Starting APU in flight

[martinlest2 58]

Hi. Is there something I am missing? How do I sart the FS9 747 APU in flight? On the ground, no problem, but it seems to take for ever when in the air - the 'avail' light doesn't come up, no matter how many times I rotate the 'start' switch: or at least, it does, but (as far as I can see) only when I am on the ground. It is certainly a lot different to when I am starting up the APU before engine start at pushback (when there is a realistic delay of a couple of minutes). Maybe I am missing a procedure? In the air , continuous Ignition is off, autostart is on ... Can't see anything in the manuals..

 

Thanks,

 

Martin

[B747tebs 80]

may i ask at what altitude you want to start your APU?

[PHXMD-11 86]

How do I sart the FS9 747 APU in flight?.

 

The 747's APU cannot be started in flight No matter what you do, it will not start. The engines are more than capable of delivering electrical and hydraulic support in almost any condition. It is available from the ground up to 20,000 feet.

 

The continuous ignition and autostart switches do not have anything to do with the APU. They provide a continuous source of electricity for the igniters and control engine start sequence.

[richardnobes14 13]

Why would you need to APU in flight?

[FLEX1978 373]

Why would you need to APU in flight?

 

Exactly :) If you ever found yourself in a position where you needed the APU in flight, you have bigger problems then trying to start the APU.

[martinlest2 58]

If pilots lose power from the engine generators for any reason (maybe all engines flame out), the APU in the real world is (and has been on many occasions) a life saver - literally. The APU generator will allow basic functions in flight that would not be possible otherwise. I start it before landing to have power to the basic systems after reaching the gate, where ground power is not available. In a real 747 the APU is startable at any time in the flight. In the PMDG version, you say not? I didn't realise that. Makes sense, because I do finally get APU power by the time I land.

 

I usually try starting the APU at about 5000' or 5 minutes to touchdown. I'll leave it until I am on the ground maybe..

 

Thanks for the replies,

 

Martin

[scandinavian13 5,192]

I'm pretty sure you can't start the APU at all in flight in the 747, regardless of altitude. If I remember correctly there's a mention in the TRCs and manuals explaining why this is the case.

 

In the case of GEN failures, you're covered by the RAT. After landing, you turn it on when cleaning the aircraft up. There's no need to turn it on prior to landing. Most airports the 747 uses are large enough that taxi is more than sufficient in length to warm it up prior to loading it with the packs.

 

EDIT:

Just remembered the 744 doesn't have a RAT because the windmilling engines provide enough to cover it.

 

Also, if you look in the manual, you'll see that the APU can operated up to 15,000' for air conditioning purposes, and cannot be started in flight.

 

The reasoning is for fire-related purposes. While you may think "engines start fires, too," the engines aren't buried in the fuselage, and they're absolutely necessary for the plane to be moving while the APU is not. Sure, a fire can affect the wing, but a fire weakening the empennage could take the tail off, and then you're royally screwed.

 

There was a thread a while back that questioned the logic of being able to fly with it on if you'd started it on the ground. The question was something along the lines of "if I can depart with it and be in the air with it on, why can't I start it in the air?"

 

The logic here is that if you depart with it, you're at 15,000' within minutes and if a fire developed, you could shut it down and return to the airport for ground-based fire suppression very quickly. If you're over the middle of the Atlantic, however, it could be a problem. While the shielding (related to fire protection) around the APU is very effective, it can only hold up for a few hours. If you're in the middle of an ocean and farther away from an airport than that can hold up, this could pose a problem. In theory, you'd only want to turn it on closer to the destination, but there's no real need for it, and there's probably a little Boeing CYA in there as well.

[sheepfu 3]

Kyle nailed it on the nose. If you look at the reports from the BA flight which experienced a four engine failure due to ash, Electrical power was reduced, but still available. The GEN drive is run off of the cold secion of the engine. Hydralics and bleed air systems work off of both the cold and hot sections of the engine. Therefore, since you cannot feather a turbine engine, if you have airspeed, you have the minimums that you need. While twin engine aircraft can do this too, they are provided with a RAT to suplement the windmilling systems. They have two turbines instead of four, so think half the power.

 

Now you are correct that some planes are required to be able to start the APU at any altitude (777). This is a ETOPS requirement which the 747 is not required to comply with.

[martinlest2 58]

OK, thanks for the replies. Will start the APU on the ground in future (I see this is procedure now I look at 747 checklists).

 

Martin

 

(A RAT would be no good on the ground BTW, even if the 747 had one - if the wind is not turning the blades (no airstream), there's no power generation).

[dskviator 4]

I'm pretty sure you can't start the APU at all in flight in the 747, regardless of altitude. If I remember correctly there's a mention in the TRCs and manuals explaining why this is the case.

 

In the case of GEN failures, you're covered by the RAT. After landing, you turn it on when cleaning the aircraft up. There's no need to turn it on prior to landing. Most airports the 747 uses are large enough that taxi is more than sufficient in length to warm it up prior to loading it with the packs.

 

 

(A RAT would be no good on the ground BTW, even if the 747 had one - if the wind is not turning the blades (no airstream), there's no power generation).

 

What Kyle meant by the previous statements was the APU could be turned on when on the gnd, he didn't mean to say the RAT was turned on, on the GND. A flow of statements, but the subject is still APU :)

[FLEX1978 373]

The 747-800 has a RAT, however , back to the APU, it still stands that Boeings logic on the -400 is you cannot switch it on in flight unless you start screwing around with circuit breakers and fool the air/ground logic.

 

Best to stick with SOP, turn the APU on when you need it, ie shortly before you arrive at the gate or 5 mins prior to pushback if on ground power. Even in the event of a 4 engine flameout you still

have 4 engines windmilling.

[kevinh 409]

In a real 747 the APU is startable at any time in the flight. In the PMDG version, you say not? I didn't realise that. Makes sense, because I do finally get APU power by the time I land.

 

I usually try starting the APU at about 5000' or 5 minutes to touchdown. I'll leave it until I am on the ground maybe..

 

Thanks for the replies,

 

Martin

In the real 747-400 you can't start the APU in flight. PMDG have modelled this correctly. However the APU will continue to run in flight if you start it on ground.

 

You may be getting confused with the 747 classic. There was nothing to stop the APU being started in flight on the -100/200/300 versions though most weren't certified for in flight use.

 

As others have said you don't really need the APU in flight. Even with a four engine flameout, the windmilling rpm is sufficient to give enough hydraulic power for the flight controls to work.

 

Plenty of time to start the APU while taxying to the stand.

[martinlest2 58]

I agree, it was my mistake to try to start the APU in flight and now I am doing it as I taxi instead. I am impressed (as ever) that PMDG included this detail in their 747. Now I have it in Windows 7 x64 and never get any OoM problems I can really appreciate what a great job they have done. Makes me want to spend the ₤4000 they ask for the 5-hour simulator experience at Heathrow!! (Almost!).

 

Yes, maybe I was getting confused with the 747-100/200/300s. I love the Ready for Pushback 747-200, which I bought on disc before they made it freeware (though you miss out on the huge booklet that came with the disc now of course). My "problem" is that I fly so many different and complex planes in FS9 (part of the pleasure: b747 one day, a320 the next, and Concorde after that - great JustFlight a/c) that I am a little bit of a jack of all trades and master of none (though I do think, thanks to PMDG, I might be able to fly a real 737-700! But that's a subject that has been aired so many times on these forums, and is out of place in this thread!!).

 

Thanks again for your time and input folks,

 

Martin

[BIGSKY 316]

I think it's an airline option.

[FLEX1978 373]

Read the thread again Jim