Airline Pilot Central Forums - Question: airplane handling at high altitude

Airline Pilot Central Forums (https://www.airlinepilotforums.com/)

- Technical (https://www.airlinepilotforums.com/technical/)

- - Question: airplane handling at high altitude (https://www.airlinepilotforums.com/technical/45636-question-airplane-handling-high-altitude.html)

Quote:

Originally Posted by UAL T38 Phlyer (Post 709579)

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.

The High Speed Buffet test was performed in-flight during a Post Maintenance test flight on B-727's coming out of a "D" Check. The purpose is to test the mach speed warning system all the way through the buffet.

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.:eek:

G'Day Mates:)

Quote:

Originally Posted by Jetjok (Post 709810)

.... Apparently McDonald Douglas feels the same way....JJ

McDonald's: makes OK hamburgers

McDonnell: made great fighters

(Douglas: made great transports, and the A-1 and A-4)

"You want to super-size your MEL? Or do you just want the in-flight happy-meal?"

:p

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.

Quote:

Originally Posted by Jetjok (Post 709810)

2. DO NOT conduct operations above 78° N Latitude.
3. DO NOT operate aircraft above 25,000 ft. MSL.
4. Fuel burn is increased by 2.7%.

If you would, a few questions from a fresh CPL holder:

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?

Quote:

Originally Posted by deltabound (Post 709573)

The "coffin corner".

Note the airspeed on the left. Redzone = overspeed. Yellow = stall.

Both = very, very bad at high altitude.

http://farm4.static.flickr.com/3616/...aee078b489.jpg

You'll note BOB is flying a .80 airplane at .77

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. :confused:

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.

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.

Awesome pic's and discussion here. Haven't had these since I was in college(since I don't fly big iron). Thanks for sharing the Concorde Pic's!!!!!

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.