Coffin Corner

[redout]

How is the correct way to escape from Coffin corner. Is it by Descending to a "safer" altitude while carefully controlling airspeed ?

[BOAC]

Well, we don't recommend climbing.

[Ayla]

Just don't go there in the first place. Know your buffet margins. If you are foolish enough to end up in that predicament your solution would work.

[BOAC]

No it won't! The definition of CC is that ANY increase or decrease in speed will result in loss of control, so basically you are Gordon Ramsey'd as we say on UK TV.

Sentence 1 rules.

[rubik101]

Surely you mean Gordon Ramsey'd. Jamie is just a cockney lad who uses slang, but he doesn't swear on TV!

Lots more power might help.

[BOAC]

Whoops! Yes I do.:O Thanks. Edited. I actually watch neither, so please may I be excused?

[redout]

BOAC what is the ans ? How does one escape without killing themselves.

[gr8shandini]

Actually, the term "coffin corner" refers to a very small window between mach buffet and stall, not loss of control. And it is possible to maintain a given airspeed while descending, is it not? This is a very high altitude phenomenon, so you'd likely be at a pretty high power setting and all you have to do is reduce throttle. In fact not many aircraft have enough motor to get themselves into this situation to begin with.

[airfoilmod]

Reduce power and.... Stall. No increase left, at max altitude, can't climb, At Vne can't descend, you going to do uh What?? Including the word Coffin is not a mistake.
rubik: more power than wing has killed many an inattentive pilot.

Airfoil

[fireflybob]

BOAC is, as ever, correct!

Why would you want to go there? You don't save fuel by climbing early above optimum.

[BOAC]

I believe the actual 'coffin corner' was a one-way street, that real control loss was supposed to happen there with ANY change in Mach or IAS. It would require consumate skill to 'lower the nose' and fly out of it without increasing Mach no or reducing IAS. I suspect it was more a notional 'media' thing rather than a real one, but I'm sure someone will know better.

Ironically, for 'rubik', "Lots more power might help." - it was the converse. Too little power stopped people from getting up there as Lear pilots have discovered the hard way.

[rcl7700]

Sounds hard to reach such a critical Coffin Corner situation where ANY changes involve a Stall or Overspeed? Seems very hard to even approach this situation without stalling or overspeeding beforehand. You will always have some margin which is the way you would get out.

If you somehow get to such a critical situation I think anybody would rather have an overspeed in the recovery process than a shaker.


rcl

[airfoilmod]

Coffin Corner "sounds hard to reach". Only if you can't imagine flying well past it. It isn't a destination, it's no man's land. It's real, have faith and believe.

Airfoil

[FE Hoppy]

In a modern Airliner you can't get near "coffin corner" without busting MMO so we should coin a new term for the limiting alt where MMO and AoA stall coincide.

Any suggestions?

How about Certifications Wall?

[airfoilmod]

Hard "Hat"

[BOAC]

redout - if you look at the bottom of the page you will see several threads about CC. Our friend John Farley posts (at #14) in one of them a good description of the situation.

Looks like your original question is best asked of a U2 pilot.

[Kiwiguy]

Reduce power and.... Stall. No increase left, at max altitude, can't climb, At Vne can't descend, you going to do uh What?? Including the word Coffin is not a mistake. rubik: more power than wing has killed many an inattentive pilot.


Airfoil you can reduce power and use attitude to maintain airspeed. Airspeed is the critical factor.

Just don't apply for a job as a carrier pilot if you don't understand that sink can be controlled EITHER by attitude or power. You add new lustre to the term plain driver ... sorry I meant plane driver. :8

[Brian Abraham]

Believe the U2 had a spread of about 10 knots.

[BOAC]

That's a big chunk of Mach, Brian - you sure? I assume that was at 'normal' operating altitude rather than any 'higher'?

[Dont Hang Up]

To recover from CC.Tumble in an uncontrolled manner out of the sky until you hit thicker air. Ultimately you will recognise the uncontrolled state has now entered a recognisable aerobatic state (spin or inverted spin). Apply standard recovey technique. Simple.

