Azza78
Topic Author
Posts: 4
Joined: Mon Sep 11, 2006 10:21 pm

Change In A/C Sound On 767

Mon Sep 11, 2006 10:35 pm

Hi everyone- been reading this forum for a long time...and learnt alot. This is my first post......

I've travelled alot on QF 767's. Whenever im on a RR powered 767, i find the a/cond seems to 'flare' when the engines are throttled back as decent begins. It flares loudly and then settles back straight away. This also happens when entering turbulence and engines are throttled back.
Interestingly i've never heard this on any of the GE powered 767's. (or any other aircraft i've travelled on)

What we be so different with the RR powered versions??

Look forward to the replies!
 
User avatar
CCA
Posts: 770
Joined: Wed Oct 02, 2002 7:29 pm

RE: Change In A/C Sound On 767

Tue Sep 12, 2006 12:51 am

The change in sound is caused by a change in the bleed supplies from the engine. In the case of the Roller the bleeds come off the intermediate stage (N2) or the high stage (N3).

When the engine is at a highish power setting the bleed of the N2 is enough for the requirements of the A/C, when the engine thrust is reduced, N2 bleed becomes insufficient so N3 bleed is used, like at idle when the engines have just been started, N3 provides more than enough bleed air.

The sound change is due to this change over in bleed requirements, affecting the aircon packs.

Rolls use 3 spools vs 2 on the GE and Pratts this is probably the reason for the difference due to the differing designs and therefore difference in bleed off takes off the engines, it also depends on the speed that the thrust is reduced, rapid thrust changes cause rapid bleed changes.

Hope this helps.

[Edited 2006-09-11 18:04:33]
P1 in A330, A340, A346, B742, B744, B748.
 
Tristarsteve
Posts: 3365
Joined: Tue Nov 22, 2005 11:04 pm

RE: Change In A/C Sound On 767

Tue Sep 12, 2006 3:28 am

On the RB211 the airflow from the HP compressor is controlled by a valve which is called a HP valve or a High-stage valve. This valve is either open or closed. It does not modulate. So at TOD when you close the throttles the air pressur in the air duct decreases to a point where the pressure sensor decides that HP air is needed. The HP controller then opens the HP valve and you hear the whoosh of air as each HP valve opens. It is a good signal that it is time to go to the loo and get a beer in before landing!
 
Azza78
Topic Author
Posts: 4
Joined: Mon Sep 11, 2006 10:21 pm

RE: Change In A/C Sound On 767

Tue Sep 12, 2006 11:16 am

Thanks for the info...i guessed it would be something like you described. Just used to bug me everytime i'd hear it and not knowing what it was!

Quoting TristarSteve (Reply 2):
It is a good signal that it is time to go to the loo and get a beer in before landing!

Have you ever tried getting a beer on decent on QF? Unless your up the front you've got no chance!

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