Greetings!Great forum relating to an even greater product.PMDG 747-400x.... taking flight simming to the next level!!Although I have ALL 475 pages of the maunal printed out and in a binder (airline style!), I cannot find any information regarding the starting/operating of the APU in flight. I have tried UNSUCESSFULLY to start it in flight, and would appreciate any insight into this.Thank you.Albert "Stubborn Swiss"

You dont use the APU in flight on the -400. You can perform a Pack to APU take off eg: if flying from a hot and high airport but the APU will shut down above 18000FT (not 100% sure of the Alt)Any reason why you would want to start the Apu in flight? there are 4 pretty huge ones hanging off the wings :)Rob

Wow..... thanks for the speedy response.Ok.... one reason: think of a situation where you lose ALL your engines. Not likely? Remember the 747 that lost ALL 4 engines when it flew into volcanic ash? (Google "747 and volcanic ash"). As a matter of fact, I believe there is more than one recorded incident.Would the ability to start and use the APU in such a situation not be beneficial?Thanks!

Well, I can't comment on the engineering aspects but hurtling earthward does provide some degree of windmilling!SOPs state that you can use the APU airborne for bleed air only, but that the APU has to be started on the ground, no electrical power is to be used airborne, the APU bleed air feed must not be used above 15 000 ft and finally the APU must be switched off above 20 000 ft.Just guessing, I imagine the aerodynamics around the inlet whilst zipping along at flight level ridiculous are somewhat different to those stationary on the ground leading to all sorts of problems if one tries to start it up airborne. Just a guess.David "jumbojock" Robertson

>>SOPs state that you can use the APU airborne for bleed air>only, but that the APU has to be started on the ground, no>electrical power is to be used airborne, the APU bleed air>feed must not be used above 15 000 ft and finally the APU must>be switched off above 20 000 ft.>David "jumbojock" Robertson OK...thanks for that info.I just kind of like to "know" such stuff :-hah I guess I'm anal like that!!

"Would the ability to start and use the APU in such a situation not be beneficial?"The APU is a jet engine.... Volcanic ash would have the same effect on this engine.Starting the APU puts a large drain on the battery, so if several attempts were made to start the APU, you might run out of battery power for the Captain's instrument displays　before getting close to the ground... Not what you want when you're flying in the dark ;)The engines were already windmilling fast enough for a relight in the BA incident, so starting the APU would probably not be much of a benefit.Cheers.Q> イアン　

Thank you..... I keep learning!That is the beauty of this forum.

The RAT (ram air turbine) will deploy if loss of electrical power results -- such as losing all 4 engines.- Jake

I thought the 747 didn't have RAT since the windmilling effect of the even one engine produces enough electrics and hydraulics - can anyone confirm?

Unless there is a military version which has a RAT, I can definitely say that the 744 hasn't got one.Having said that, the electrics and hydraulics produced by windmilling engines is dependent on rpms. If the N2 or N3 rotor of an engine drops below 50~57% (depending on the engine), it will not produce electrical power. Airspeed needs to be quite high to do this (If you're trying to glide to a distant airport after a 4-engine failure, you may not have sufficient airspeed for power generation).For engine-driven hydraulics, the airspeed must be above 140kts or so.Your aicraft would be in serious trouble with only one engine running. You wouldn't be able to maintain altitude. However, you should be able to power most electrical systems, including the two electrically-driven hydraulic pumps (for satisfactory flight control) and essential navigation/communication.Cheers.Q> Ян

Hey Jake your thinking of the A340. Rob

The inability to use APU to run utilities in flight in the Boeing 747-400 series aircraft is a serious deficiency which should be corrected ASAP. There have been incidents where engine power or electric power was totally lost. In one case the crew had just minutes of battery power when they landed. That's because it happened right after takeoff, from Bangkok, lucky!

Or maybe the lazy cabin crew should have thought about pouring liquid waste down the drains when it's against company policy. Also it happened during decsent, as the aircaft pitched down the liquid ran into the electric bay.Rob

Let's get our facts straight shall we? This incident happened at the end of a long flight, during descent into Bangkok. The water was affecting the ability of elec power to come on certain parts of the sync bus (i.e. it was an electrical bus control issue as well as a generator control issue). In this case, having APU power probably would not have helped (note here that the APU generator controllers are on the same rack as the engine generator controllers and the Bus power controllers).This particular event was not caused by pouring liquid waste down the drains (assuming the other poster means coffee and tea, rather than pure water). The problem would have happened with pure water, also (there is certainly no policy in the manual against this). The problem was partly caused by a failure of the de-icing system on a tube connected to the external "grey water" drain mast... and partly by flaws in the *multiple* layers of water protection between the galley and the equipment racks below...and partly by flaws in design.The anti-icing system on these connecting tubes and the drain masts is periodically checked, so unless a continuous drain heat monitoring system is installed and water barriers magically become immune to ageing, it may happen again to this and any other type of aircraft (unfortunately, due to lack of space, galleys seem to be placed in proximity to equipment racks on almost all types of aircraft (Boeing, Airbus, etc)).Less serious similar events have happened before in other parts of the 747 series and the waterproof barriers were upgraded*, but even so, something of this magnitude was not foreseen (considering the multiple layers of protection already in place and the levels of redundancy built into the electrical system).My 2c's worth... :(Cheers.Q>*continuous metal panelling was placed over another equipment rack (and riveted in place). Unfortuately, this also creates problems as it makes proper wiring inspections and modifications almost impossible.

"Or maybe the lazy cabin crew should have thought about pouring liquid waste down the drains when it's against company policy. "Where do you suggest they put it? I've even seen liquids pouring from cracks in the removeable galley garbage containers (on a number of different airlines). Plastic linings in garbage containers and plastic/fibreglass containers stowed under galley benchtops are also not immune to puncture/ageing/abuse.There is a series of filters/grilles between the galley sinks and the drain system which should catch larger obstructive particles in liquids. If there is a blockage, the liquids will simply back up into the sink to the point where flight attendents can't pour anything more down there. It's highly unlikely that they would continue to pour liquids down the sink until it started overflowing onto the benchtops and onto the floor. This, perhaps, is another clue that the flight attendants were not responsible for the Bangkok incident. BTW, the lavatory sinks (basins) are also linked to the galley drain system. Perhaps you (as a passenger) might also be responsible for blocking up the drain system?In summary..The consequences of our actions always seem obvious in hindsight. Before we start accusing people of laziness, let's get off our own lazy asses and try to get a better understanding of ALL the factors which lead to these type of incidents (as in most cases, the causes of accidents usually start at management levels and filter down to the sometimes underpaid, undervalued and abused masses).Regards.Q>