Re: Boeing 777 Wing

From:         shevell@leland.stanford.edu (Richard Shevell)
Organization: Stanford University, Dept. of Aero/Astro
Date:         20 Nov 94 01:59:02 
References:   1 2
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In article <airliners.1994.1686@ohare.Chicago.COM>, kls@ohare.Chicago.COM
(Karl Swartz) wrote:

> From what I've heard, the 777 has met or exceeded its fuel consumtion
> and performance goals from day one, in contrast to the well-publicized
> problems with the MD-11.  I don't really know how the A340 did, but
> shortfalls are fairly common.

To follow up and clarify these remarks, there are two levels of performance
values.  The first is the 'nominal' which is the expected or best guess
level.  This is normally the median of performance level possibilities. 
The second level is the guarantee level, usually 5% lower for range
estimates. To miss the latter often involves financial penalties stipulated
in the contract with the buyer.  In the case of the MD-11 the drag was
essentially on nominal but both the Pratt & Whitney and the GE engines were
high in specific fuel consumption by about 5 to 6% compared to nominal. 
Douglas has found ways to reduce the drag and the engine manufacturers have
recovered part of their deficiencies. The total improvement makes the MD-11
right on its original nominal or 5% better than the guaranteed.  The 777
according to Aviation Week of October 10,1994 is right on the nominal which
is 5% better than the guarantee.  Thus both aircraft meet their nominal
predictions.  None of this is a real comparison.  To obtain that one would
need to have the actual specific range (miles flown per pound of fuel)
figures at corresponding flight weights and then correct for the number of
seats in each airplane.  Since the 777 comes later and may have more
efficient engines, one would expect it to have some advantage.  

-- 
Richard Shevell
Email: shevell.leland.stanford.edu