Windshift causes altitude loss enroute

[punkalouver]

An Eva Airways Corp. Boeing 747-400, was operating as EVA651 in the vicinity of Whitehorse, YT at FL430 when they encountered windshear. EVA651 required a descent of 500 feet in order to regain airspeed. At approximately 80 nm west EVA651 encountered a second windshear event resulting in a loss of 1200 feet due to a wind change of about 60 knots over a distance of 5 nm.


Would an aircraft like this have autothrust to compensate for a loss like this over 5 miles or is it too much of a change in wind?

[Pilot Pete]

When you are in the upper reaches of the envelope you don't have much to play with. Autothrust (which they would have had) or no autothrust makes little difference, you need to avoid a stall, which it sounds like this crew did.:rolleyes:

PP

[barit1]

These guys (Pinnacle RJ (http://www.ntsb.gov/ntsb/brief.asp?ev_id=20041015X01633&key=1)) put themselves in the coffin corner and weren't astute enough to stay out of trouble.

The EVA crew found themselves in nearly the same circumstance, but did the right (safe) thing.

[ahramin]

Yes. Sounds like a good job.

Not as dramatic as pinnacle but Air Canada Jazz had an RJ event going into Calgary which did not go so smoothly either.

http://www.tsb.gc.ca/en/reports/air/2005/a05w0109/a05w0109.pdf

[FullWings]

Would an aircraft like this have autothrust to compensate for a loss like this over 5 miles or is it too much of a change in wind?
At over 8 miles a minute in still air, that's only 30-40 seconds. At the altitude they were operating at, there probably wasn't that much excess thrust available anyway; certainly not enough to counter a c.2kt/sec airspeed loss.

Another factor is that you soon end up "behind the drag curve" which makes it even more difficult (maybe impossible) to accelerate. It may reach a point where you're going to go down, whatever, so it's better to start a controlled descent than have the airframe decide to do it!

[Old Smokey]

I don't know the full details of the Eva "event", but Fullwings said it all with respect to flight behind the drag curve.

In a previous life during test flying, the exercise was to take the aircraft as high as possible, i.e. to "nudge" coffin corner to examine aircraft charasteristics.

At that altitude, minimum drag speed was defined by Mach Number (Mmd). It was found that, at the altitude being examined, MCT JUST held M0.67 (Below Mmd) in level flight. We did encounter quite mild wind shear, and the Mach Number crept inexorably downwards, with full Takeoff thrust unable to arrest the deceleration. The only solution was to PUSH quite assertively with about a 1200 ft altitude loss.

Flight near the Maximum Operating Altitude is for nerds. Other "fun" things I've done in testing like deliberately trying to deep stall T Tail aircraft seem like kiddy stuff compared to fiddling with Coffin Corner. Glad I don't do it any more:ok:

Regards,

Old Smokey

[misd-agin]

Had to do that, once, to maintain minimum speed for that altitude, in a mountain wave at FL430 crossing the Rockies SW of KDEN.

Could the a/c have flown slower without stalling? Yes. But minimum speeds are there for a reason.

In a 'block altitude', we descended 700' at max cont. thrust at the minimum airspeed, bottomed out, and climbed right back up to FL 430.