High/Low Altitude APU starts.

[newifr]

Hello All

I am studying for my ATPL and came across an article regarding the APU. The article went on to say that should you loose all your generators, you could attempt one APU start but it also recommended that the start attempt should be done at a lower altitude around 22,000 ft.

Why the lower altitude, is it because it's a fuel flow, airflow or electrcal limitation?

Looked at google but all it would say is that it recommended a start at a lower altitude, no reason.

Thanks

newifr

[Jonty]

The air wont be thick enough to start the APU, and you don't want to run the batteries down trying.

On the aircraft I fly the APU battery start limit is 25000ft.

[Slasher]

An APU is simply a turbine engine and has its own start
envelope, so pressure altitude comes into play here like
what Jonty stated.

Actually there's maximum ceiling for start and a recommended
level. The recommended level is to be sure of a start in the event
you're on batteries only.

[Nepotisim]

The 767 & A330 can start up to the service ceiling, around FL400. They can only supply air until about FL200.

In fact they are started regularly to test this feature as it is a requirement for ETOPS. If they don't start they have to be rectified and ETOPS is no longer valid until rectification and a valid inflight start is carried out.

The reasons for a lower start would be thicker air and oil supposedly thinner.

[westhawk]

The maximum altitude for APU starts will be specific to the particular airplane model. The same basic APU model may have different start limitations in different airplane models.

The limiting factor for a particular APU starting altitude limitation is most likely to be air density. In the less dense air at higher altitude, a gas turbine runs at a higher EGT for a particular RPM and load demand than it would at a lower altitude where the air has a higher density.

For certification purposes, the airframe manufacturer establishes these limitations in accordance with the procedures set forth by the certification authority. In all cases, the maximum starting and operating altitudes for an APU should be equal to or less than those tested and certified by the APU manufacturer. The placement of the APU air intake on the airframe may well affect this limitation.

On the HS125-800XP I flew for awhile, APU operation was limited to 30,000' while starting was limited to 13,000' ( IIRC.) I remember how we always got warmer starts at KTEX. (Telluride, CO elev 9,000')

Regards,

Westhawk

[Tail-take-off]

Some aircraft do indeed have an APU start envelope that goes right up to the aircraft ceiling. Of those that do some also have a maximum altitude for assured APU start. If you loose 1 generator it would be no big deal to attempt an APU start at the max altitude for APU start. However if you are on battery power only you want to be sure it will start & not drain the batteries unnecessarily.

Indeed the A320 proceedure for loss of both generators recommends not starting the APU & sticking with the Ram Air Turbine (RAT) powered emegency generator.

[rudderrudderrat]

Indeed the A320 proceedure for loss of both generators recommends not starting the APU & sticking with the Ram Air Turbine (RAT) powered emegency generator.
Only if you wanted to remain in ELEC EMER Config.
If you didn't - why not start the APU iaw QRH 1.16?
ENG Dual Failure - Fuel Remaining

[Slasher]

The ELEC EMER CONFIG is a bit different from a loss of both
donks rudder.

If you've lost all yer electrics due to a bus prob that threw off
your gennys then it wouldn't be wise to fire up the APU as its
likely that genny will be booted off line too. But if you've simply
lost all your gennys (say a scenario where GEN 1 fails then you
have an IDG overheat No 2), then yep I'd risk an APU start.

Having lost both your donks with fuel remaining means again
you've only lost your gennys, so that's why its worth having a
crack at the APU.

Don't forget if you manage to get a generator on line the RAT
will continue to power the DC and AC ESS buses irrespective,
so your minimum 140kt lim still applies.