Overheard on channel 9 ...

Date:         15 Apr 97 03:22:39 
From:         Andrew Goldfinger <Andy.Goldfinger@jhuapl.edu>
Organization: apl
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Recently, on a United flight, I was listening to channel 9 (cockpit
communications).  An airliner of unknown type was cleared to FL 410.  It
then requested that it be sent no higher that 390.  The controller asked
why: "don't you have the performance for 410?"  "Yes," came the reply,
"but not in the light chop that is reported there."

Could someone explain what was going on here?  Is FL 410 so close to the
service ceiling that a little chop (and presumably some gusts from the
rear) are problematic?