Use of APU in classic british airliners

[Sebastian]

I was astonished to read in the tutorials of the BAC and the Trident that the APU should be shut off just after the take-off and so should be reengaged prior to the approach. It is written that during the take-off and approach phase the APU should feed the packs.

That is new for me. I am familar with the Boeing procedures. And even in the classic ones (take the 727 for example) the APU has to be turned off right after engine start. The packs (then fed by the engines) would then be turned off for the take-off run (to gain some extra power).

Further more, a real Boeing pilot told me that it wouldn`t have been possible to start the APU in some thousand feet of altitude.

Is this procedure really different in british jets or just somekind of a simplification?

[DaveB]

Hi Sebastian

I have no knowledge of how Boeings operate but in the case of the 'American' operated One-Elevens.. it was usual to turn off the APU above 1500ft AFL on flights of more than one hours duration. On flights of one hour or less, the APU remained on throughout the flight.. primairly to cut down thermal cycling and to reduce mechanical wear on the starter and associated components ;-) APU status was then part of the 'Before Landing' checks.

ATB

DaveB :tab:

[NigelC]

Also, the 1-11 runway performance dropped away quite steeply once you got above 24c or so. You need every li'l bit of oomph you can get, so you don't want to spill air from the compressors for cabin conditioning. Boeing perf was somewhat better.

[Sebastian]

That statement explains everything, Dave. Thank you.

Also, the 1-11 runway performance dropped away quite steeply once you got above 24c or so. You need every li'l bit of oomph you can get, so you don't want to spill air from the compressors for cabin conditioning. Boeing perf was somewhat better.

Regarding the Boeing - that are not a subject here - turning off the packs for take-off is part of the checklist in any Boeing, even today (or done automatically by auto-setting of the packs). For these few minutes the passengers will have to live with a decent increase of cabin temperature.

The APU-usage is also limited to a certain altitude in the case that it is used airborne. I think it was 18000 ft. Maybe british APUs are/were of higher standard. And passengers can enjoy climated air all the flight...

[DispatchDragon]

Sorry Sebastian

Because of the placement of the APU on B727s (in the main gear bay ) and the APU intake door on the wing
its impossible to run the APU in flight

As Nigel stated the 1-11 was severely WAT limited (the 500 more than others) above 24-28C so ALL takeoffs
were "bleeds" (Pack) OFF The 727 - the Engineer had the option based on WAT/Load - However with most
of the charter companies I worked for - the policy was "off for takeoff' no matter what in an effort to save
wera and tear on expensive engines.

What Dave says applies even today -- The 737NG/A320 family can all run their APUs in flight and they will
take a substantial Electric or pneumatic load from the engines - A standard MEL (Minimum Equipment List)
item for an inoperative Generator or disconnected CSD on an engine is to run the APU in flight infact
it is required by law.

Casting my mind back to Dan Air Daze -- the 1-11s APU was one of the most effective and trouble free
units (but this was in the early 70s when 1-11s were still fairly young) I can remember - the worst by
far were the early A320 APUs which shed turbine blades at the drop of a hat and if cold soaked would
not start , (A bit of an inconvience when your operating Stuttgart-Keflavik.Pontiac in winter).

So the manual and DaveB are right -- APU off (was usually part of the climb checklist once the aircraft was cleaned
up - and one in the approach checks) -- When Dan Air operated the Luton-Leeds-Glasgow sched service with a 1-11
it never went off.


Leif

[Sebastian]

Sorry, that I wrote obvious nonsense

I am only an armchair pilot :lazy:

[DaveB]

Sebastian..

I like nothing more than to learn something new.. every day if possible and that usually happens on here ;-)

ATB

DaveB :tab:

[DaveB]

See.. something else I've learned! Cheers Mr T

ATB

DaveB :tab:

[Chris Trott]

And to add to Leif's items, part of the 737's certification was devoted to specifically create the operational envelope for the APU. This testing went on for all aircraft built by Boeing after that and most of their aircraft have APU envelopes that go up to and in the case of the 777 above 10,000 feet for starting and as high as 20,000 feet for operation. In fact, Southwest even today operates the APU through takeoff and into climb during the summer for passenger comfort. The difference in fuel consumption between shutting down the APU just prior to takeoff and just after is fairly negligible anyway.

In addition, it is much more preferable to start and operate the APU as it has a full-power hydraulic pump allowing for full operation of the secondary flight surfaces (leading and trailing edge flaps/slats, spoilers, etc) and partial pressurization making it possible to control the cabin descent better and less uncomfortable for passengers and crew. This means that on twins especially, the loss of an engine does not necessarily mean the loss of redundancy in the operation of the hydraulic, pneumatic, and electrical systems as the APU systems can substitute fully for them.

Heck, just look at the 747's APU. It is so good, it can start 2 engines at the same time and still supply air to 1 air conditioning pack, keeping you from having to completely shut down the airflow into the cabin during engine start.

[DispatchDragon]

Didnt mean to sound condescending Sebastian :kissu:

As Dave says - there is so much knowledge here at CBFS - we can all learn something enw everyday -- Its one of the joys of coming
here
And my bad -- The APU intake for the 727 is in the mainwheel bay right hand side -- the exhaust is out over the wing



Leif