[airfoilmod]

reducing power and increasing pitch won't "fly". The increase in AoA is what causes the Stall, obvious. CC, "perceptually" sounds like a perfect balance of all vectors. Contrarily, it is a very dynamic configuration. No "Trades" left. Reducing power, Stall; +AoA, Stall; Fart, Stall.

Aviators don't "land" on the boat.

If the definition of Coffin Corner is "anything you do results in departing aerodynamic flight", then the thing you propose will cause departure. Defining terms is the first step in a discussion? Unless, like ssg you insist on ignoring the terms and proposing situations that don't meet the standards of a civil discussion. What if? Let's say, consider this. A fool's argument. Starting a talk without understanding the proposal is like launching with the control lock fixed and the pitot covered.

Airfoil

[Lemurian]

In a modern Airliner you can't get near "coffin corner" without busting MMO so we should coin a new term for the limiting alt where MMO and AoA stall coincide.
No, normally it is reached at a much lower Mach than Mmo.
Actually, the term "coffin corner" refers to a very small window between mach buffet and stall, not loss of control.
That is the accepted concept nowadays, in the FMS era.
Airfoil you can reduce power and use attitude to maintain airspeed. Airspeed is the critical factor.
Very much so and anyone who's used a HUD can testify to that principle.

Coffin corner in fact is a sensationalist description of approaching the *maximum reachable ceiling* at a given aircraft weight.
The Turcat formula shows that there is a point at which Cl max will be reached and will prevent a further climb whatever the available thrust as one would hit at the same time high and low stall.
It is to be remarked that this ceiling would be attained at only one Mach number.
Now, an airliner is normally operated at altitudes where there is a g factor protection ( as g increases the "apparent" weight ) and one would find those graphs in any QRH. Those protected graphs also exist ,in an easily exploitable form, in the FMS computations of MAX ALT.
So, in real airliner pilot's life, one would be flying with a reduced spread of Mach numbers and that spread will increase at the same altitude as we burn fuel and the airplane gets lighter and lighter. For instrance on a 60T A-320, that altitude (Mach and stall buffet) is 39,000 ft / Mach .78.
At 37,000 ft,flight is possible between .73 and .82 Mach (Mmo)
At 35,000 ft, it is between .70 and .82 Mach.....etc... etc....
From this, the inference is that the best *escape* will be to slowly reduce power (it's almost infinitesimal in the beginning) and start losing altitude at the Mach number one was at. And, yes, maintain Mach with the pitch control (what we do ususally in FL CH or OPEN DES, btw), and very soon, the margins will increase on both "sides" of that Mach.

[airfoilmod]

Wonderful explanation. Coffin Corner is sensationalized. I think you describe how to avoid it perfectly. My point is that, by definition, once you meet its parameters, no escape is possible. 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. Neither argument is wrong. You are certainly a very qualified Captain, you have my complete respect. This is a forum, not a cockpit.

Respectfully, Airfoil

[gr8shandini]

Airfoil,

You're normally on top of the aerodynamics questions, but I'm gonna have to disagree with you on this one. Take the term "coffin corner" out of the picture and think about maintaining an airspeed while reducing power. You do this every time you begin a descent for landing. Pull the power back, nose over slightly and you can maintain any speed you want right out to Vne. Or 1.05 Vstall for that matter. Just control airspeed with pitch and vertical speed with power. Standard "backside" stuff that you learned way back in your PPL days.

I'm a bit intrigued, actually, at how often the coffin corner comes up in this forum. It's fun to think about when you're talking U2s and such, but it happens at altitudes so far above what's reasonable for flight efficiency, I can't imagine an airline driver getting anywhere near it in normal ops.

[BOAC]

I think a few here are getting confused between 'coffin corner' and airliner max altitude, and as 'airfoil' rightly says, the real CC is a 'dead' end. There was no controllable way out of a true CC. An airliner, however, with a few knots between high speed and low speed control problems is a different kettle of fish, and of course we can all fly our way out of that. If the U2 had a 10kt margin as BA said, that also was not 'CC', but just a place to be, how shall I put it, a leetle bit careful. It is not really clear what the OP meant when the thread opened.

