B737NG APU(Allied Signal 131-9B) Start Altitude

[B738HT]

Hi everyone,
I was just confused after reading a tech book since it says there's no maximum recommended start altitude for B737NG but 25000ft for Classics; The QRH has no specific instruction.

Could anyone tell me should I believe the tech book or QRH?

[Yeelep]

The 737NG APU can be started at 41,000 feet or below.

[oceancrosser]

Isn't that an option that needs to be purchased?

[latetonite]

No. Standard.

[Evenrude]

The 25000' is only a recommendation in the case of "Loss of both engine driven generators". The reason for this is that you only have battery power remaining and a successful APU start is essential.

[flyingchanges]

Because your 30 minutes of battery life only includes one APU start attempt.

[B738HT]

You mean a reliable successful restart?

[B738HT]

The APU model?

[B738HT]

Making sense. I'm wondering if the NG's APU has better performance for starting in air than Classics.

[B738HT]

I believe one APU start just needs 5-7 min battery life

[latetonite]

And once it is started, you do not have to worry about battery life..