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Why are the APU GEN & APU BLEED usually left on?

Posted: Mon May 20, 2019 7:09 pm
by Martinlest
The title says it all, more or less! Why do these switches tend to be left on for the whole flight (certainly in all the real-world cockpit videos I have seen), even though the APU itself has been turned off after engine startup?

Is there a difference between Airbus and Boeing general practice in this?

Thanks.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 1:13 am
by DL_Mech
Many cockpit switches are left in the "on" position to keep solenoids in the circuit from burning out. For example, engine driven hydraulic pumps are electrically powered in the "off" position. When the switch is selected off, it provides a ground to complete the pump shutoff circuit. Keeping the solenoid not energized (normal operation) increases the life of the solenoid.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 3:26 am
by fr8mech
Simply put, it’s designed that way to reduce flight crew workload.

On older Boeings (757/767), and I’ll assume the newer ones, too, the APU ECU commands the bleed valve to close, irrespective of bleed switch position, when the APU is selected off. Conversely, the ECU commands the valve to open when the APU reaches operating speed, only if the switch is on.

I’m fuzzy on the electrical side component nomenclature, since I haven’t looked at that in a while, but the same principle applies. The APU GEN switch is always left on. The APU GEN will automatically take the buss only if the buss is unpowered. In order for the APU GEN to pickup a buss, the source on that buss must be lost or deselected.

If this thread is still active later this week, I can get a little more into the weeds.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 4:00 am
by zeke
On Airbus the apu bleed is turned on when needed after the apu is available. The APU gen contractor is normally closed all the time, the button will open the contact.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 4:19 am
by Starlionblue
It's all pretty seamless on the 'bus. With the APU on, when the external power is switched off on the overhead, APU gen takes up the load seamlessly through NBPT (No Break Power Transfer). Same when the APU is switched off, the engine gens take over.

When starting engines, the packs are automatically switched off. We don't actually touch the pack pushbuttons.

"Dark panel". If you look up and see no white lights, all is well. A concept pioneered by Fokker I think.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 2:36 pm
by Martinlest
Thanks a lot for the replies.

Flying 'training level' aircraft in X-Plane 11 (in particular the ToLiss a319 and the FF B767) I am trying to get this as near to the 'real thing' as I can! I have long appreciated the different ways that Airbuses and Boeings deal with these things - the Boeings, in general, seem to need a bit more manual switch input (esp. the packs) than the Airbuses, which latter automate these things more (though I don't have a 'high-level' B777 or B787: thy may behave differently being much newer)..

I was specifically interested in the B767 for this question. Watching an in-cockpit (real world) video, I noticed that my procedure wasn't quite the same with respect to the APU. Seems then that the APU GEN and the APU BLEED switches are both left on in the older Boeings, once they have been turned on (from, say, cold & dark cockpit)?

Martin

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 2:49 pm
by Lpbri
An exception to this is on a 737. The APU generator power breakers are always off unless manually selected on. The switch is a momentary action switch. Every time the APU is started, the APBs must always be manually closed.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 2:53 pm
by Martinlest
Re. the 737 that's what I had expected (from what I have seen): thanks for the confirmation.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Tue May 21, 2019 4:23 pm
by fr8mech
Martinlest wrote:

I was specifically interested in the B767 for this question. Watching an in-cockpit (real world) video, I noticed that my procedure wasn't quite the same with respect to the APU. Seems then that the APU GEN and the APU BLEED switches are both left on in the older Boeings, once they have been turned on (from, say, cold & dark cockpit)?

Martin


Yes, on the B767 those switches are left on.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Thu May 23, 2019 5:06 am
by Horstroad
The MD11 is an exception again... as always.
APU bleed and power are off after APU start. At least if you start the APU with the dedicated APU start/stop pushbutton.
However The APU automatically starts when the APU PWR pushbutton is pressed and in this case the APU takes over the main AC busses when power is available. When the APU is switched on with the APU PWR pushbutton, it automatically shuts down after engine start (when all three main AC busses are powered by their onside IDG).

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Thu May 23, 2019 8:02 am
by fr8mech
Martinlest wrote:
Seems then that the APU GEN and the APU BLEED switches are both left on in the older Boeings, once they have been turned on (from, say, cold & dark cockpit)?

Martin


I'd like to add that on the B757/B767 neither switch is selected OFF unless directed by checklist or maintenance procedure. They are always left ON.

Re: Why are the APU GEN & APU BLEED usually left on?

Posted: Thu May 23, 2019 7:21 pm
by Tristarsteve
As a side note.
On the B757/B767 the APU bleed and the engine bleeds are controlled by the electronics. The rule is only APU bleed or Engine bleed not both.
The engine bleed valves are air powered open, and spring loaded shut. But quite often when the engines are shut down, especially if the APU is not running, then the spring will not shut the valve, and the ON flowbar will stay on. Then when you start the APU the APU bleed will not open as the engine bleed is 'Not shut'.
So how do you start the engines?. Well Boeing has thought of this and operating the engine start switch to start will override the logic and open the APU bleed valve.
I found this out the hard way after manually closing an engine bleed valve on the ramp, then finding it was not necessary!

ps the engine bleed valves are extremely hard to access inside the pylons.