I suggest that those 'younger' posters do a bit of historical research on the early jets and their problems, particularly the early straightwing mil jets. A lot of useful lessons were learned.

[FE Hoppy]

Lemurian

I find the most telling sentence in your explanation to be :
It is to be remarked that this ceiling would be attained at only one Mach number.

Would it be correct to say that at a higher Mach number one would for the same mass and altitude require a lower Cl to maintain 1 g flight and therfore the limiting factor is in fact thrust?

With more thrust at the same altitude a higher Mach number could be obtained and thus a lower Cl.

Could recovery from this ceiling not be safely be obtained by lowering the nose and making an accelerating descent. As in doing so Cl is reduced and therefore high speed margin increased?

[airfoilmod]

Everything you write, along with Lemurian, is indisputable. However, you are also not fully taking in the definition of Coffin Corner. You both describe manouvers that pull you back from the precipice. Reducing power inside the box causes a Stall, which is the A/C's honest response to lost lift; it wants to gain speed and refly. But you are at Vne, so the Stall becomes disastrous. Likewise, if you alter Pitch up or down you tumble out of control. BOAC said it best: it is a "one way street".

best, Airfoil

[Lemurian]

I can't imagine an airline driver getting anywhere near it in normal ops.
It has happened a few times in the pre-FMC era : an underestimation of weight would be enough to be too high . A jet upset is another example when the airplane is submitted to a g-factor higher than planned...
An incident where the influence of flying too close to the max aerodynamic ceiling had a part was the China Air 747 : High altitude, high AoA and an engine flame out....made interesting aerobatics.
This is a forum, not a cockpit.
...and it should reflect what we experience in our lives in the cockpit.
That term that you readily coined as CC has absolutely no official existence in our manuals, but we use terms like :
"Buffet Ceiling",. "Buffet Onset", "Buffet Limited Altitudes", "En-Route Maneuver limits"...etc...CC is just club bar crap (but it came from real-life experience of those gallant test pilots of the fifties who went up there to find out ) and its official name is "LIFT CEILING"
Coffin Corner "sounds hard to reach". Only if you can't imagine flying well past it.
You can't fly past it unless you tried to zoom it from a lower altitude, but you'd be stalling anyway...
This discussion is a perfect demonstration on how imprecise, unofficial undefined concepts can spoil a technological approach to a problem : is a corner a zone or a dot ?
If it's a zone, refer to the buffet margins graph of your airplane
If it's the summit of your curve - i.e a dot -, you are talking of a situation that is best described as accidental severe upset....see your training manual or get the Boeing or Airbus texts on upset recovery.
But you are at Vne, so the Stall becomes disastrous. Likewise, if you alter Pitch up or down you tumble out of control.
And now you start your usual uninformed blurb : You are NOWHERE NEAR Vne, as the parameter that causes the phenomenon is not Mach number, M, but the Clmax . M² part of the lift equation....Therefore, your explanation is in error...

[fantom]

Lemurian...

Please pm me if you were tcx.

[gr8shandini]

I guess it all depends on how you define when you're "in a corner." When I was punished in grade school, we weren't required to actually touch the line where the walls come together, so being in the vicinity is close enough to use the term in my mind.

I suppose that if you were flying an aircraft that had both a wicked Mach tuck tendency and deplorable stall characteristics, the very tip of the coffin corner situation could be truly harrowing. But as was stated earlier, in order to get to that singular point where you're well and truly screwed, you'd have to make a very deliberate effort at getting there as the critical speeds converge (barring some crazy NF-104 type zoom maneuver that punts you into ballistic territory). Not something you're likely to wander into.

And if either of the two critical cases prove to be controllable, you could either ride the stall or put up with the Mach buffet until you're low enough to have some breathing room again.

