One thing I have
never managed to understand, or managed to get a satisfactory explanation for:
The DUAL BLEED light illuminates whenever the APU bleed air valve is open and the position of the engine bleed air switch and isolation valve would permit possible backpressure of the APU. Therefore, thrust must be limited to idle with the DUAL BLEED light illuminated.
Basically, my doubt is: do we want to protect the engine or the APU?
According to the manual, we want to protect the engine: APU pressure could go into the 9th stage modulating and shutoff valve.
So why on earth do we have to limit thrust to idle?
Increasing thrust would help preventing this problem!
Years ago I was told it's the APU we want to protect, actually....
