APU in-flight start

[Markus Vitzethum]

Hi all,

I always thought that starting the APU in-flight is not possible. Ground start only and only available up to FL220 or so.

While flying one of the original situations, "Reproduction 001 - Volcanic Ash.situ" I got down to FL200 or lower and by then have lost all 4 engines due to volcanic ash. I am on battery power and have only the Captains PFD, ND and the upper EICAS.

For some reason I tried to start the APU and was much surprised that it actually started and became available some 10-20 seconds later on. I just flew the scenario another time and can reproduce this. (Sure this does not help with the engines but you have a lot more systems available.   )

But why am I able to get an in-flight start in this case? (version 10.0.2 here). (In other situs with all engines running I get the expected behaviour that the APU does not start in-flight.)
Is there any system logic preventing in-flight start unavailable on battery power?

Markus

[Hardy Heinlin]

Hi,

some engineers say it is possible, some say something else (like most manuals do). There is no general answer on this subject, unfortunately.

There is a certain air-ground relay involved that inhibits APU start in flight (F22 and H8 on P7). When you lose certain busses, the relay relaxes. This has various consequences. E.g. you cannot move the gear lever from OFF to UP anymore, unless you push the lock override button.


Cheers,

|-|ardy

[Avi]

The APU can be started when the APU door is open. To open the APU door (when setting the APU selector to ON) you need an Air/Ground relay (specific relay of many) to be in ground mode.
When there is no power and all of the Air/Ground relays are de-energized, some are in Air mode (even on the ground) and some are in Ground mode (even in flight).
The Air/Ground relay in APU door logic/circuit belongs to the second group. That means, when you lost power, the relay relaxed, it was in Ground mode and you could open the door and start the APU even in flight.

In PSX you can start the APU in flight by pulling out a single circuit breaker. Good luck finding which one

Cheers,

[Hardy Heinlin]

In PSX you can start the APU in flight by pulling out a single circuit breaker. Good luck finding which one

Too late. I already revealed it :-) Our messages crossed.

[John H Watson]

Getting APU electrics on the busses in flight involves:
Opening the APU door
Starting the APU
AVAIL light illuminating
Pushing the AVAIL light
Auxilliary Power Breaker (APB) closure.
The re-application of APU power to the busses (re-energising all the systems, including the Air/Ground Systems)

Each comes with it's own set of complex logics. e.g. the APB will not close if the main engines are above a certain rpm and the main engine GCBs are closed. In some manuals, this logic is described as "Airmode", which can lead to confusion (Normally airmode is defined by gear tilt).

Note that the APU Generator AVAIL lights are controlled by gear tilt (among other things). This leads to the interesting case where you can be barrelling down the runway for takeoff with the APU running (and the APU generators showing AVAIL), but you can't select the APU Generators on.

Of course, having certain air/ground CB's pulled might cause all kinds of complications in the air and after landing. e.g. will the pitot probes be fully heated, will the Yaw Damper system work properly, will the pressurisation system work properly, etc. The cure may be worse than the ailment )

Rgds
JHW

[Grebleem]

I tried this situation today in a KLM 747 simulator. The behavior of the simulator was almost exactly the same as PSX.
The only difference is that both AVAIL lights will come on after starting the APU, but after selecting one generator the other AVAIL light will extinguish.

Cheers,
Bastiaan

[John H Watson]

The only difference is that both AVAIL lights will come on after starting the APU, but after selecting one generator the other AVAIL light will extinguish.

Interesting.  Now we have to find out why (if the big sim is correct or if it is programmed for a particular airline/aircraft).

Did you pull any circuit breakers?

The first APU generator ON powered the ship (minus a few non-essential busses which won't operate in the air). That doesn't narrow down the investigation process much  

[John H Watson]

Bastiaan, did you try pushing the Gen #2 button even though the AVAIL light was not illuminated?

Hardy, the AVAIL signal travels from the respective AGCU, onside BCU, Alternate Air/Ground relay R230 to the respective AVAIL lights. Both #1 and #2 AVAIL light signals go through the same air/ground relay R230. When the first APU Gen comes online, it powers the Alternate Air/Ground System via CB C97 [P7 F23] (DC Bus 2).

With the Alternate Air/Ground system powered and the aircaft in the air, neither of the AVAIL lights should illuminate.

I think this was discussed on the old beta forum (searchword "R230")

Rgds
JHW

[Hardy Heinlin]

With the Alternate Air/Ground system powered and the aircaft in the air, neither of the AVAIL lights should illuminate.

They don't illuminate in PSX either. The APU provides bleed air only.

Perhaps a bug in the KLM simulator? :-)


Cheers,

|-|ardy

[John H Watson]

They don't illuminate in PSX either.

I just tried this scenario and the #2 AVAIL light didn't extinguish when I selected the #1 APU Gen online.

I pulled P7 F22 (primary system) to get the APU started in the air (all engines off), then selected the #1 APU Gen online. Wouldn't the Alternate Air/Ground System be powered in this situation?

[Grebleem]

Good morning,

We didn't pull any circuit breakers. It was just a demo from the instructeur that the APU can be started in flight when the AC busses are not powered, it is not a procedure in the books.
We did not try to select the second generator, it is possible that it is a KLM simulator bug. But the instructeur told us that we only can connect one generator.  Flying N-4 with smoke in the cockpit and a oxygen mask kept us busy ;-)

The big advantage is with the APU running you get most of the systems back, like the auto pilot. But if the APU won't start, you've wasted a lot of battery power. Something to think about...

Furthermore, I must say I am VERY impressed with the PSX simulator. I almost completed my type rating on the 747-400 and the systems behaves identical to the real simulator!

Cheers,
Bastiaan

[Christian Adrigan]

Flying N-4 with smoke in the cockpit and a oxygen mask kept us busy ;-)

Hi, what is "Flying N-4"?

Cheers
Chris

[Grebleem]

BTW, in PSX you don't have to pull any circuit breakers to start the APU. Just let 4 engine fail, start the APU and after 30 seconds the AVAIL lights come on.

[Grebleem]

N-4 = flying with four engines out...

[John H Watson]

BTW, in PSX you don't have to pull any circuit breakers to start the APU. Just let 4 engine fail, start the APU and after 30 seconds the AVAIL lights come on.

My mistake. Either circuit breaker H8 on P7 can be unpowered or circuit breaker F22 on P7 can be unpowered to get the APU door circuit into GND mode. With Battery/Standby Power only, circuit breaker P7 H8 is unpowered.

So where did we get the idea that a circuit breaker had to be pulled? The APU starter motor circuit doesn't seem to have any air-ground logic in it.

[jtsjc1]

N-4 = flying with four engines out...

Nice a 250+ ton glider!

[Hardy Heinlin]

So where did we get the idea that a circuit breaker had to be pulled?

Just for the case when you want to keep an engine running (that's what I did in my last test).

With N-4 I get the AVAIL lights, else I don't.

[John H Watson]

Just so we're on the same page...

N-4 scenario as per the first post. APU started.
Both AVAIL lights on.
APU Gen #1 selected (as in Grebleem's post)
Is APU Gen #2 AVAIL light still illuminated?

[Hardy Heinlin]

Yes, still. The system behaves like on the ground.

[John H Watson]

Well, the KLM sim and the Boeing Wiring Schematics say that APU Gen #2 AVAIL light shouldn't illuminate.

Rgds
JHW