Question: airplane handling at high altitude
 

In a thread somewhere, there was a comment stating pilots flying large/heavy jets at high altitudes should hand-fly some to get the feel of how the aircraft handles. While I have fair experience flying a straight wing in the low flight levels, I am wondering how a swept wing handles in rarefied air. Is it akin to flying a small airplane near its service ceiling (semi-slow flight, sluggish to control inputs, unforgiving of uncoordinated flight)?

Thanks
J

I Have hand flown both the DC-10 and the 737-700 and 800 at and above Fl 400. They both fly fine and will fly a 30 degree bank without issue, however we typically will limit the bank to around 20 degrees and most of the turns or altitude changes required at those altitudes are gradual. The yaw damper will keep the aircraft coordinated in flight.
Some older aircraft like the B 707 had more of a dutch roll tendency during flight due to the wing sweep and I believe not a very good yaw damper. The big difference at altitude is the stall buffet margin, which means the air speed operating envelope is quite narrow as compared to low altitude, the envelope might be 20 kts between MMO- max speed and the stall margin and the aircraft has weight limits it must be under before it can climb to the higher altitudes.

When I was a new co-pilot on the KC-10 (DC-10), part of our training was high altitude handling and we had to fly a portion of a flight at cruise without the use of the AP. During UPT, I also hand flew the T-1 (Beech 400A) without the AP.

In my experience, and anyone can correct me if I say anything out to lunch, both airplanes became pitch sensitive. In the DC-10, you literally had to fingertip fly the yoke. I had to fly a KC-10 for 2 weeks without an AP. At FL420, one quick click on the trim switches caused enough pitch change that it would measure on the VVI and you could feel the nose pitch up and down ... almost like light chop turbulence.

As you get higher, your turn radius increases significantly. Also, at a given IAS speed, you could probably bank up to 45 degrees without any buffet, but in the higher altitudes, the buffet may start at 15 degrees. I have seen students who were too aggressive in a turn at high altitude, and their faster than normal rate of bank caused us to get some buffet.

In the T-1 especially, dutch roll was much more pronounced at the higher altitudes. In fact, I think the IPs would demo turning off the yaw dampers to show this effect.

Another consideration is performance. It is very easy to get yourself on the back side of the power curve, and thus, your speed may decrease as you try to hold an altitude. In my current airplane, we call these HALAs (High Altitude Loss of Airspeed). If you get on the wrong side of the power curve, a descent of 1000 FPM may not be enough to get your speed back (MD-88 HALA simulator demo). If you allow this scenario to happen, and ATC can't give you a clearance promptly, they aren't going to like you very much when you come screaming down through the altitudes looking for airspeed. Speed management becomes very important. Reference Pinnacle Airlines Flight 3701.

I hope this answers your question.

J-
I don't think hand flying a heavy jet at the top of the envelope is very useful. Its not like you can do any aggressive maneuvering. The whole objective would be to stay smooth and not induce any wing loads whatsoever. If you needed to turn, you'd do it as gently as possible.
My "heavy" experience is with the 767 series. 200,300, and 400s. Its a very forgiving airplane. Boeing gave it a robust wing. Having said that, you can still get that "balanced on the head of a pin" feeling when you are up high and you get some turbulence or mountain wave. Its not really something you "feel" through the yoke, though. Its more from watching the airspeed and power settings and kind of a "seat of the pants" thing.
The 767-400 avionics has very good high speed and low speed buffet limits displayed on the airspeed tape. When those "hooks" and "zippers" start to close in on you from above and below, it definitely makes you uncomfortable.

I've had a couple flights over the years where we told ATC "we need to descend. NOW!" because the mountain wave was pushing the envelope beyond my comfort limit. I don't know anybody who has actually had a loss of control. It has happened though. An American A-300 had one a couple years ago if I recall. Lost a lot of altitude and some empennage parts before they got it back.

Now the MD-88 -- that's another story. They are pigs at the top of the envelope. The wing on that airplane is designed for a DC-9 and its barely adequate at 3700 ft let alone 37,000. :)

Boeing builds airplanes. McDonnell builds character.

upper 30's it is all about airspeed.

It is called "coffin corner" for a reason...

.........................................

The others have summed it up fairly well. The highest I've ever flown and airplane (CRJ series) by hand is FL270, due to the RVSM restriction. I've had the A/P disconnect on me at FL350-380 a few times, and I've left it off for a minute to retrim the airplane. It is just like someone else described as balancing on a pin, and I'll completely agree with KC10FAT about fingertip flying the airplane. It requires a constant nudge here, a nudge there. The CRJ trims itself out very well, but if the flight attendant is in the back with a cart, and she rolls it all the way to the front, I can feel it, and I will have to retrim.

At 31,600 the flight director switches to Half Bank mode, and all turns are made at half what we normally bank at. Airspeed, of course as others have mentioned, is crucial.

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

Nice!
 

Bellerophon:

Very Nice!

(For those who didn't see it before it got removed for format error, it was a Concorde cockpit shot at FL600 and Mach 2.00)

Josh:

In any jet airplane, flying near the top of its envelope, the things you notice are:

1. Low Thrust Available. This means pitch changes have to be small, smooth, and controlled, or you starting bleeding/gaining airspeed easily (more losing than gaining).

2. Change in Pitch-Feel. In any aircraft, as you approach transonic speeds (.80 and up), there is a small but noticable shift of the center of pressure aft. In any aircraft that is supersonic, the center of pressure will be at 50% mean chord.

This means the tail moment-arm becomes shorter as you fly faster, and it may take more control deflection to achieve the same result...which means it "feels" different.

I can't touch Bellerophon's Mach 2.00 at FL600, but I've done 1.8 at FL520 in the F-4... I was stirring the stick like I was baking a cake, and I felt like I was standing on a basketball...it was pretty unstable, even with 500 knots indicated.

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.

In a turn (g-load on the aircraft), angle of attack is increased, so the airflow over the top of the wing accelerates even more...which means the high-speed buffet can happen at lower indicated Mach-numbers, regardless of Indicated. This is why, as KC10 said, bank has to be limited at high-altitude.

4. Turn Radius, as KC10 mentioned. Air Force formula is "mach minus 2." Example-- 0.80 Mach: 8-2=6-mile turn radius at 30 bank.