737 APU start failure suggestion

[BaconRash]

We are getting more and more APU faults during starts...typically around one in twenty five starts.

Never happens on the first sector.... almost exclusively happens when OAT is 25 degrees C or over. BITE Fault indication is invariably "slow acceleration".

Engineers advise it is a problem which occurs with our type of APU as it gets older. Advice is to start it when cold. Typically around 5000'...sure enough they start every time at altitude.

Apart from the 5 mins plus of extra fuel can anybody see any technical issues with starting APUs between 10,000 and 5,000'?

I know the problem needs fixing...but in the mean time I am not over keen on using air start carts three times a month if it can be avoided by an early APU start.

[aviatorhi]

There are many airports between 5000 and 10000 with 737s starting their APUs on them.

Should not have any associated issues. Depending on the specific type of APU certain tweaks may be made to provide for quicker acceleration.

[BOAC]

Apart from the 5 mins plus of extra fuel - less than 2 kg? I reckon if it works, do it.

[Flying Wild]

Wish I'd thought of starting prior to landing today. Instead we had a failed first start, followed by a nice tongue of flame from the exhaust followed by an overspeed warning on the next go.

Love those air starts in 31C heat...

[A37575]

There is often too much accent on perceived fuel savings with APU operation. Suggest the APU manufacturer be asked for advice if starts unreliable on the ground. Don't always take the word of engineers in operational matters. Opinions are a dime a dozen. Facts are more rare.
Until your maintenance organisation can fix the progblem for good suggest you start the APU at (say) 10,000 ft. With no load on the APU fuel consumption is minimal.
Keep in mind that in- flight start of the APU is recommended if flying in moderate to heavy rain, hail or sleet. This is to hedge all bets if double flameout occurs. This applies at any stage of flight.

[Checkboard]

Flying the 737 with Ansett, in Australia, we operated to a lot of remote (i.e. no engineering/fire or ATC coverage) destinations. It was SOP to start the APU on descent every time - it gave a more reliable start, and if the start failed you could make a second attempt immediately (no waiting for pooled fuel to dissipate). A bit less fire risk on a second start for that reason as well. :ok:

[sudden Winds]

I do not normally start the apu in flight unless part of a procedure or my judgement calls for it. One of the reasons is that, as soon as I turn it on, I have one more possibility of a fire.
It looks like you fly NGs, so we're talking about the same apus here. If the apu fails to start on the ground, and as long as the FAULT light extinguishes after 5 min, you can give it a second try, while asking for a gpu during taxi in.

Now, if the place where you´re going lacks pneumatic capability that's a different thing, but otherwise, I just leave it alone in flight.

That being said, I don´t think there is anything wrong or unsafe with temporarily starting them in flight until a solution to the problem is found.

Regards,
SW.

[grounded27]

There are many airports between 5000 and 10000 with 737s starting their APUs on them.

Should not have any associated issues. Depending on the specific type of APU certain tweaks may be made to provide for quicker acceleration.

I do not believe that to be the issue. Thinner air makes it harder to start. I believe the advantage to be a cold soaked APU and 5K being a good altitude to balance fuel burn v/s a nice cold APU.

[c100driver]

Actually it is the ram pressure in flight that assists the APU starter to turn the APU.

The B737 200 that we flew in our airline had a history of starter failures on the ground and the interim solution was to in-flight start until an upgraded starter was available some years later.

[sky-738]

say APU has two staters , one is for air start the other is for electric start ,correct?

[sb_sfo]

say APU has two staters , one is for air start the other is for electric start ,correct?

Are you still talking about 737s? The triple 7 APU has a pneumatic starter as well as an electric starter, not sure about smaller Boeings, though I tend to doubt it...

[Yeelep]

737 up to NG has a starter only, NG's have a starter/generator.