Coffin Corner

[Jenna2011]

Footage shows otherwise

Interesting you don't use this anonymous fellow's name. Yeager has had to deal with jealousy his whole career - from people trying to take him down a notch.

However, there is much footage re this flight. And it shows that the thrusters failed and your alleged friend's alleged analysis is incorrect.

[BOAC]

Jenna - welcome to the thread, if a little late. It is nearly 4 years since the thread 'died'. Can you substantiate that? Any links to the data or footage? Bob's article seems pretty conclusive.

I assume you refer to gr8's post, and I wonder if gr8 had read Brian's link to Bob's 'blog'?

[John Farley]

May I suggest that sixty years ago to finish up flying an aeroplane that would stall if you lost two knots and would go out of control due to Mach number if you gained two, was a trickier business than with a more modern design today?

Aircraft behaviour with high mach loss of control has become much less severe in later years.

To go back to the original question the answer has to be to lower the nose a fraction and then once the knots go up by one ease the power back. Just flying really.

[Wizofoz]

I also noted, in this necro-thread, that what SN3GUPPY said was-err-wrong.

Max ceiling of an aircraft can be EITHER minimum residual rate of climb (and what that value is can vary between customer and regulator) OR minimum buffet boundary.

We fly both GE and RR powered 777s. The RR versions have no buffet boundary problems right up to max altitude, as they run out of thrust before it becomes an issue.

The GE aircraft are exactly the opposite. They have no problem maintaining nearly 1000fpm up to there ceiling- which is defined by a minimum buffer between high and low speed buffet.

[cwatters]

Put the wheels and nose down? Cross fingers the wheels are still there later.

[henra]

Originally Posted by airfoilmod

If one reduces power, infinitessimaly as you will, you will infinitessimaly Stall. I don't disagree with anything you write, but it ignore's the definition of the concept, sensational or not.

But what if you synchronously, infinitesimally lower the nose and reduce power in a synchronised manner at the same time ?
Thereby reducing pitch and altitude while exactly maintaining speed (IAS) and solwly but surely reducing Mach number ?
Admittedly a purely theoretical excercise.

Hint: The way you got there must be the way you get out of there. Any combination of speed, Mach and altitude you had while climbing should be equally feasible when descending (AoA would be even ever so slightly less upon descent).
But in theory I do not see why it should not be possible.

regards,
henra

[Sillypeoples]

I'd love to fly a plane where CC could be a 'problem' for the obvious reasons that I don't think are obvious to many of the posters on this forum, and hence why most of us are stuck, by the FAA, flying big fat wings, with lower service ceilings, burning lot's of fuel and taking all day long to get to our destination.

[rudderrudderrat]

wing load

Hi henra,

Bob Smith (1928 - 2010)
"...maintaining 70-degrees to the null angle of attack."

I think the technique of using reduced wing loading (with the thrust vector at 70 degs) used to get them beyond "coffin corner". Once the wing tried to support the full weight of the aircraft, they were in deep trouble.

[sevenstrokeroll]

SO, THEY CALL IT COFFIN CORNER FOR A REASON

so, like windshear...avoid, avoid AVOID.

and most planes now, operated in a conventional manner probably won't get too darn close to the coffin corner

but I always worry about those guys climbing to avoid turbulence and then getting a big surprise when they really hit the oscillating ventilator.

thick air may be worse for fuel consumption but it is usually a little bit safer.

[henra]

Originally Posted by rudderrudderrat

Hi henra,

Bob Smith (1928 - 2010)
"...maintaining 70-degrees to the null angle of attack."

I think the technique of using reduced wing loading (with the thrust vector at 70 degs) used to get them beyond "coffin corner". Once the wing tried to support the full weight of the aircraft, they were in deep trouble.

OK, that's a tricky case where you might indeed achieve an even theoretically unrecoverable coffin corner.
But it depends if the plane when completely chopping power and keeping the same lowered wing load will be accelerating before you can start re-loading the wing. Depends on excess thrust on the way up and drag on the way down. (High SET and low drag would be the ticket to the ultimate -and quite surely fatal- coffin corner experience with this approach)

[cosmo kramer]

Very theoretical, but I would suggest that initiating a descent, the nose-over would cause a reduction in required lift (curved flight path) and AoA would therefore be reduced, increasing the margin to stall (effectively temporary increasing the altitude of coffins corner).

