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

From:         piersdorff.michael@ic.gc.ca
Organization: The Communications Research Centre
Date:         10 Feb 96 12:30:51 
References:   1 2
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kls@ohare.Chicago.COM (Karl Swartz) wrote:

>I read this and scoffed, as I did when they talked about instruments
>on the 777 being unreadable due to engine vibrations.  Sounds pretty
>absurd, right?  Just shut the engine down!

>Then, nearly at the end of the article, they mentioned windmilling
>engines and it dawned on me that just shutting the engine down isn't
>good enough, since it will still windmill and thus still produce a
>pretty decent vibration if it's massively out of balance.

One of the major issues in trying to read instruments in a vibrating
environment has to do with the elasticity of the human spine.  I
recall reading somewhere that the spine has a natural frequency at
about three cycles per second.  At this point, the head would be
bouncing up and down so violently that focussing on anything would be
impossible.  So even a glass cockpit, which doesn't suffer the
problems of vibrating needles making them unreliable indicators, can
become useless in certain circumstances.

The engine certification tests must show containment in the engine
casing of rotor blade and disk fragments in the event of worst-case
failure, and the recent PBS series on the B777 clearly showed some of
these tests.  The effect on the airframe and its inhabitants of
continued vibration from a windmilling engine with a blade failure may
be something else again.