So the 747 had been sitting on the tarmac with engines running for about 10 to 15 minutes in sub zero temperatures (24F) with light snow in P3D v4, and the APU was off all the way from there to about 38,000 ft. Just for shits and giggles, I tried to start the APU at 38,000 ft. Well, it didn't start. I understand no QRH procedure calls for this, but...why not? Its EGT was at -42C, no N1, no N2, and oil quantity was at 0.79. I'm pretty sure other aircraft's APUs can start at 38,000 ft......right? Or is the extreme altitude the issue? As I understand it the APU is started by an electric motor, not bleed air, so this shouldn't be a problem, yes? Is there a possibility of the APU inlet door freezing over?

20 minutes ago, Robert3512 said:

Just for [removed] giggles, I tried to start the APU at 38,000 ft. Well, it didn't start.

Can't start the APU in the air.

20 minutes ago, Robert3512 said:

but...why not?

You have 4 engines, and 4 gens, why risk an additional source of fire? You have plenty off of the windmilling fans anyway.

21 minutes ago, Robert3512 said:

I'm pretty sure other aircraft's APUs can start at 38,000 ft......right? Or is the extreme altitude the issue?

Depends on the APU, the door design, and the plane.

21 minutes ago, Robert3512 said:

As I understand it the APU is started by an electric motor, not bleed air, so this shouldn't be a problem, yes?

In theory, provided it was wired to allow it...but it isn't.

The 744 apu inlet door needs an "on ground" signal to open. I believe there is logic in the APU controller which inhibits APU start if the door is not open. It may be possible, however, to simulate an on-ground signal by pulling certain circuit breakers in flight. The aircraft electrical system has been modified to allow APU electrical power on to the busses if all engine generators are below operating speed, so if the APU can be started in the air, you may be able to get power on the busses (but it's not going to be much help if all the engines aren't powered up... You're still going down). Also, with those circuit breakers pulled, other systems will be behaving abnormally.

The Classic 747 used to have the ability to start in the air, but this ability was removed on many airlines, probably due to safety reasons (fire hazard, etc). Twin jets traditionally have the ability to start the APU in the air (because they only had one generator per engine), but aircraft like the 777 have two generators per engine.

The 744 APU is not really designed to be started in the air. Like most engines, they are difficult to fire up at high altitudes. Also, an APU which has been cold-soaked for long periods doesn't like starting. It has to thaw out first. Sometimes APUs can't even be started on the ground after long cold flights.

 

Although the APU cannot be started in flight and isn't certified to do so, from an operational point of view it can be left running up to 20,000ft after takeoff and will provide sufficient bleed air to power one air conditioning pack up to 15,000ft. Electrical power is not available in flight from the APU generators, but if the TOW is >660,000lbs (>300,000Kgs) then the 'APU to Pack' takeoff procedure is often used in order to gain approximately 6,600lbs (2,000kgs) in weight on a suitable runway.

Bertie Goddard

38 minutes ago, berts said:

Although the APU cannot be started in flight and isn't certified to do so, from an operational point of view it can be left running up to 20,000ft after takeoff and will provide sufficient bleed air to power one air conditioning pack up to 15,000ft. Electrical power is not available in flight from the APU generators, but if the TOW is >660,000lbs (>300,000Kgs) then the 'APU to Pack' takeoff procedure is often used in order to gain approximately 6,600lbs (2,000kgs) in weight on a suitable runway.

Bertie Goddard

I've heard that one operator will takeoff with all packs off and turn them on after acceleration height, the impact on pax is negligible.

QFA uses all packs OFF for every takeoff.... BA uses packs OFF at 300T or greater MTOW... the cabin does indeed climb very slowly....plenty of time to get the packs on (typically by 1500').

 

50 minutes ago, lasnubes said:

QFA uses all packs OFF for every takeoff.... BA uses packs OFF at 300T or greater MTOW... the cabin does indeed climb very slowly....plenty of time to get the packs on (typically by 1500').

 

According to the information I have been given, wherever possible BA will use the APU to PACK procedure and derated power whenever the actual TOW >300,000kgs.   They will only use PACKS OFF procedures on certain Performance Weight Limited runways where it is advantageous to do so.

Bertie

I see...

It is my understanding that BA only uses APU to PACK when necessary (think very hot or very cold). Still, it is a viable option and one worth practicing. As my wife often reminds me, I am frequently wrong. ;)

On 3/6/2018 at 11:31 PM, downscc said:

I've heard that one operator will takeoff with all packs off and turn them on after acceleration height, the impact on pax is negligible.

Dan, I wonder if they leave them off at high airfields like Nairobi and Bogota, or freezing places like Anchorage in winter or hot ones like Kuwait in summer?  The adverse effect this could have on one's Gin and Tonic doesn't bear thinking about, does it?!

Bertie 

APU to pack take off can be routinely used when required for performance, but the packs off technique isn't really recommended as it causes a heavy overpressure hit to the system when the pack are selected back on which can require maintenance action to fix/reset.

 

Jon Bunting