APU question

[SpringHeeledJack]

I saw this photo and noticed that the APU seemed to be on in-flight and wondered why this might be ?

Photos: Boeing 747-436 Aircraft Pictures | !!!!!!!!!!!!!!

Any answers appreciated by a curious mind



SHJ

[Denti]

Probably just water from a drain mast, don't think that is the APU.

[SpringHeeledJack]

Thanks, so all in all a very lucky shot as the two aircraft were closing, just as the vent was opening



SHJ

[ampclamp]

Could have been a promotional shot with APU running to make it look pretty with the con trails. Or maybe just an in flight APU start test ?

[IFixPlanes]

IIRC, ther are no inflight APU starts possible on the 747.

[ampclamp]

Interesting thank you. I'm sure I have seen pics of early an early 747 with an apu con trail.

Are / were all 747s variants the same in not being able to use the apu in flight?

[IFixPlanes]

I wrote "inflight APU starts". If you start it on ground, you could use it in air. (B747-4 and -8)

If you took a closer lock at the picture, you will find that the contrail is not in line with the APU exhaust.
I am with Denti (#2): Water out of a drain mast.

[Engineer_aus]

The APU is not on, as the contrail is off to the side and not directly where the APU exhaust is. The APU may be operated from the ground into flight. There can be a MEL on one of the engines bleed systems which then calls for the use of the APU to be on providing bleed air.

[IFixPlanes]

... There can be a MEL on one of the engines bleed systems which then calls for the use of the APU to be on providing bleed air.

Uhhh..., definitely not.
1. The 747 is equipped with 4 engines.
2. One bleed system INOP (B747) does not require a alternate bleed source.
3. APU bleed is only available up to 15,000 feet.
4. The APU can be operated in flight up to 20,000 feet.
5. ...