Re: Overheard on channel 9 ...

Date:         15 Apr 97 12:56:41 
From:         "D Snow" <>
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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?

Depending on the a/c type.  If he was over his 1.3G or 1.5G Buffet
margin weight for FL410, then yes.  If any turbulence had been
reported, then the crew must take that into account as well.  Since,
in your example, light chop was reported, then the crew MUST at
least be under their Max Cruise Thrust Limited Weight for a 1.3G
buffet, or even 1.5G if the captain felt it was necessary.

Some aircraft in general have such a small margin between low-speed
stall and the high speed mach buffet margin (the coffin corner).  A
B727 around 125,000 lbs at FL390 only has less than 20 kts between lo-
speed and hi-speed buffet, at 1.3G buffet.

Douglas Snow
Tech Support - Cyberlink Inc.