Why B777 APU Is Not Used On Takeoff???

[AeroTech]

Hi,

I have some questions regarding B777 APU:

-Is it true that the APU is not used during pack off takeoff? If yes, why there is such restriction?

- The APU is fitted with 2 starters (electric & pneumatic). Why there are 2 starters?
In case of all engines flame-out (no pneumatic power for the pneumatic starter), is the electric starter supplied by the battery? (separate battery
than the aircraft battery?)

-Up what maximum altitude can the APU provide pneumatic power, and pneumatic & electric power (in the same time)?

Feedback appreciated. Thank you.
Best regards.

[mutt]

1- The APU is not certified for use during takeoff. The reason that we were given by Boeings FLOE department was that they didnt want the APU inlet open and deflecting airflow over the rudder.


Mutt

[vapilot2004]

The Boeing 777 APU can be started and provide electrical power up to the maximum certified altitude of the 777.

Bleed Air is available up to FL220.

The air turbine starter (ATS) will automatically engage when there is sufficient bleed air available - otherwise the electric starter will be commanded on. The electric starter has a dedicated battery. 2 starters = redundancy.

Whether the APU runs on takeoff is at the Captain's discretion or subject to the airlines' SOPs.

EDIT: Mr. Mutt, I didn't know of the FLO engineer's advisory. Bulletin I guess ? I stand corrected here.

Something you didn't ask but I offer:
the 777 APU will autostart when airborne if all 4 generators (main and backups) fail.

[huckleberry58]

1. The 777 APU can be used during take off.
2. The APU isn't used for a packs off take off because the packs can be switched off just prior to take off and then switched on again just after airborne, that equates to around 2minutes with the packs off and the cabin temp won't change noticeably.

[Old Smokey]

Mutt, "The APU is not certified for use during takeoff". Ummm, I was not aware of any such limitation. In fact our MEL allows for it's continuous operation for dispatch on Non-ETOPS flights with a generator inoperative.

Maybe my colleagues and I are guilty of an AFM infringement or 2 :}

Do you have a reference from Boeing on the matter? T'would be appreciated.:O

Regards,

Old Smokey

[mach 84]

besides all the electrical conditions where you have to have the apu running continousely, there are also some circumstances you have to use apu bleed air as a backup during take off. for instance if you have only one center hydraulic primary pump working. if you look it up in the mel there is quite a lot of switching to do before take off!
in case of engine failure during take off one engine could not supply enough beed air for the hydraulic demand pumps to get the gear up at normal speed.

[Old Smokey]

Good point Mach 84, an MEL example for one of several combinations of Center System Pump failure -

Center System Demand Air Driven Pumps (ADP)
One may be selected OFF provided:
.1 Both center system Alternating Current Motor Pumps (ACMPs) operate normally,
.2 Associated ADP is verified not running when selected OFF,
.3 Remaining ADP heater operates normally,
.5 APU is used as ADP air source during takeoff, and
.6 Appropriate performance adjustments are applied.

Item .5. refers to APU operation during Takeoff. Fairly hefty (at least in the commercial sense) 1st segment weight corrections apply due to slower gear retraction, even with the APU back-up of the failed system.

Regards,

Old Smokey

[mutt]

Just read the FCOM 7.10.3

Option:
A-TO1. A-TO2 - APU to pack or APU to pack derated

I guess that we didnt buy that option! :\ :\

In 1997 when we were selecting the aircraft, we had long discussions with Boeing FLOE regarding PACKS OFF takeoff for additional weight with the APU providing air conditioning, it wasnt possible for the reason that i stated above!

Sorry about the mis-information.

Mutt

[Old Smokey]

Thanks Mutt, you had me worried for a while.

Options are options are options, and indeed, no two B777s are the same.:D

Regards,

Old Smokey

[Straight & Level]

The B777-300ER is, however, approved for APU-to-Pack for takeoff.