737 APU starting

[LEM]

One little problem happened a few days ago to confuse my already confused knowledge:

After a scheduled airplane change, we arrived at the new one and found it completely dark with an engineer struggling in the cockpit.
Apparently, as he told me, somebody :rolleyes: had forgotten it with the battery on... so the battery was depleted and the APU couldn't be started.

Long delays waiting for a GPU, of course, at LIRF :mad:

Then the AC GPU came and he was able to start the APU.

This confuses me a bit: doesn't the APU require Battery power to be started?

We were told in the classroom the Apu starting excludes the battery charger because it is limited to 40 amps, and only the battery can provide the sudden huge amps amount to crank the APU, as in theory it can provide a million amps in one millisecond...

This seems to be confirmed by the BAT ammeter going full negative every time we start the APU normally, with ground power already on line.

So how on earth was it possible to start the APU the other night using an AC ground cart?


:confused:

[IFixPlanes]

Hi Lem
There is a BIG difference between APU- starting 737CL vs. 737NG.
737CL use 28V DC to turn the starter on the APU. When 100% speed is reached you can get AC by the APU generator.
737NG APUs have combinated starter/generator. This means that during starting he acts like a starter. Later it acts like a generator.
The starter/generator needs AC to turn the APU. Now two other new parts come into action. The start power unit (SPU) and the start converter unit (SCU). The SPU provides the SCU with 270V DC. The SCU converts the 270V DC to AC and feed the starter/generator.

And here finally the answer to your question about AC ground cart:
The SPU can use batterie (28V DC) OR "AC TRANSFER BUS 1" (115V AC)

HIH :E

Ingoxx

[Flight Detent]

Hi LEM,

I agree, the 737NG APU can be started with either aircraft battery or AC power, with AC power being the preferred when it is available - you'll know it's using AC power to start when the "BATT DISCHARGE" light doesn't come on during the start.

Further, I do hope you had the aircraft battery changed for a fully charged one prior to departure, just in case you had to rely on it for standby power.

If your airplanes have twin batteries fitted, the auxiliary battery is ONLY used during standby power operations, so if one of the batteries is not fully charged, the advertised time available on batteries could be somewhat less than expected, and if you don't carry two batteries, well.......

[xetroV]

The 737 300/400 has an auxiliary 28V DC power receptacle located near the battery that can be used to start the APU if the battery is depleted.

[LEM]

Hi IFixPlanes,
As you say The SPU can use batterie (28V DC) OR "AC TRANSFER BUS 1" (115V AC) was the missing link in my brain...
But not only in my brain :mad:
I wonder why they provide us with so miserable manuals.
The above information is available NOWHERE to pilots, and please correct me if I'm missing something.


Hi FlightDetent,
I agree, the 737NG APU can be started with either aircraft battery or AC power, with AC power being the preferred when it is available - you'll know it's using AC power to start when the "BATT DISCHARGE" light doesn't come on during the start.
We have a classic fleet, however some of our airplanes have the NG ammeter panel and some the APS2000 apu.

I don't know if these airplanes are the same as the NG, in this particular case, however I've always seen the BAT DISCHARGE light coming on when starting the APU using the GPU, or while taxying after landing...

That's also why I thought that APU starting requires exclusively battery power, even if AC is totally available.

But I must add that I'm not sure if those with the NG ammeter have the APS2000, or Garret 129.
Our fleet is completely wixed up, I'll check further...

Thanks for your help.

[IFixPlanes]

Hi LEM
Even when you have an "Digital" electrical power control panel in your 737CL, underneath it is "old fashion". :hmm:
Normally 737NGs use Allied Signal 131 APUs.

Ingo