Starting APU in flight in the 747?

[tunnelcat 1]

Can the APU be started in flight on the PMDG 747? If so, how? I can only get it started on the ground. Is it altittude dependent or even possible to do in this aircraft? It would be nice to have backup electrical and bleed air in a major (simulated) emergency.Kim Kraemer

[scandinavian13 5,192]

The manual (Chapt.11, page 65) states that the APU can be operated up to 15,000 for air conditioning purposes.I couldn't find anything else about speed restrictions or anything of that sort.Regards,Kyle Rodgers

[lbarber 20]

APU won't start in flight on the 744, but can be used after takeoff up to 15,000 feet. The theory goes that in the event of a quad engine failure, the windmilling of the engines will be enough to provide essential electric and hydraulic power.However, I wouldn't recommend trying that at home :(-Lee Barberhttp://home.nc.rr.com/uniuniunium/dvasig.jpg

[tunnelcat 1]

Thanks for the information. I did find that in the PMDG manual but was curious if it was possible to do in an extreme situation. Corelock has not been not simulated with sudden engine failures in FS9 but I have heard of this in several real life aviation accidents.Kim Kraemer

[Qavion 19]

Some suggested on another forum that the APU could be started by pulling various circuit breakers, but I didn't follow up his claim."the windmilling of the engines will be enough to provide essential electric"Electrics are not guaranteed. N2 windmilling speeds need to be quite high. EDP hydraulics are available down to 140kts(?), I recall. The British Airways 747 pilots who flew through a volcanic ash cloud over Indonesia said they had more electrics than they expected, but I remember that the lights were flickering, so the supply may not have been constant.The 744 APU is not designed to start inflight. Depending on software, the APU, if still running after takeoff, will supply electric power if all 4 engine generators fail.Cheers.Q>

[Check_Airman 3]

Q,Do you have any idea why Boeing decided to inhibit starting the APU in the air? Seems like the'd have to do extra work for such an inhibition. Why the extra effort?Paul

[Tabs 2,790]

If you need power after shutdown at the gate, just start it while you're taxiing in, it's on the checklist I believe...

[Qavion 19]

Not sure. There is only a single extinguishing bottle, but if it was so much of a fire hazard, why would it be allowed for takeoff? Perhaps APU starting is statistically more dangerous than APU running/stopping?( But you would only use it in extreme emergencies anyway, I'd imagine... so you would probably want to take that chance).Now that the tail has a HST, perhaps it was considered to be more of a risk than a benefit? Cheers.Q>

[teeps 0]

Funnily enough, this was covered in the last copy of PC Pilot that I read. Apparently the APU sits within a fireproof shield which is good for three or four hours of raging APU inferno if something goes wrong. The 747 may fly routes which take it more than four hours away from a suitable airfield, so the APU is not allowed to start in flight because if something did go wrong the potential consequences of the APU fire shield failing half an hour before landing (for instance) are worse than the limitation of not having in-flight start, especially as having four engines makes complete electrical failure unlikely.

[Qavion 19]

All true.I'm just wondering what might happen, however, if the engine disintegrated. Engine blades have a tendency to puncture just about any material... Did you read about the 767 engine which disintegrated on a test run and one bit(blade?) flew out, puncturing the cowl, then the fuselage, then the cowl of the other engine :(Almost enough to make you want to give up flying :)Cheers.Q>

[markadeane 272]

> Almost enough to make you want to give up flying :)Almost but not quite!!!The pictures of that incident were scary though, it took me a while to realise that the piece of compressor embedded in the number 2 engine was from the number 1 engine! if it had gone up into the centre fuel tank one shudders to think of the outcome.

[teeps 0]

Haven't heard about that one, but in my former job years ago I went to one of the North Sea gas rigs a couple of times. They were using gas turbine compressors to pump the natural gas back to shore. The compressor room was a big steel box, but it still had holes in various places from a time when one of the turbines threw a blade...

[Jet Tech 31]

The scariest thing about that incident is the amount of damage to the wing tank access plates and wing plank skin from the discarded blades. There were several large holes that propogated the fire that caused the most extensive damage. That and the fact that one piece of the engine was found 2500 ft away across both active runways, laying near the AOA fence south of the airport. Read the NTSB report, its very interesting reading.RegardsPaul Gollnick :-cool Technical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.comhttp://www.precisionmanuals.com/images/forum/devteam.jpg

[tunnelcat 1]

Thanks for all the replies to my original question of starting the APU in flight on the 747-400. Apparently the APU cannot be used as a backup power source in flight and since the 747 does not have a R.A.T. (ram air turbine) that can be deployed in the case of total engine failure, you have to rely on engine windmilling. A four-engine failure can happen. In 1982, a 747 flew through a volcanic ash cloud at night that disabled all four of the engines at 37,000 feet. Fortunately one engine was windmilling with a sufficient speed to run the associated generator drive. They barely managed to restart three engines after they cleared the ash cloud and land the aircraft in one piece. The front of the plane looked like it had been sandblasted, including the cockpit windows. All of the engines had compressor blades that were coated with melted ash that had fused to all hot surfaces inside the combustion chambers. It was amazing three engines even restarted, let alone rotate after ingesting all that ash. Even the pitot tubes clogged up! Thankfully, we don't have that hazard in flight sim. You can read about this incident in the book 'EMERGENCY, Crisis on the Flight Deck' by Stanley Stewart, which I think is still in print.Kim Kraemer

[Qavion 19]

"A four-engine failure can happen. In 1982, a 747 flew through a volcanic ash cloud "In this case, I'm sure the APU would have failed also... The APU inlet door would have made an excellent scoop for volcanic ash :(Cheers.Q>