Boeing has changed the design of the landing gear selector valve at the 737NG by adding a new feature --> the manual extend solenoid.
Even if the lever is in the UP position, if you open the access door to the manual extension handles, the microswitch gives power to the the manual extend solenoid. This moves the manual extend solenoid valve to the ALT position what causes the bypass valve to move in the BYPASS position. Now it doesn't matter on which position the landing gear lever is positioned because the UP-line of actuators are connected to the return line.
A nice tip for trouble shooting when you didnīt get up your LG after lift off:
Pull the CB "ALTN EXTEND SOL" (P6-3 D16).
If you can now retract your LG, the microswitch has defectively switched to "access door open" (normally caused by dirt). Make a log book entry and finish your flight.
But in this situation donīt forget to press in the CB or move your LG-Lever to OFF before manual extend the LG.
Englisch is not my native language so please forgive me if i write it down in a in a little bit confusing way ;-)