B747 cruise power settings
 

Has anyone ever heard of retarding thrust lever settings on 2 outboard engines whilst increasing the inboards in order to save fuel. Something to do with engine thrust specific fuel burn and the curve. Has anyone got any information on this?

empire4,
Not in cruise, but in low level holding, with two at idle and two at whatever was required to maintain the hold speed. The claim was about 8% reduction in holding ff at 1500'. I could never measure the difference when we tried it, but it did work on the P&W JT3D-3B powered B707-320.
In cruise, anywhere near optimum, there is not too much thrust to spare to even consider such a technique.

Certainly with 'basic' axial flow engines like the RR Avon it was reckoned the sfc was far better with RPMs in the 90's than the 80's (design factor?). Thus it was sometimes 'procedure' on the BAC Lightning to close down an engine when short of fuel (often:)) to push the other engine up into the 90's. I also heard 'a story' that the Nimrod shut down 2 on patrol loiter.

I can remember that 'procedure' on the Lightning often having unhappy consequences, and I am sure BOAC can too. The closed down engine would invariably windmill, and the engine-driven fuel pumps would try to pump the non-existent fuel that had now been shut off by the HP cock. The result was often a clattering loss of integrity of the fuel system, with large fuel leaks resulting when the HP cock was re-opened and the engine re-lit for the rendezvous with the tanker or the recovery to base.

Given the proximity of pooled leaked fuel and hot bits in the Lightning, the result was all too often a Martin-Baker let-down, a paddle in the North Sea and the welcome appearance of a bright yellow helicopter.

So, idle all you like, but shut-down I'd rather you didn't.

No-one never told me that, Guv!:)

I recall, from my early engineering days, fuel system guys passing on lessons related to dead heading fuel pumping systems. I think the key problem was thermal expansion of the fuel that became sealed between low pressure and high pressure shut-off valves; the heat generated by windmilling pumps may be added to by heat transfer from the engine lubrication oil system.
The threat has also been proven to exist in test cell fuel systems :hmm:

Thanks for the replies gents.

Empire4:

After 16-17 years on the Classics...no.

Hi, Talking about B747 & engine power settings etc. I am not a qualified aeronautical engineer and this question might look stupid, but Would a B747 by any chance have two different variants of the same engine -"RB211-524H2-T-" and "RB211-524G2-T-. I have come across an issue related to engines and Navigation database software loading and related queries.

Gothani, the permitted engine intermix is specified in the Type Certificate Data Sheet. See note 6 for permitted combinations.
TCDS A20WE Rev 58 Boeing Company, The

empire4

No (similar experience to Desert185, 15-16 years B747-100,-200 and-400).

Not stupid at all!

I think that all four engines on a particular airframe have to be either 'G' or 'H'. When I was on the -400 there was a sticker on the instrument panel which said:

'This aircraft is 'G' rated' ;or
'This aircraft is 'H' rated.

The pilots had to check that the FMC IDENT page reflected the same engine rating.

My understanding is that all -400 on that fleet are now 'G' rated, so the sticker has probably been removed.

The '-T' suffix relates to the modification of the core to 'RR Trent' standards. The intermix of -T with non-T on the same airframe was permitted. There would be a note in the tech-log with words to the effect of, "No 2 engine has been modified with a Trent core. All procedures and limitations are the same but pilots may notice lower EGT and Fuel Flow when compared to engines Nos 1, 3 and 4".

Never heard or the holding technique involving retarding symmetrical engines and advancing the others. It sounds perfectly plausible from a technical standpoint but was never SOP.

Back in the old days - pre FADEC - ratings intermix was fairly common. To change the thrust rating of the engine you literally needed to replace the fuel control. So intermix was allowed, typically combined with specific "part power trim" procedures to minimize the resultant throttle stagger (and in those days, the 747 had a flight engineer who would make small throttle adjustments to line of the thrust settings).
Since the incorporation of FADEC, changing the engine rating is simple - basically changing a plug on the FADEC and relabeling the engine data plate - an hour or so job. Plus, on the 747-400 the resultant throttle stagger with rating intermix is a significant work load impact with a two crew flight deck. As a result Boeing has never certified ratings intermix on the 747-400, or any other model with FADEC engines (this is not to say it's never happened - it has - but it was never an approved configuration).
As eckhard noted, the "-T" variant of the RB211-524 is a performance improvement (better fuel burn, lower EGT) but doesn't change the engine thrust rating. "-T" and non-T engines can be freely intermixed, however you need to have -T engines across the wing before you can program the FMC to take advantage of the improved fuel burn.