RAT 5
14th Oct 2003, 23:54
Any bright ideas out there? I'm searching for an answer to solve my confusion. Any ideas will be welcome, but if no-one knows the absolute answer, does anyone have a contact at Boeing for me to ask?
Engine Fire/Sev Damage etc QRH.
After the engine has been isolated via Recall actions it continues:
"ISLN valve switch...................................... Close
Pack Switch (affected side)..........................OFF
APU bleed switch.........................................OFF
APU......................................................... .....Start & on bus."
Seems OK. There is no mention of which engine is still running. If it was the left engine then there is a possibility of that bleed back pressuring the APU; If it was the right engine, the ISLN valve would have opened if left in auto. Hence closing the APU bleed valve first before starting the APU.
Now compare the Engine Failure/Shutdown QRH. One engine is still running, the other has failed or been shutdown. The QRH continues:
"Engine start lever..........................................Cutoff
APU......................................................... ........Start & on bus.
Pack (effected side).........................................OFF"
You will see that in this case the only protection for the APU is the one way valve in the APU bleed line.
Why is there a difference in the sequence of actions? In the Engine Bleeds Off configuration the ISLN valve is always closed before opening the APU bleed to protect it. In the Engine Fire/Sev damage case this is also true. Why not in the Failure case? In the latter case you will get a DUAL BLeed light. Following the QRH for that will lead you to turn off the APU bleed. But why not just make life simple and follow the other procedures?
Thanks to all the gurus out there for some bright ideas.
I'm stumped.
A post script.
I appreciate that the Dual Bleed light will ony come on if the APU bleed is ON. In normal cases it will be closed in the After Starts. However, it is possible to be using the APU bleed in flight.
Engine Fire/Sev Damage etc QRH.
After the engine has been isolated via Recall actions it continues:
"ISLN valve switch...................................... Close
Pack Switch (affected side)..........................OFF
APU bleed switch.........................................OFF
APU......................................................... .....Start & on bus."
Seems OK. There is no mention of which engine is still running. If it was the left engine then there is a possibility of that bleed back pressuring the APU; If it was the right engine, the ISLN valve would have opened if left in auto. Hence closing the APU bleed valve first before starting the APU.
Now compare the Engine Failure/Shutdown QRH. One engine is still running, the other has failed or been shutdown. The QRH continues:
"Engine start lever..........................................Cutoff
APU......................................................... ........Start & on bus.
Pack (effected side).........................................OFF"
You will see that in this case the only protection for the APU is the one way valve in the APU bleed line.
Why is there a difference in the sequence of actions? In the Engine Bleeds Off configuration the ISLN valve is always closed before opening the APU bleed to protect it. In the Engine Fire/Sev damage case this is also true. Why not in the Failure case? In the latter case you will get a DUAL BLeed light. Following the QRH for that will lead you to turn off the APU bleed. But why not just make life simple and follow the other procedures?
Thanks to all the gurus out there for some bright ideas.
I'm stumped.
A post script.
I appreciate that the Dual Bleed light will ony come on if the APU bleed is ON. In normal cases it will be closed in the After Starts. However, it is possible to be using the APU bleed in flight.