What is the difference if you have both the engine driven (hydraulic) pump in the ON position and the air....?

Question

......... driven pump also ON (so both pumps are constantly working)?

Answer 1

Both pumps are variable displacement so even when running they may not be supplying any fluid to the system, ie running in idle. Both EDP and ADP supply pressure module so ADP only supplies pressure to system if demand exceeds output of EDP. If switch in auto ADP supplies system if EDP pressure falls below 1400psi for more than 0.3 seconds.

Answer 2

dont forget the accumulators, which take up the slack.

its alll backup....

just image what happened when the L1011 had an uncontained wheel burst that cut through all 3 redundant lines in the rudder......sayonara.