Re: 777 engine imbalance (Business Week, 1/29)

From:         Chuanga@cris.com (H Andrew Chuang)
Organization: Concentric Internet Services
Date:         10 Feb 96 12:30:51 
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In article <airliners.1996.177@ohare.Chicago.COM>,
Karl Swartz <kls@ohare.Chicago.COM> wrote:
>
>It still seems like they ought to be able to simulate the situation
>reasonable well, since they have vibration data from test-stand runs
>of the engines with intentionally failed blades.  This information
>could be used to appropriately shake an airframe on the group, or so
>it would seem.

Last summer, the GE90 failed a blade-out and bird-strike tests resulted
in the delay of the certification of the B777/GE90 combination.  British
Airways had to wait an extra two months to get its first B777 (but
received a hefty sum of penalty payment from Boeing and GE).  According
to AvLeak, in one of the failed tests (I think it was the bird-strike
test), due to the impact, some platforms (spacers between the fan blades)
cut into trailing fan blades.  Two or three fan blades were severed and
resulted in unacceptable engine imbalance.  GE later fixed the problem
by redesigning the platform and passed the tests late last summer.

I don't know the exact FAA rules.  However, from the GE incidence, I
would think that engine imbalance is a critical criteron in the blade-out
and bird-strike tests.  Thus, I don't quite follow why Business Week made
an issue out of this.  Since these tests require the engine to maintain
certain level of thrust, the vibration that one would experience in
windmilling cannot be any worse than that at 80 or 90% speed.

--
  H Andrew Chuang (chuanga@cris.com)