A lot of speculation above! This is the true answer to this,although it may sound a bit unlikely at times!
The Dual Bleed light is illuminated when the APU bleed valve is open and the engine bleeds are selected on,regardless if the APU is running or not.
The reason for engine idle power only in this scenario is as follows.
There was an instance years ago when the APU bleed valve was open [not sure if the APU was running or not] and the APU bleed air duct check valve failed in the open position.
This resulted in engine air back-feeding through the [failed] open apu check valve,which is,in fact,a simple flapper valve,into the APU and causing it to drive the compressor blades.
This resulted in a overspeed condition which the crew were unaware of and an uncontained failure of the APU was the result.Not sure of the exact consequences this caused but it obviously would have had the potential for severe problems.
This is the reason for lower engine power.It is obviously irrelevant if the APU is running or not with these circumstances as the engines will produce higher duct pressure at cruise power than the APU.Also remember that normally the APU will not be running in flight.
By the way,before someone says the APU bleed valve cant be open unless the APU is above 95%,just remember that it can fail in the open position.
Last edited by eng123; 30th Sep 2003 at 19:23.