[Lemurian]

Would it be correct to say that at a higher Mach number one would for the same mass and altitude require a lower Cl to maintain 1 g flight and therefore the limiting factor is in fact thrust?
Absolutely correct. That's the gist of high altitude flying.
With more thrust at the same altitude a higher Mach number could be obtained and thus a lower Cl.
Yes, but at our lift ceiling, we are at Clmax. No level flight is achievable, whatever the thrust.
Could recovery from this ceiling not be safely be obtained by lowering the nose and making an accelerating descent. As in doing so Cl is reduced and therefore high speed margin increased?
Yes, as immediately upon lowering the nose - and begining the descent-, one is in fact reducing the AoA, hence the Cl and therefore escape the situation. Acceleration comes a bit later, as we see the buffet margins widening with diminishing altitude.
Google a very simple paper made by Airbus with "Getting to grips with aircraft performance". A very nice paper for reminding oneself of basic theory.

[Brian Abraham]

BOAC - Sorry, been away, hence late reply. Pat Halloran was an F-84 pilot with some 100 combat missions over Korea when he joined the Air Force’s U-2 program in 1957. Before moving on to fly the SR-71 Blackbird, Halloran logged 1,600 hours in the U-2. Here’s what he has to say about piloting it.

The Coffin Corner

“[Maximum and minimum airspeeds] began to converge as you went higher and higher. It wasn’t too bad when you first leveled off, but as you continued through the flight and [went] higher, the indicated airspeeds would get lower and lower, and pretty soon you were down to, oh, maybe a 10-knot window in which you had to continue the rest of the flight. If you exceeded or bounced off the limits, either too fast or too slow, you got pretty much the same indication, so it was difficult to tell which you were encountering: approaching stall or approaching Mach limit.”

AmericanHeritage.com / U-2 (http://www.americanheritage.com/articles/magazine/it/2007/3/2007_3_40.shtml)

It is believed that at least three aircraft were lost (in the USA) due to loss of control resulting from transgressing the buffet boundary. The aircraft had low structual limits and was particularly fragile.

The Chuck Yeager Approach
To recover from CC.Tumble in an uncontrolled manner out of the sky until you hit thicker air. Ultimately you will recognise the uncontrolled state has now entered a recognisable aerobatic state (spin or inverted spin). Apply standard recovey technique. Simple.
I guess you are talking of Chuck's efforts in the NF-104. Didn't turn out too well for him, his only possible recovery action was to bail out. Lucky to survive considering the injuries/circumstances of the bail out. Accident showed his good side, coolness under stress, and his bad side, impetuosity.
NF104 | Spin, Crash & Rescue (http://www.nf104.com/stories/stories_12.html)

[gr8shandini]

Actually, I heard another, not nearly as favorable account of the NF-104 incident from an engineer on the program. It's hard to say where the truth lies as Yeager has a - let's say - disagreeable personality and makes lots of enemies.

According to this guy, Yeager actually lost control of the aircraft twice. It was known that the F-104 had an unrecoverable spin mode with the engine out (apparently, the motor was big enough to generate a stabilizing gyroscopic force) and since the a flameout was a high probability, the NF-104s were fitted with spin chutes. So after the initial departure from controlled flight, he deployed the chute and recovered the aircraft. Instead of descending with the chute deployed until he reached the re-light envelope as they briefed, he decided to cut the chute to get down faster. But without hydraulics, the flight controls were still configured in the position that caused the initial spin and he immediately entered the second one which he had to bail out from.

Between the unauthorized record attempt (he didn't quite steal the airplane like in "The Right Stuff," but it's close) and the failure to follow procedure for the spin recovery / relight, it would have been end of career for any other pilot. But I guess being a national hero buys you a few "get out of jail" cards.

[Pugilistic Animus]

Chuck at 84 flies sick kids [Make -a - Wish] in a Ford Tri-motor---he still puts up about 200 hrs a year---so, I guess he's repentant---still a noteworthy aviator today---can't comment on his personality, but he still does good work


PA

[BOAC]

Thanks for that Brian - I bow to the U2 guys' experiences of course but was just observing that 10kts is not THAT tight a margin, but I guess they may have flown just that little bit higher - I would :) and, of course, they were not just 'sitting there' looking at the view!

