APU-to-Pack takeoff

[Will]

Regarding an APU-to-Pack takeoff, this isn't something I know anything about... it's just been referenced a time or two on the forums, but without much explanation. Can someone fill me on on what it is and why you would do it?

I'm guessing the purpose is to provide pressurization and air-conditioning, while getting the performance boost of a 2-pack-off takeoff, with the only penalty being wear and tear on the pack and the APU, plus the APU fuel burn. Is that right? Any considerations, checklists, procedures, limitations etc. to be aware of? (Other than the general altitude limitation for in-flight APU use?)

How common is it to do this?

Thanks guys.

[michelvsr]

Hi,

As you say, the main purpose of this procedure is to preserve takeoff performance when limited while maintaining air conditionning in the cabin. For exemple, a typical case is takeoff from Mexico City in the afternoon  at the maximum operationnal takeoff weight. Airport elevation is 7300 ft and temperature around 25°C that mean ISA +25 !

Below is an extract of the relative Supplementary Procedure from the FCOM


747 Flight Crew Operations Manual
Supplementary Procedures Chapter SP
Air Systems Section 2

747Supplementary Procedures SP.2 Air Systems
Air Conditioning Packs

APU-to-Pack Takeoff
After engine start:
LEFT and RIGHT ISOLATION valve switches - OFF
Leave APU running to supply air to pack 2.

Before takeoff:
PACKS 1 and 3 control selectors - OFF

After takeoff:
PACK control selector (One only) - NORM
After engine thrust is reduced from takeoff to climb, position one
Pack Control selector to NORM.

PACK control selector (Remaining pack) - NORM
When cabin pressurization stabilizes, position remaining Pack
Control selector to NORM.

LEFT and RIGHT ISOLATION valve switches - ON

APU selector - OFF

Happy landings
Michel

[DougSnow]

However, not every -400 could use it.  The UA -400 AFM states that at least one pack had to be running for takeoff, AFM p 1-14 , if a specific service bulletin had not yet been complied with. 

When we had the TNT 400s, there were no field elevation restrictions to APU TO PACK.  On a 3 engine ferry, one pack had to be ON.

On our 777s, we cant use APU TO PACK above 6900 ft pressure altitude, thankfully it doesn't go to MMMX.

Why?  I dont know, just how the aircraft was certified.

[Britjet]

BA didn't use this technique. The SOP was all packs on with weights of 300,000Kg or less. With weights above this (most of the time) the packs would be selected OFF as a procedure just before take-off, and switched back ON (one pack initially, then the others) after initial thrust reduction and before 3000ft.

It was just a procedure, not even a checklist item. There was no discernible cabin pressure change.
The overall benefit of all packs OFF is about 2000kg increase to the TOPL (Take-Off Performance Limit), so very little really.

I surmise that having a pack ON supplying just the cabin is more of a passenger comfort thing.

Of course, closing isolation valves in the pneumatic system does risk losing leading edge flaps normal operation, as well as wing anti-ice in the event of a loss of all bleeds on the same side, but hopefully not much chance of that? Until the day....!

Peter