Throttle-up. Perceived discrepancy in FOB
Quote from autoflight:
Is anyone claiming that the crew opened the thrust levers to max and kept them there?
[Unquote]
Quote from AAIB Bulletin S1-2008 (Feb18):
The engines failed to respond to further demands for increased thrust from both auto-throttles, and subsequent movement of the thrust levers fully forward by the flight crew.
[Unquote]
Does that go any way towards answering your question?
CONF iture, Thanks for reminding me about the "250 kts 12D before LAM". I also think they are unlikely to have entered the hold at much above 230 kts, decelerating to their ideal holding IAS. I'm guessing this might have been about 210 - 220 kts, as they were light.
So I think it is likely that a significant amount of thrust would have been used in the hold (but see my previous post, above) to average only 400 ft/min ROD. By the way, a typical time taken from exit of the LAM hold to touchdown is 10 - 15 mins, depending on traffic. Even taking the quickest case, they are still likely to have used thrust (above idle) again while flying at a steady 160 kts, normally required by ATC for the segment between (roughly) 7 miles and 4 miles from the runway.
But, in the absence of the FDR/QAR data, this is all speculation...
Quote from CONF iture (discussing/quoting AAIB Bulletin):
“the total fuel on board was indicated 10500kg” and even if “both of the eng spar valves were found to be open, allowing the fuel leak evident at the accident site” they avoided to mention how much fuel they drained from the main tanks.
[Unquote]
You seem to be suggesting that the AAIB's economy in words may have been designed to cover some doubt they had about the indicated fuel quantity (FOB) on landing. I think it is likely that the indicated FOB the AAIB stated would have represented the lower of the following 2 figures:
1) total FOB in real time as measured by the FQI sysyem;
2) departure FOB (as entered into the FMS by the crew at Beijing) minus fuel used (as measured by the engine flowmeters); known as the CALCULATED FOB.
Method (2) does not, of course, take into account any leaks; nor (probably) any APU fuel consumption. The latter is not normally used after engine start; the former self-evident post-flight.
The stated FOB on landing being 3600 kgs higher than the F/P destination fuel could be accounted for by a number of (presently unknown) factors in combination. A ZFW lower than planned (as you are thinking?) would have saved 300-400 kgs of fuel per tonne on a 10-hour flight. Again, as you imply, we do not know how much flight time was saved (if any, due to better wind-components and/or direct routings). Contingency fuel is - for the purpose of calculating F/P FOB at destination - assumed NOT to be burned, so is probably irrelevant.
Finally, you will have noticed that TOD was at FL400, which is probably higher than planned, saving fuel for the last part of the cruise with their light payload. Also, the F/P included a temporary descent from FL341 to FL318 early on, which would have been costly in fuel; they avoided this at the request of ATC. The latter is the only known source of fuel saving for us at this stage.
In summary, there is already much known scope for fuel saving, and the unknowns may have even more to offer. I see no reason to infer (as I assume you are) that the AAIB may harbour doubts about the validity of the indicated FOB they published.
Last edited by Chris Scott; 28th Feb 2008 at 01:29.
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