"Qavion; actually some 767's have one HMG (now called HDG by the new Boeing books). The HMG comes on-line automatically (after a 10-15 second delay) with the loss of both left and right AC buses. To power the HMG the center hydraulic system's ADP (Air Demand Pump) is signaled to operate (the selector must be in Auto)."
Agreed, "None"... but since there is probably insufficient bleed air in this scenerio to run an ADP, I only mentioned the 3 HMG-type 767. The windmilling engines and the EDP's are more likely, however, to provide power to the HMG's on the L & R Hydraulic systems (or have I overlooked something obvious?).
Regards.
Q.