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QAVION
28th Feb 2002, 13:05
Just a few "simple" questions about 767 engine trimming.... :)

1. Should both engines on a GE-powered 767 run at exactly the same N1 in (normal) flight when being controlled by the Thrust Management Computer? (i.e. down to the last decimal point)... or is there generally a split?

2. If there is an N1 split, are FADEC engines any closer (rpm wise) than non-FADEC? (or does EEC engine trimming produce the same N1 anyway?). By FADEC, I mean no steel cables running from the cockpit to the engines.

3. When trimming is applied to compensate for bleed air demands, what are the EEC's(?) looking at? Pressures sensed at the engine? Bleed duct pressures? Valve positions? Cockpit switch positions? All of the above?

3. If bleed requirements are assessed at the engine.... does the EEC on, say the left engine, trim only the left engine... or does the TMC look at the bleed requirements for both engines, average them and then set N1's on both engines to the same value. If the Bleed Isolation Valves are normally closed for flight, presumably this would mean that each engine would have a different bleed demand placed on it(?).

Thanks in advance.

Cheers.. .Q.

[ 28 February 2002: Message edited by: QAVION ]</p>

Frank M
2nd Mar 2002, 02:18
Hi Q,

I do fly the 767 with GE CF6-80B6F engines and during cruise there is no split between Left and Right engine. This means same N1, indeed down to the last decimal. I can't help you to the answers to the bleed demand trimming questions as our manuals aren't that comprehensive.

Frank.

QAVION
2nd Mar 2002, 10:41
Thanks, Frank <img src="smile.gif" border="0">

Also, I think I may have found what I'm looking for regards the engine bleeds, so I think we can safely put this one to bed.

Cheers.. .Q.

P.S. By the way, do the -F models suffer from the N1 split that earlier model 767 GE's sometimes suffer during the first half of descent?

Golden Rivet
2nd Mar 2002, 15:05
Only one EEC discretes card serves both engines via the TMC, so both engines receive the same bleed demand information.

Regards

Dan Winterland
2nd Mar 2002, 16:07
On the GE powered 744, the TMS senses the throttles are close (not sure of the parameters, but half an inch rings a bell) and assumes the power settings should be matched. Although this is of course for 4 throttles, so I don't know if its the same on the 767.

Also, you can bring up the bleed valve positions on the lower EICAS screen from the CMC menu on the centre CDU.

QAVION
6th Mar 2002, 09:06
"Only one EEC discretes card serves both engines via the TMC, so both engines receive the same bleed demand information.". .. .Just trying to make sense of this, GR... (I thought I understood what was going on, but...). .. .I think I'm getting different trimming concepts muddled here..... .. .*You have trimming which compensates for slight differences in the position of two or more throttle levers (i.e. the type mentioned by Dan and discussed at length in a previous message thread on 747-400's). This happens during non-A/T and A/T controlled thrust ops on FADEC engines.. .*You have idle speed adjustment when Anti-Ice is on or when the flaps are }/= 23 deg.. .*Then you have trimming (if this is the right word) for (other) pneumatic/bleed demands.. .. .It's the last one that is giving me the most problems. What exactly does it do to the engine? Does it boost the N1 of the engine to a higher level.... or does it simply allow the engine speed to get back to where it was before you slowed it down by sucking off lots of bleed air from the compressor? (or am I simplifying things too much here by only mentioning engine speed). Does it allow the engine to brace itself for the anticipated extra bleed demand (so that the rpms remain the same when the bleed demands are made)?. .. .Re the EEC Discretes card: I see from my gathered information that, on FADEC engines at least, the Discretes card sends ECS/bleed valve data to the EEC via the TMC. Conceivably, with today's technology, a summary of all the demands/bleed valve positions could be relayed to the EEC from the TMC on databusses and the EEC could easily determine which bits of data are important to it and have each engine respond accordingly/uniquely, rather than have both engines responding to an "average" bleed demand determined by the TMC. Of course, with things running normally on the aircraft, bleed demands will be pretty much the same for both engines (even with the Bleed Isolation Valves closed) and individual engine responses might be a bit of overkill. But in other, non-normal, situations....?. .. .On CF6 FADEC engines, there seems to be no direct output to the EEC from the Discretes card. Does this mean the engine cannot compensate for bleed demands without the TMC being operative?. .. .Anyway, my aplogies for the rambling.. .. .Regards.. .Q.

Golden Rivet
7th Mar 2002, 04:00
Think you need a beer !. .. .Regards

QAVION
8th Mar 2002, 06:01
"Think you need a beer !". .. .I do too, but that's not going to answer my question GR..... .. .To cut a long story short... If the N1 remains at the same value with bleed demands after trimming, what's the point of having a discretes card?.... I'm sure the EEC is quite capable of sensing any shortfall in engine speed as a result of bleed demands and boosting it back to the level required by the TMC or thrust lever. No?. .. .Thnx. .Q.

HPSOV
9th Mar 2002, 05:10
Heya QAV.. .As you know most of our 767's have the CF680C2B6, and the newest one has the FADEC "F" at the end.. .On the non-FADEC aircraft there will be a split in N1 values, and you must manually equalise the engines yourself, which will result in staggered thrust levers.. .On the FADEC aircraft, provided the thrust levers are within one "knobs" width of each other, the FADEC system will ensure both engines produce exactly the same N1.. .I am unsure if the FADEC aircraft suffer from the N1 split during descent, however I doubt they would. I am getting very tired of the Idle Disagree non-normal checklist.....

744PNF
9th Mar 2002, 17:02
Hi all,. .. .about a year ago I visited the flight deck during a 767 Air Canada flight from Munich to Toronto and asked the pilot why the No. 1 lever was a bit forward of the No. 2 one. He explained that this was due to one Recirculation Fan being inop, which kind of surprised me as a possible reason for staggering thrust levers. Judging from the appearance of the aircraft (flaking paint etc.) it was an older one that presumably didn't have FADEC engines. From my understanding, with one of the newer (FADEC-engined) 767s the handling of this situation would have been different from the one described above, right?