Radimz From Czech Republic, joined Dec 2006, 8 posts, RR: 0 Posted (8 years 8 months 3 weeks 2 days 15 hours ago) and read 7613 times:
I have a question: what happens with the generator on a B 747-400 when the engine is shuted down due to a flameout or a engine malfunction? Can it generate power with windmilling engines?
CFMTurboFan From Canada, joined May 2007, 41 posts, RR: 0
Reply 1, posted (8 years 8 months 3 weeks 2 days 15 hours ago) and read 7596 times:
The generator which is typically reffered to as an IDG (Intergrated Drive Generator) is mounted on an engines accessory drive gearbox. Gearboxes are driven by the HP and in some cases the IP compressor spools via a radial drive shaft to the accessory drive gearbox. The IDG is driven by the gearbox.
IDGs normally need to rotate around 7000rpm output to the internal generator (not sure if this is correct, correct me if it's not) to produce the required electrical frequency for the aircraft. This power is fed into the aircraft main buses via 4 heavy duty wires running to the IDG.
If an engine flames out, or is shut down, then it will not provide the required rotation via the gearbox to the IDG to produce the required power and therefore will not supply power.
PhilSquares From , joined Dec 1969, posts, RR:
Reply 3, posted (8 years 8 months 3 weeks 2 days 14 hours ago) and read 7546 times:
Quoting 777WT (Reply 2): Then if that's the case, then howcome 747's don't need RAT since 4 windmilling engines is more than enough to provide hyd power?
Providing windmilling hydraulic power and having the IDG work are two different things. First windmilling hydraulic will get you the pressure needed but it might not get you the volume. That is why Boeing recommends against an attempt at landing the aircraft if all engines are flamed out. You will most likely get the gear and some flaps out, but as you slow the windmilling will also be less reliable.
The IDG acts as a transmission. It provides a constant RPM range to the generator thus allowing it to operate over a very narrow volt and frequency range (115v +/-2 and 400HZ +/-5). The problem is the generators don't come on line until just about at the end of the start cycle. In a windmilling situation, you won't have even that rotation. So, no IDG.
As for no RAT, the odds of losing all 4 electrical systems is fairly remote. Even on one generator, with selective equipement off, the aircraft will fly all day.
Bellerophon From United Kingdom, joined May 2002, 586 posts, RR: 58
Reply 10, posted (8 years 8 months 3 weeks 2 days 12 hours ago) and read 7444 times:
...what happens with the generator on a B 747-400....Can it generate power with windmilling engines?...
CFMTurboFan and PhilSquares have both answered your question already, but I'd just like to mention a point that follows on from what they have said.
With all four engines operating normally, but at flight idle power - for example during descent from cruise altitude - it can be harder than you might think to spot an engine failure, should an engine quietly just run down. Initially a failed engine will be indicating roughly the same EPR, EGT and Fuel Flow as the other engines, whilst they are at flight idle, there will be little or no yaw or roll affecting the aircraft and the control column will have little displacement from its normal position.
In such a situation, it is highly likely that the first EICAS warning you will get following, say, a #4 engine run-down, will be an Electrical warning, such as "ELEC DRIVE 4" or "ELEC GEN OFF 4", as the IDG drops off-line, because, as has been said, a windmilling engine will not be turning fast enough to keep its IDG on-line.
Odd though it may sound, following an electrical generator type warning, most B747 pilots will first make a quick check that the associated engine is running normally, before actioning the electrical QRH!
Only on the classic. On the 747-400 the APU can be left running in flight, but not started in flight.
As for a RAT, as has been stated on numerous previous threads the primary purpose of a RAT is to supply hydraulic power for flight controls. In aircraft where electrical power is also required for flight controls (e.g. FBW Airbus types) there is also a hydraulically powered generator installed. The VC10 was an exception as it had two RATs one electrical and one hydraulic.
All the 747 flight controls need is hydraulic power and the windmilling engines are enough for that. Standby electrical power is available from the battery for a limited time. So the 747 does not need a RAT.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
N8076U From United States of America, joined Jun 2006, 425 posts, RR: 8
Reply 14, posted (8 years 8 months 3 weeks 2 days 11 hours ago) and read 7350 times:
The APU on the 747-400 can't be started in flight. But if it was already running while on the ground and the aircraft takes off, it will continue running, up to a certain altitude. It can provide bleed air while in flight, but as soon as the aircraft goes into air mode, the APU generators cannot be used.
KELPkid From United States of America, joined Nov 2005, 6818 posts, RR: 3
Reply 18, posted (8 years 8 months 3 weeks 2 days 5 hours ago) and read 7177 times:
Quoting Starlionblue (Reply 17): The Gimli Glider incident would have been one case where Boeing could recommend all it wanted. The landing would have been attempted without any engines on.
If I recall correctly, until the Gimli Glider incident, Boeing had no procedures in the manual covering a dual flameout, and hence no best rate of glide speed in the 767 manual (the Gimli Glider flight crew were just guessing on that one...). It was because the engineers figured the possibilities of that happening were very remote...
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
PhilSquares From , joined Dec 1969, posts, RR:
Reply 23, posted (8 years 8 months 3 weeks 2 days 2 hours ago) and read 7084 times:
Quoting Starlionblue (Reply 21): It has several. But electrical power alone doesn't do that much good. What is needed is hydraulic pressure to control the flight surfaces. AFAIK the batteries cannot provide drive the hydraulic pumps.
Windmilling engines will provide more than ample hydraulic pressure to the PRIMARY flight controls. The problem is when you have to slow down, the pumps are volumetric and can't power the flight controls and extend the flaps.
I will let all the "experts" on here take it from now on because they have so much more experience flying the 744, oh that was MSFS......
The APU on the 747 could be started in flight, but only after a mod was done. It was not part of the standard 747 delivery package. The 744 APU can not be started in flight. You should have ESS AC and DC for about 25 minutes.