View Full Version : 747-400 engine starthttp://www.pprune.org/forums/images/infopop/icons/icon5.gif

10th Jul 2004, 12:02
Dear friends,

I'm an aviation enthusiast. I'm wondering how many engines may be started simultaneously by means of APU on 747-400. Moreover, are there difference in terms of bleed air power between the 744 APU and an external source? Does it exist an external source of bleed air that may allow start of 4 engines simultaneously?
Thank you in advance.
Best regards,


10th Jul 2004, 12:35
RR start 1 off the APU.
GE can start 2 off APU.

You have a job to get 1 started off external air let alone 4!

Although they should give you the same pressure 35psi or more the APU gives a far higher rate of flow.

Capt Fathom
10th Jul 2004, 12:58
Actually, with Autostart Operative, you can start 2 engines simultaneously, RR & GE engines.

10th Jul 2004, 13:26
i onced watched a virgin 747-400 dvd and the captain said on there "the 747 400 has a excellent apu and can therefore start 2 engines at once "

fire wall
10th Jul 2004, 14:31
Cloudskimmer, it is entirely dependant on how tired the APU is.....or whether you have one at all as we have been finding out recently. Output psi can vary markedly and same goes for the gnd air source.

10th Jul 2004, 15:49

In days of yore, I hear tell around here, they used to have an APU assisted start system on that wonderful Vulcan Bomber called a "ripple start" wherein all four engines could be started simultaneously (you know, all together at once at the same time. ;) ) and all run up to idle in around 7 seconds.

Pretty cool eh, I'm sure there is an oldie 'round here who can give us the goss. :ok:

Now that'd save a bit of mucking around nowadays.:E

10th Jul 2004, 19:13
The SOP for our -400s with GE engines is to start 2 at a time (4+3, then 2+1) with the APU. One pack can also be running; and the demand pumps are also on, so #1 ADP is taking air as well. I have yet to find an air cart that could do it -- most of them barely start 1, with the pack off!

Also, the APU air is automatically regulated, and will attempt to maintain a duct pressure of approx 30 psi during the start. Occasionally an APU will not maintain duct pressure, so we'll turn off the pack, but that will also generate a squawk for maintenance. With an air cart, dips to 24 psi or less are common.

10th Jul 2004, 19:57
We have aircraft with dual APUs for an immediate start of all four engines................


11th Jul 2004, 00:12
Thank you very much, friends!
Have a nice sunday,