Immediately thereafter, once established in the descend (straight downwards flight path, lift and weight again balanced out, with the original AoA), the aircraft would be out of coffins corner and stall speed would start to reduce.

Hence, it would in theory only be dangerous to maintain the altitude and either increase or decrease speed. I don't see any reason why flying up to coffins corner with a perfectly accurate speed, level off and curving/descending back out would not be possible. ...in an ideal theoretical world.

The "coffin" in coffins corner probably more likely comes from someone trying to either establish it or break it during test flights in the '50'ies where no computer models where available to make accurate predictions.

[FullWings]

I've never been there but I'd assume if you just took the power off (gently) and let the aeroplane find its own way down for a bit, it'd work out in the end. Stalling or running into mach buffet doesn't automatically kill you - after all, you've got plenty of height to recover with and as you descend into denser air, things will return towards normality. AF447 must have been close to coffin corner at the apogee of their climb but I think most agree that the aircraft was recoverable even after it had been comprehensively stalled.

Many years ago we had an incident where one of our crews used the ZFW for performance calculations instead of the TOW. Unfortunately, it was a LH tankering sector, so the computed speeds ended up c.23kts too slow. After an uneventful takeoff and climb out (1kt above the stick shake at times from the FDR), they looked at the FMC OPT/MAX ALT and attempted a climb up to 370/390. Somewhere in the low-to-mid thirties, after some time trying, the airframe just wouldn't go any higher and was wobbling around a bit. This was perplexing but they went down to a lower level and landed at destination about 6-7hrs later at Vref-18, put "poor performance in cruise" in the tech. log and went to the hotel. Thank goodness for modern wing sections and FBW...

[rudderrudderrat]

Hi FullWings,

landed at destination about 6-7hrs later at Vref-18,

Did they ignore all other clues like flying below VLS, and "Check gross weight" messages?

[Shaka Zulu]

Airbus isn't the only manufacturer that makes FBW aircraft. On earlier FMC versions of the 777 the t/o weight box in the perf init page wasn't blanked unless a zfw was entered. The newer ones do not allow you to put the zfw in the tow box.

[FullWings]

Did they ignore all other clues like flying below VLS, and "Check gross weight" messages?

As Shaka Zulu has surmised, they weren't flying an Airbus, so didn't get those messages. The aeroplane *was* providing plenty of other clues but...

When I said "performance", I meant in-flight performance as in the old ZFW/GW mixup. Luckily the thrust reduction was done on gross weight otherwise you'd have read about it in the papers.

[kinteafrokunta]

FullWings...are you sure this really happened? Are you pulling our legs? This could never have happened; the pilots were so meticulously picked and trained to the most superior standards in the world. Are you talking about an outfit in darkest Africa or the slippery slopes of Asia?

[thermostat]

Redout, to answer you question simply, Think of an inverted "V" or an "A" without the horizontal line. You will notice that the two legs are far apart at the bottom and converge toward the top. The bottom part is sea level and the top (where the lines meet) is your high altitude limit. So at sea level there is a wide space between max speed and stall speed. However as the aircraft climbs, the two lines get narrower which means the difference between max and min speeds is getting less. Eventually you get to an altitude where if you increase or decrease your speed the aircraft can no longer maintain lift. It will either stall or fall. So, how to get out of that situation? Know the performance of the 'plane, know the max safe altitude for a given weight and don't go there. I think you answered the question yourself, you must descend to a lower altitude to increase the margin of safety. Unfortunately turbulence at altitude can cause speed fluctuations which can cause an upset to occur.
Hope that helps.
T

[Natstrackalpha]

Dead meat? . . and yet . . ?

Maybe there is a way - can I suggest, 1/. Reduce power but do not hold att.
2/. aircraft starts to descend following reduction in power - ad nauseam.

Voila! There we have a descending aircraft into air which is becoming increasingly more dense with reduction in altitude. Balance the ROD with the reduction in power. So that the lift over the wings . . . . well, you know what I mean.

. . . .Alas - too late !
===================

But what if you synchronously, infinitesimally lower the nose and reduce power in a synchronised manner at the same time ?
Thereby reducing pitch and altitude while exactly maintaining speed (IAS) and solwly but surely reducing Mach number ?
Admittedly a purely theoretical excercise.