787 APU on for takeoff?

[Check Airman]

Hi,

Why do the 787 crews seem to leave the APU on for takeoff? I've noticed it with AeroMexico, LAN and LOT.

http://www.!!!!!!!!!!!!!!/photo/LOT--...ner/2470243/L/

[slam525i]

I think this came up before and the consensus was that the APU is "off" but in cool-down mode.

[ItsMeFromEarth]

After engine start APU is switched OFF, then 3 minutes after the APU switch to be in ON Position to allow APU Door to open to increase cooling, after takeoff APU switch to OFF.

[nitpicker330]

Really? Seems a bit silly as that was not the way in the 777 and I can't see why it would be different on the 787.....the cool down schedule is automatic.

[sleeve of wizard]

On the ground, when the APU inlet door is closed after APU shutdown, hot air can collect in the APU. The hot air can cause a rotor imbalance or bowed rotor. In rare cases, a bowed rotor can cause rotor rub that might lead to a hung start and APU damage.
To prevent a bowed rotor hung start on the ground, the APU door should be kept open for at least 40mins after APU shutdown. This prevents hit air from being trapped in the APU and allows the APU to be started at any time with no risk of a bowed rotor hung start.
If takeoff occurs during the 40min wait, the APU door can be closed at a convenient time after takeoff.

[DJ77]

Could be APU-to Packs procedure.

[Check Airman]

Thanks guys

[Swedish Steve]

The APU is being modified to get round this. We have only one left to do, then tit will be back to normal.

[stilton]

'tit back to normal'



Checklist complete.

[Gillegan]

Could be APU-to Packs procedure.

The 787 does not feed the A/C Packs with bleed air from the engines or APU. Air for the A/C system is provided by 4 electrically powered Cabin Air Compressors (CAC's).

[DJ77]

Thanks Gillegan for correcting me.
Perhaps there will be APU-to-CACs (electrically) someday.

[Brian Fantana]

So it was due to a design fault that requires a modification from Boeing, probably costing them quite a bit of money.

[gas path]

Actually the APS5000 apu is manufactured by Hamilton Sundstrand so will not cost Boeing.

[underfire]

Bad RAT, bad APU....how ARE the batteries doing these days?

[ItsMeFromEarth]

how ARE the batteries doing these days?

Smoking good

To prevent a bowed rotor hung start on the ground, the APU door should be kept open for at least 40mins after APU shutdown

I think that you have to leave it 40 minutes if you restarted the APU by mistake (going all the way to START instead of ON position)

[sleeve of wizard]

Its me from earth.

Nope that's not what the FCOM Bulletin says.

[ItsMeFromEarth]

Correct......
I any case.... I wrote I think......