The transfer valve will operate automatically with these conditions present:
1. One main landing gear not up.
2. Landing gear lever not down.
3. Engine 1 N2 less than 50%.
4. System B pressure available at the valve.
5. Aircraft in the air.
The valve exists to allow system B EDP to supply retract pressure to the landing gear when system A EDP is offline (Engine 1 N2 less than 50%). The EMDP (System A in this case) flow rate is too low to get the gear up in a satisfactory time during a single engine climb out after takeoff, with the obvious drag issues created.
To the OP: System A quantity has no bearing on the valve operation.
Last edited by Gas Bags; 5th October 2013 at 04:46.