APU operation during low vis Takeoff

[Adsto]

Hello all,

Am trying to find out why NG/BBJ operators have the APU running during low visibility takeoffs. I can't find a regulatory requirement and am wondering whether it is done because we can, and it doesn't otherwise affect system operation. If in the event of an engine or IDG failure, the APU is available for quick selection.

Can anyone help out with more information. Any regulatory requirements?

Thanks,

[speed freek]

Probably that too, but I reckon it's to do with returning to the field, if necessary. Since its low vis ops, CAT III would more than likely be used, and the autoland requires the APU to be running in case of a power source failure. I guess having it on during the t/o means that the crew have one less thing to worry about when they return.

Just a guess......

Cheers

[BOAC]

I would put it down to airmanship, Captains' discretion - whatever you wish to call it. It may be an airline SOP?

OOI DanAir always lit the APU for Cat2/3 approaches, BA did not, and that was BEFORE Boeing banned routine airborne lightups.

I cannot quite see, however, why you would be 'coming back' to a Cat3 airfield in a hurry?

[Right Way Up]

What about an APU fire that won't go out!

[BOAC]

You're not a lawyer by any chance, are you...................?

[Flight Detent]

Just to throw a cat amongst the NG's -
Our NG's are required to have the APU shutdown during all takeoffs, but for an entirely different reason!

Cheers, FD

[Shaka Zulu]

Southern Hemisphere that's why FD

[Dehavillanddriver]

Detent,

I assume that you are QF, what is the reason behind that?

[Flight Detent]

We have a modified cowling on the underside of the APU, for some additional equipment.
Though this additional housing is designed to withstand quite hard contact with the runway surface, the possibility exists that it could impinge into the APU compartment as a result of a really hard hit.
Therefore the APU is required to be shutdown during all takeoffs - from a safety viewpoint!

Errrr.....No, I'm not QF......but I was in Australian Airlines operating B727 a few years ago, does that count?

And Shaka, it all goes anticlockwise here, which means we don't get a real winter, as you know it, here in BNE!

Cheers

[Adsto]

Thanks all for the good input.

Flight Detent,

you mentioned that your prohibited from TO with the APU running due to risk of tailstrike affecting APU performance. Does this mean that your MEL (or MMEL) reflects that you can't dispatch with a faulty IDG or other maintenance problem that prevents electrical power from one engine?

Thanks,

[Flight Detent]

Adsto,

Yes, that is true, both IDGs are required to do the job!

Cheers, FD

[barit1]

APU performance credit

I've not studied the 747 AFM for many years, but I recall that baseline takeoff performance required the APU running, and if it were not, then a small (500# ??) TOGW penalty applied. I believe this was due to the small, but measurable thrust from APU exhaust.

And at one time (mid 70's) an alternative APU was being considered that would have put out a bit more thrust.

Can someone verify this for 744?

[Centaurus]

BOAC. "Boeing banned all airborne routine APU light up's"

Can you quote an authoritive Boeing source for that statement? Is there a problem in airborne APU light ups - if so, what is it?

[Flight Detent]

I never heard of any Boeing ban of inflight starts of B747 APUs, then again, I only operated the SP/-100/-200/-300 Classics till about the end of 2002.

I KNOW there is no Boeing ban on B737NG inflight APU starts, believe me here!

Cheers, FD

[The Puzzler]

Puzzle me this....

We operate our APU for all low vis take offs and landings. This is because it is immediately available in case of a failure eg IDG. As for the risk of an APU fire, it is an accepted risk.

Cheers

[Empty Cruise]

@ barit1...

...if my (rather new-found but already) failing memory serves me right, I think you get a 2nd segment & climb penalty of 20(?) kg for departure with the APU running (737 classic) due to drag from the inlet door being open

Ze devil iz in ze detailz

Brgds,
Empty

[Phil Squares]

barit1...Not true. You do get additional performance with a packs off take off. However, that's due to the lack of bleed air being taken off of the engines.


Flight Detent...there is no 747 certified for inflight start of the APU. However, if the inlet door is modified, you can have it operate in flight.

That is true of all 747/744.

[BOAC]

Centaurus- no, sorry, I cannot, but it came in in BA I guess in the early 2000s when only lights as directed by the QRH were permitted.

[barit1]

barit1...Not true. You do get additional performance with a packs off take off. However, that's due to the lack of bleed air being taken off of the engines.

There's still some bleed air driving accessories (Hydraulics???) but you are right about packs-off takeoffs. (That's pretty standard on most types I'm aware of)

Are you referring to Boeing's own charts, or to company-produced charts - which may contain some conservatism for the sake of simplicity? (That's not uncommon when an operator's routes don't require every last kg. of Boeing's performance tables...)

[LEM]

Hi BOAC and Centaurus!

Buried deeply in my paper mess was the copy of the bulletin, I found it!

It is dated Sept 1st 2000, but that was the date my (now) Company put it in force, not necessarily the same as Boeing's.

It states that a 732 lost ALL electrical power while attempting a routine APU start on base leg in VMC (luckily!).

The airplane battery was nearly dead due to the loss of electrolyte caused by a continuous overcharge condition from the battery charger. Additionally, both GCUs had shorted diodes which allowed internal GCU fuses to open during the APU start attempt, thereby disabling both generators.

Up to now the fault has not been duplicated.

As an interim recommendation, waiting for the final result, Boeing recommends as a precaution to preclude this malfunction from occurring, that routine APU inflight starts should not be attempted.

So, Gentlemen, assuming this bullletin is still in force ( ), nothing is FORBIDDEN, and I think it is left to Captain's judjment to start it or not before a CAT III takeoff or approach.

Regards, LEM