APU Master ON before takeoff

[MD83FO]

What are the consequences of having the APU master ON before takeoff, and back OFF sometime after?

[Roj approved]

What are the consequences of having the APU master ON before takeoff, and back OFF sometime after?

The flap will be open

[Tomaski]

The flap will be open

Unless the APU is still running from the before taxi/engine start phase. In that case, the ram air door will have cycled closed. Having the master switch on without the APU running will cause the ram air door to open which will add somewhat to the drag profile and increase chance of FOD ingestion without any real benefit.

[vilas]

What are the consequences of having the APU master ON before takeoff, and back OFF sometime after?
And why would you do that?

[Tomaski]

And why would you do that?

If someone is actually putting the master switch on without the APU running, then I'm guessing they want to get a leg up on a possible APU start after takeoff (engine failure, generator drops off line, etc) since they won't have to wait for the ram air door to cycle open. My thought would be if they are really that concerned, then just start the APU. That said, I understand some airlines track APU usage by flight crews and discourage unnecessary use. This may be one person's technique of threading the needle between having the APU readily available during takeoff and not getting a call from their Flight Department about excessive APU use.

[vilas]

If someone is actually putting the master switch on without the APU running, then I'm guessing they want to get a leg up on a possible APU start after takeoff (engine failure, generator drops off line, etc) since they won't have to wait for the ram air door to cycle open I doubt if this could be the reason. EFATO is not such a deadly emergency that you require APU in a split second. ECAM or STATUS don't even mention APU start. Starting of APU is only mentioned in FCOM Abnormal procedure There cannot be procedures for phobias.

[Uplinker]

Maybe the OP pressed the APU master switch by mistake before T/O and wonders if there are any consequences?

Many moons ago, at TOD inbound to Heathrow in our BAe146, I noticed the APU was still running. Ooops ! :)

[Tomaski]

I doubt if this could be the reason. EFATO is not such a deadly emergency that you require APU in a split second. ECAM or STATUS don't even mention APU start. Starting of APU is only mentioned in FCOM Abnormal procedure There cannot be procedures for phobias.

I never said it was a good reason, but I can see the thought process that gets you there.

[matsemann]

Many moons ago, at TOD inbound to Heathrow in our BAe146, I noticed the APU was still running. Ooops ! :)
All 5 of them?

[IAEdude]

All 5 of them?

Brilliant :D

[FlightDetent]

What are the consequences of having the APU master ON before takeoff, and back OFF sometime after? Perplexing idea, but I flown under rules that mandated APU on for LVP and similar. 🤷🏼 (now imagine if such idea came from the NAA perhaps you'd be pressed to reinvent the wheel a little).

1) The flap will be open. Looking at the MEL items where the APU is left running, there may be no performance penalty. Hm, actually it would not be thrusting nor sucking even a little, with the open inlet in this scenario causing net parasitic drag - so perhaps check with CDL and the OEM is needed to formally tick all the boxes.

2) Some less insightful people might think it is actually a good idea. 🤔

3) Something happens in the electric system with Master SW on (cue emerg elec proc), so worthwhile researching if that is relevant and with consequences or not.

4) the wear of the switch push-button and system contractors is increased.

(recorded in order of appearance).

[Uplinker]

All 5 of them?

Ha ha !

Remind me how many successful civil jet airliners Norway has designed and built ?

:)

[matsemann]

Ha ha !

Remind me how many successful civil jet airliners Norway has designed and built ?

:)
None, we are just too busy drilling for oil ;)

[FlyingStone]

If someone is actually putting the master switch on without the APU running, then I'm guessing they want to get a leg up on a possible APU start after takeoff (engine failure, generator drops off line, etc) since they won't have to wait for the ram air door to cycle open..

Most modern-ish aircraft work perfectly fine on single generator, bar the ovens and IFE - which isn’t a concern unless you are a certain European airline that likes to fly across Europe on a single engine.

[FlightDetent]

On a late 80's designed jet, dropping 1 ENG GEN cuts the IFE, aft galley and half of the fwd galley. There one oven and one coffee-maker remain operative - guessing that without them it would be a real emergency.

[Tomaski]

Been away from the MD-80 for awhile, but what I recall from this situation is the concern over something happening to that single engine-driven generator until the APU started. With one generator, the APU had an electric starter assist. Without, you were down to a windmilling start (to preserve the battery) which could take a while if you were not well inside the starting envelope. Not a huge problem in VMC conditions, but IMC was a different story.

[MD83FO]

thanks for the injunctions, my train of thought is, ok, mitigation of bird damage on initial climb when warranted, accelerate above 3000' where most bird impacts occur, secondly be ready for a dual engine failure and emergency electrical config by having the APU ready to start, it doesn't seem to affect in any way normal operation.
the A350 starts the APU automatically when needed.

[vilas]

thanks for the injunctions, my train of thought is, ok, mitigation of bird damage on initial climb when warranted, accelerate above 3000' where most bird impacts occur, secondly be ready for a dual engine failure and emergency electrical config by having the APU ready to start, it doesn't seem to affect in any way normal operation.
the A350 starts the APU automatically when needed.
In dual engine flame out all you have to do is press three buttons the MASTER, ON and BLEED and in 40 seconds you will have it. APU bleed is pressure sensitive switch only opens at correct pressure.