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Guest Flyingingle

747-400 Queen of the Skies APU

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Guest Flyingingle

Does anyone know if there is any reason that the APU cannot be started in flight...surely this isn't correct.Also is it possible to track a VOR radial using the Autopilot? The MCP panel has a [LOC]aliser button but not the traditional VOR/LOC button.Many thanks in advance for any helps/tips received.

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Guest s_L_Y_F_a

you could set up a VOR station in th LEGS page as your current waypoint and then enter an inrecept course which LNAV is going to fly. This of course works only with inbound radials.

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Ms/Mr NamelessPlease adhere to the forum rules and sign your posts with your real name. Thank you.>Does anyone know if there is any reason that the APU cannot>be started in flight...surely this isn't correct.Surely it can. IIRC this is modelled after the real thing. I'll let others explain why. With four engines you hardly ever need an extra APU in flight as you might lose one or max two engines. I've only heard of one instant where all four engines were lost. And that was the BA flight flying through volcanic ash. The APU wouldn't have been any good in that situation anyway as it is a gas turbine as well and wouldn't have started either.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Guest Peter Bowcut

In-flight APU start capability is a carrier option for commercial airline customers with the 747-400, and most (all?) choose not to have it, so in this regard the PMDG sim is very accurate. :) As was stated earlier, this capability simply isn't needed with four engines. The reason the APU on the -400 can't be started in-flight is that the intake inlet on the rear fuselage doesn't have a 'scoop' that pops up during startup as on some other airliners, but rather a 'pouch' that sort of folds in for starting the APU on the ground. This makes the 747 aerodynamically cleaner in flight at the sacrifice of the loss of inflight APU start capability (the 'pouch' inlet works fine on the ground but doesn't have enough inlet airflow inflight for safe APU start, whereas the 'scoop' on other aircraft do. The APU can however be started on the ground and left running for the duration of the flight, but I think only Air Force One ever does this. Basically the 747 APU intake pouch can facilitate the APU running inflight, just not the start up sequence.

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To expand on your answer, the APU can be started on the ground and left running up to a maximum altitude of 20,000 feet for generator use. APU bleed air is available in flight for one pack up to 15,000 feet.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

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"left running up to a maximum altitude of 20,000 feet for generator use."Normally the APU generators trip offline when the aircraft takes off (although there is an option to have the generators operative if all the engine generators fail).The APU itself is only certified to run up to 20,000'.Cheers.Q>

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