B787-8 APU

[gfmb]

From Stockholm Arlanda Airport extranet:

"In conjunction with the introduction of its B787-8, British Airways has applied for an exemption to use an auxiliary power unit (APU) throughout the turnaround process as recommended by Boeing."

Link: AI-121-2013 Exemption for use of APU - Swedavia Extranet

Anybody who knows the reason/s why behind "...as recommended by Boeing..."?

(BA is using the B787 on one flight between LHR-ARN-LHR per day during 09AUG-21SEP13 as part of the introduction of the aircraft)

[squarehole]

This is pure guesswork but I guess the extra time is used to assess APU performance and as an APU is effectively an engine the extra running time will allow a mechanical run in period to ensure long term efficiency.

[archae86]

shaft bowing?

My guess is that this is a reference to shut-down/startup restrictions related to the shaft bowiing problem.

[Ian W]

Archae86

My guess is that this is a reference to shut-down/startup restrictions related to the shaft bowiing problem.

This may well be the case as the turn around time for a short 'shuttle' flight may be a lot less than for the expected routine flights of the 787; 45 - 60 minutes compared to 90 to 180 minutes.

[EEngr]

In the linked article I see a reference to an "environmental permit". Perhaps they are trying to minimize the airport's carbon footprint by encouraging the use of ground power.

[Mr @ Spotty M]

It might be to save on the requirement for the two GPUs that would be required for the turn round.
Positioning the extra GPU and the removal may impact the access around the aircraft for the short turn round also.
Keeping the APU running also eliminates the APU failing to start, which is required to start the engines.
If the APU fails to start, you will then have to find a third GPU quickly for engine start-up or you will end up with a delay, which BA does not want.

[FE Hoppy]

How many GPUs does that thing need???!!!

[Mr @ Spotty M]

If you only have 90 KVA producing units, you will need three for engine starting.

[Swedish Steve]

My guess is that this is a reference to shut-down/startup restrictions related to the shaft bowiing problem.

Correct. The BA780/! has a 1hr 15min gate stop. The rules with the APU cooling cycles don't leave any leeway if you shut it down.
Over on T5 (at ARN) we shut down the Qatar B787 APU, but that has around 2 hrs gate time and it works OK.
B787 must have two 90KVA GPU at the fwd connections at all times, or you get load shedding. It says in the book that you can start one engine with two GPUs, but we haven't tried yet. The APU easily starts both at once.

[TURIN]

In addition to the above, Ground power has a tendancy to drop off line under heavy loads. At the moment, an unexpected and non normal shutdown of the 787 computer systems will cause hours of resets and tests before dispatch.
Also, if you want air conditioning on ground, and there is no conditioned air available on stand, then either a 3rd GPU is required or run the APU.


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[nicolai]

Originally Posted by TURIN

In addition to the above, Ground power has a tendancy to drop off line under heavy loads. At the moment, an unexpected and non normal shutdown of the 787 computer systems will cause hours of resets and tests before dispatch.
Also, if you want air conditioning on ground, and there is no conditioned air available on stand, then either a 3rd GPU is required or run the APU.

With the carbon bootprint (and non-CO2 emissions like soot and nitrous oxides) from (perhaps) three diesel GPUs grumbling away next to the aircraft, perhaps running a nice new clean turbine APU is looking like a better idea!

(Some of the ground power might come from fixed installations powered from the building mains but how many stands have 3 of those available, or even two - and how much dirtier is a diesel GPU than a Jet A1 APU?)

[Swedish Steve]

Here in Scandinavia, and most of Europe, running the APU on the ground is not an option. That is why we got the dispensation in the first place. On every other aircraft at ARN, the APU is shut down as soon as ground power is attached, and started again when the door is shut for departure.
Every gate has one FEGP, and they are very reliable, and nearly all the widebody gates here have two FEGP. It is not a good idea to run the packs from ground power, other systems will load shed, so the third GPU will only be required for an engine start. If it comes to it we will attempt a start from the two FEGP first and see how it goes. The AMM says it will work.

You can only use modern diesel GPUs on the B787, older ones do not have stable enough frequency. Modern GPUs are very quiet, and much preferable to an APU.
Get out on the ramp at ARN one day. Sometimes you can hear the birds singing it is so quiet!

[nicolai]

Originally Posted by Swedish Steve

Get out on the ramp at ARN one day. Sometimes you can hear the birds singing it is so quiet!

Sounds very pleasant! Thanks for the info.

[Pub User]

The 787 APU is extremely quiet, and in fact is drowned-out by the CACs & Packs running when on the ground. Two GPUs would probably produce as much noise.