Topic Author
Posts: 73
Joined: Tue Aug 02, 2005 7:51 am

757/67 More APU Questions

Thu Apr 06, 2006 8:30 am


Maybe someone here can help me with these questions referenced to a Garret GTCP331:

1) APU EGT rise:
I was told that normal EGT for a stabilized APU is 370 deg (99 RPM) and on the
first start, it may reach 650 deg at 35 RPM
Now, in this hypothetical case, immediately after APU shut down (0 RPM), EGT
would be around 250 deg.
In case a restart is attempted, I guess EGT would first go down quite a
number , maybe to 140 deg, while RPM increase from 0 to 16 %, and then would
reach a final peak of around 750 deg because it was hot when the restart
begun. It this a valid reasoning?
Besides, at stabilized RPM (99%), EGT would have come down to the same 350
deg, or it will stabilize at a higher temp, around 450-500 deg (GEN OFF and
BLEED OFF) because of having reached a hotter peak?

2) APU Bleed air duct pressure
What would be the numbers for a wealthy APU ? (AT MSL)
Duct pressure with APU Bleed Valve On and
- Packs Off
- one Pack On
- both Packs On
- Engine Start - Packs Off
- Engine Start leaving one or both Packs On

Many thanks
Posts: 420
Joined: Mon Feb 07, 2005 5:22 am

RE: 757/67 More APU Questions

Thu Apr 06, 2006 11:10 am

Quoting Taguilo (Thread starter):
- Engine Start leaving one or both Packs On

I don´t know the 757/767 but all aircrafts I know shut down the packs during main engine start with the APU. So I assume the 757/767 will do the same...
This job sucks!!! I love this job!!!
Posts: 3426
Joined: Tue Nov 22, 2005 11:04 pm

RE: 757/67 More APU Questions

Thu Apr 06, 2006 5:13 pm

The APU ECU controls the start EGT. The EGT rises to the max value and stays there as the APU accelerates. So you will always see the same EGT on start, if it is hor it will start a bit slower.
Posts: 45
Joined: Wed Feb 25, 2004 11:03 pm

RE: 757/67 More APU Questions

Thu Apr 06, 2006 7:28 pm

A strong APU duct press should be 33-37 psi with packs off, and engine start,(im not sure what other airlines do, but on mine, we shut down packs for engine start), and about 17-19 psi with both packs running.
Topic Author
Posts: 73
Joined: Tue Aug 02, 2005 7:51 am

RE: 757/67 More APU Questions

Fri Apr 07, 2006 9:13 pm

Thank you guys!


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