Question: airplane handling at high altitude
11-10-2009 | 01:47 PM
#21
Snakes & Nape
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From: B-767 Captain
Wild Ride
3. High-Speed Mach Buffet. When you approach the critical Mach of a wing, the airflow going over the top of the wing will speed-up to the point it is supersonic. This typically happens in a rapid on/off/on/off fashion, and is manifested as a rumble or buffet. It occurs because the temporarily supersonic air separates from the wing, then re-attaches...just like a conventional angle-of-attack stall.
After going through the Boeing school and having a Boeing test pilot riding along as a "reference", I've done this "test" under controlled circumstances starting in the low 30,000'. Believe me, when the high speed buffet begins, there is no doubt what is happening and one would not want to get into a high speed buffet near the coffin corner..... especially in a three-holer !
Ask "Hoot" Gibson for those who remember him and their adventures in a 727.
G'Day Mates
11-10-2009 | 02:10 PM
#22
Moderate Moderator
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From: Curator at Static Display
11-10-2009 | 02:55 PM
#23
Thx Age 65
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From: MD11CAP
The MD-11 is a weird Beast. Not having flown a Boeing product designed after 1980 I don't really have a comparison, but the integration of the FMS and the auto flight is so total that I think it leads to that MEL language.
11-10-2009 | 02:59 PM
#24
Gets Weekends Off
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From: the correct seat
Why wouldn't you be able to hand-fly above 78N and FL250?
Would the fuel burn increase just because the AP is smoother with corrections? If so, wouldn't the fuel burn depend more on pilot technique?
11-10-2009 | 05:33 PM
#25
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From: A320 CA
11-10-2009 | 08:44 PM
#26
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From: 767A (Ret)
Any U-2 guys
I heard a presentation by one of the early U-2 test pilots who talked about their margin between high-speed and low-speed buffet. He said it got narrower as you climbed, to as little as 5 knots ("the throat"), and then widened as you climbed further. This was many years ago, and later U-2s with a different wing may have been less critical. 
11-10-2009 | 09:07 PM
#27
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From: Curator at Static Display
Tropopause
Tom:
It widened because they flew above the tropopause, and OAT went up, thereby increasing the speed of sound...which made the Mach number go down for any given indicated speed. Pretty sure MMO is the limiting high-speed end of the throat.
Didn't think about that it could happen until you mentioned it.
That's pretty dang high.
It widened because they flew above the tropopause, and OAT went up, thereby increasing the speed of sound...which made the Mach number go down for any given indicated speed. Pretty sure MMO is the limiting high-speed end of the throat.
Didn't think about that it could happen until you mentioned it.
That's pretty dang high.
11-10-2009 | 09:59 PM
#28
Gets Weekends Off
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From: 767A (Ret)
Powered glider
Thanks Phlyer,
He also showed a cockpit photo, and I recall that the IAS was less than 100 knots. Don't know exactly what phase of flight that was -- maybe climb.
He also showed a cockpit photo, and I recall that the IAS was less than 100 knots. Don't know exactly what phase of flight that was -- maybe climb.
Last edited by tomgoodman; 11-11-2009 at 06:25 AM.
11-11-2009 | 12:37 PM
#30
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I routinely practice hand flying the 737 at altitude with little difficulty.
Flying the 727, it's favorite altitude seemed to be FL310. Above that, it required constant pitch trim. At FL370, it become more of an effort.
I wondered if the movement of the cart and passengers up and down the aisle played a significant role in having to constantly adjust the trim.
Flying the 727, it's favorite altitude seemed to be FL310. Above that, it required constant pitch trim. At FL370, it become more of an effort.
I wondered if the movement of the cart and passengers up and down the aisle played a significant role in having to constantly adjust the trim.
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