[airfoilmod]

Flies out of O-17, Grass Valley, I see him occasionally coming and going, usually on Saturday mornings when he flies his friend Bart Riebe's Aviat Husky with a friend. He has a well developed command presence, people generally just go quiet when he enters the room or walks on the Ramp. In my experience, he wouldn' talk to just anyone who isn't with media. He is gruff, but very articulate with a real West Virginia accent. Looks very well for his years. He settled here years ago after retirement, his wife, Glennis, was born here (She has passed away). I've stood next to him and listened, (closely), he is a true hero, quite reserved, and keeps to himself in the neighborhood.

Airfoil

[SNS3Guppy]

No, normally it is reached at a much lower Mach than Mmo.


An aircraft at it's performance ceiling isn't at it's upper buffet margin for any reason but coincidence. When an aircraft reaches it's maximum ceiling, given available power and temperature, it's limited in further climb by power. It is not limited by an upper buffet margin. While one mach number may be the magic number for getting to that altitude, that one mach number does NOT represent the maximum speed. A higher mach number will be achievable (assuming one is not in "coffin corner"). This higher mach number's maximum value will be determined by the buffet boundary and will be the upper limiting number.

Simply because the airplane can't climb any more doesn't mean it's reached it's upper limiting value. You're right; the airplane will generally run out of performance prior to reaching Vmo, or such limiting number at which mach effects such as buffeting shall be found. The fact is that most airliners and even many corporate jets simply don't have the power to push themselves to "coffin corner." Certainly at some point they run out of available power and can't climb any more, and certainly a point may be reached where the best angle for climb coincides with or arrives very close to the stall point; the aircrat has reached it's absolute ceiling. This will seldom approximate or equate to the "coffin corner," in which the mach buffet boundary is very close to the low speed buffet (or departure, as the case may be).

[Brian Abraham]

10kts is not THAT tight a margin
Perhaps some other aircraft characteristic made it a tight margin. In all the writings of U2 ops the buffet margin is something oft mentioned and with much respect paid. But then again, perhaps it was skygod talk meant to invoke awe.

[BOAC]

meant to invoke awe - mere mention of U2 does that alone for me:). I would imagine any sort of control loss at that low IAS and with high inertia is something to be very wary of!

[chornedsnorkack]

Let´s have a look at it this way:

A plane at a given altitude (air density) and low Mach number has a Cl which depends on angle of attack, and that Cl has a maximum. That is where the plane stalls: any increase of AoA causes drop of lift. And decrease of AoA also causes decrease of lift. A plane flies at stall speed when its total weight is equal to the local maximum of lift for that air density and true airspeed. Right?

Note that at stall, Cd is substantial. A plane can have lower Cd, and bigger L/D, by flying faster. It is perfectly possible for a plane to have no thrust at all, or to have thrust which, at stall, is less than the drag at stall.

Now, with changes of Mach number the Cl will change. Generally, in nearsonic region, it decreases, so that supersonic Clmax is lower than subsonic Clmax. But not zero.

In nearsonic region the Clmax decreases with Mach number so fast that the product Clmax times V squared has a local maximum - increase of airspeed decreases the maximum lift over the given airspeed.

This is a local maximum - in supersonic range the Clmax may decrease further, but the lift shall increase with V squared, without bound.

The local maximum of lift equalling the total weight defines the coffin corner. Right?

Now, the available thrust changes depending on air density and true airspeed. Since the coffin corner is at stall (a high drag condition anyway), it makes sense that the engines may not provide the thrust necessary to reach and sustain coffin corner altitudes...

[Jenna2011]

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]

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]

Hi henra,

Bob Smith (1928 - 2010) (http://www.nf104.com/stories/stories_12.html)
"...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]

Hi henra,

Bob Smith (1928 - 2010) (http://www.nf104.com/stories/stories_12.html)
"...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... :confused:

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]

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.