So I fly a Boeing product long haul.
Cold soaked APU.
Some use the following technique: after landing check turn APU switch to ON but not start till just prior to parking.
ON position opens the APU door and allows the cold soaked APU to come up to ambient temperatures.
Thats their explanation anyway, seems reasonable.
Was corrected yesterday by a Captain who calls that an incorrect technique and not what Boeing recommends. Advocates an immediate start after landing.
Any guidance from the maintenance Gods?
Any difference between Honeywell or P&W APU’s?
Mechanical sympathy would dictate some ‘warming’ time prior to start.
Ideas? Opinions?

Then I’ve got 3 remaining :8

About half the people I fly with seem to do it so I’m guessing it’s tribal law.

this! x1000

A quick look in the FCOM will dispel any confusion, however this sounds very much like what we would call were I work, a “technique” a rather illogical one at that.

Yup, sounds like homegrown bulls**t. Seen a few such things in my career. Usually associated with new bosses.

I was unfortunate enough to have flown classic B737-400 into places without ground power to support the aircraft for a decade some 20years ago. Coupled with ‘not-so-sharp’ grease monkeys at home base workshop, APU start success( on the ground) was a an issue. Low engine bleed pressure, sticky APU start valve were some excuses those grease monkeys mentioned. I know nothing because I am just a pilot. Carrying extra fuel for one engine running transit would put landing performance beyond the curve on the 5600ft runway.

Starting the APU on descend successfully put some worries to rest. No scientific proof but it seems APU has better start success in the descend compare to on the ground.

I would not recommend this practice to anyone in this era where everyone is waiting to shoot you bum-bum if you deviate from the books.

We did airstart 90% of the time in the 73 because of shoddy equipment.

If it’s -25C then well I guess your warming it up from -52C cold soak to -25C.


Nothing in the MX manual that I have access to and nothing in the FOM except “start APU”.

There are a lot of home grown techniques that make sense but I’m not sure this is one of them.

is it a feel good or does it actually make a difference.

The 737CL apu seems to be the more troubling of all the twin engine airplanes, I definitely see the failed starts on the ground with this type; it eventually gets cranking after a few tries but can understand why a pilot might start on descent, into a remote or poorly served port. But all other twins seem to be reliable enough, in my experience.
The mx requirement on A330 to select Master switch ON and wait approx 3mins before start, is to prime the APU fuel system, the fuel pump specifically if I recall correctly, to prevent cavitation. But that is a mx req.

if an operator/airplane is ETOPS certified, the apu is condition monitored and inflight starts may be required from time to time in addition to other apu reliability tests and checks as part of the normal mx program... these inflight test starts will occur specifically whilst the apu is cold soaked to ensure a successful start, if not, it gets looked at. This is what SHOULD be happening. but I recognise not everyone does the same thing and there are differences between some mx programs and mx staff competency.

Bottom line, I would recommend sticking to official guidance. If your apu doesn’t work the way it should whilst cold soaked, on the ground, then this is exactly where you want to find out about it! Log it, get it fixed! Don’t be part of the problem.

Well...metal conducts heat fairly well and the purpose is to reduce thermal stresses I suppose.
Last night we taxied 19 minutes....:}