VC10er
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Climbing Too High, What Happens?

Sun Feb 26, 2012 3:41 am

If a commercial jet liner, say a 747, climbs up beyond it's max crusing altitude far up enough so it can climb no further, will the aircraft keep going until the pilot stops it? And if it just allowed kept going would it tumble out of the sky or level off on it's own?
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FlyMKG
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 4:23 am

To answer your question check out this wikipedia article on the coffin corner. http://en.wikipedia.org/wiki/Coffin_corner_%28aviation%29 It should answer everything.

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trav110
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 8:03 am

If you keep trying to climb the air around you gets so thin that the wings aren't able to provide enough lift, and you eventually start to descend, It's a very dangerous situation to put oneself in that also risks inflight breakup.
 
vikkyvik
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 8:23 am

Quoting VC10er (Thread starter):
If a commercial jet liner, say a 747, climbs up beyond it's max crusing altitude far up enough so it can climb no further, will the aircraft keep going until the pilot stops it? And if it just allowed kept going would it tumble out of the sky or level off on it's own?

If you let the aircraft climb at its max climb rate at all times, you'll do a large arc that will level off at the absolute ceiling (it's asymptotic, so you never actually get there). From there, the aircraft can fly level or descend, but can't climb (well, technically you can, since you'll be burning off fuel as you go, but never mind that).

Climb rate is based on excess thrust - so your absolute ceiling is the altitude at which your thrust can only equal your drag, but can't exceed it. I THINK you may still have some stall and critical-Mach margin (though it may not be much). For instance, if you're flying level at the absolute ceiling, and you pitch up, the airplane may not stall just yet, but you'll start descending because you increased the drag through the pitch-up.

That's what I remember, at least. Corrections welcome.
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Starlionblue
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 8:31 am

As previously mentioned, you enter coffin corner. Go faster and you encounter mach buffet (overspeed). Go slower and you stall. Eventually the two curves meet or become so close that any turbulence will make you either stall or buffet you to structural problems.

The altitude of coffin corner is dependent on the wing and the engines basically. If you have a wing like a U-2 spyplane you can go higher than a F-104 Starfighter at the same speed. Then again a Starfighter has higher thrust so can go faster...
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saab2000
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 12:25 pm

The certified ceiling for my airplane is FL410. I have never been there but we now have a company restriction of FL370 for safety purposes so it's a bit moot. The airplane barely climbs at the higher altitudes anyway.

Most of the folks I've talked with who've been at FL410 in the CRJ200 describe the feeling as if they were standing on a basketball. The margin between overspeed (Mach buffet) and stall is very narrow and so it's just not a good place to be. That said, the later model CRJs can get there more easily with more power and more wing.

The others describe coffin corner well, so there's no need to go into that, but that's basically one of the reasons for a certified ceiling. There are others too, like speed of emergency descent, but that isn't really the question here. Business jets go to the upper 40s and even FL510 on a regular basis. I can't even imagine!
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 2:19 pm

Pinnacle flight 3701 gives you another hint, beside the obvious coffin corner topic. There, the engines overheated and quitted.

Is the inside-outside pressure difference important? Is any airliner able to climb that high to make this a problem?


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VC10er
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 4:04 pm

Thank you all. I really know nothing about extreme flying conditions, I just know a bit more than the average person. But a novice for sure!

I would find it safe to presume a pilot for a 747 or any other commercial jet for a safe airline would never push their ac up to a dangerous place intentionally for kicks. I would presume test pilots do some risky stuff. But I was really curious if instruments were busted and a plane drifted upwards, would the thinner air automatically level the plane off before going so high it would stall, break up or tumble out of the sky!

Also, in some airline magazines which feature their fleet and add spec like max speed and max cruising height (36,000 ft) is that height well below the ac's real altitude the ac is capable of?
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tdscanuck
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 6:02 pm

Quoting flyingturtle (Reply 6):
Is the inside-outside pressure difference important? Is any airliner able to climb that high to make this a problem?

Most pressurized aircraft have positive and negative pressure relief valves. If you climb too high and the delta-pressure gets above what the fuselage is designed to handle, the positive pressure relief valves will open and the cabin will start to climb to hold a constant delta-P (cabin altitude will climb). If you let it get high enough you'll get cabin altitude warnings and have to go on oxygen.

Quoting VC10er (Reply 7):
But I was really curious if instruments were busted and a plane drifted upwards, would the thinner air automatically level the plane off before going so high it would stall, break up or tumble out of the sky!

It depends on the balance between wing performance and engine thrust which will happen first. But, provided you don't cross the mach buffet or stall lines first, the aiplane will level off where the engine thrust degrades to the point that it equals drag.

Quoting VC10er (Reply 7):
Also, in some airline magazines which feature their fleet and add spec like max speed and max cruising height (36,000 ft) is that height well below the ac's real altitude the ac is capable of?

Yes, the airplane is capable of higher (sometimes considerably higher).

Tom.
 
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RE: Climbing Too High, What Happens?

Sun Feb 26, 2012 6:29 pm

Quoting VC10er (Reply 7):


I would find it safe to presume a pilot for a 747 or any other commercial jet for a safe airline would never push their ac up to a dangerous place intentionally for kicks. I would presume test pilots do some risky stuff. But I was really curious if instruments were busted and a plane drifted upwards, would the thinner air automatically level the plane off before going so high it would stall, break up or tumble out of the sky!

It really depends on the situation. If a pilot continues to input a climb in a 747, it will still climb. The 747 has no altitude protection. However the airplane will stop climbing because it does not have enough thrust to continue climbing. The airplane will fly with a nose up attitude. If the pilot continues to try to climb the airplane, it will eventually stall.

A 747 will not reach the coffin corner. It will begin slowing down as it does not have enough thrust to reach the coffin corner. The airplane will slow down and end up in a very high attitude before stalling. Very few commercial airplanes can reach the coffin corner. One of the few planes that flies anywhere near it is the U-2.

High altitude stalls are very dangerous. AF447 and Pinnacle 3701 show what happens when a pilot inputs a climb above what the airplane can do. In both cases the airplane did not have enough thrust and lost speed. Pinnacle 3701 stalled and had an engine flame out. AF447, just stalled.

If the instruments failed, then it is a human factors situation. In AF447 they crashed the plane. In other situations the pilots may recover. In other situations the pilots would react to the stall warnings and push the nose down. The stall warning and prevention systems will typically push the nose of the airplane down and has to be overridden.
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 3:26 am

Quoting Roseflyer (Reply 9):
High altitude stalls are very dangerous.

They're not really dangerous - the recovery is even easier than from a stall at low altitude - but you do have to know what to do differently. First and foremost: don't even try to quickly get back up to your altitude the way you would at a lower altitude. It's not going to work, and that's where you'll get yourself into trouble. Just lower the nose, add full power, and wait. The airspeed will build up slowly, to a point where you can return to level flight. You'll lose several thousand feet in the process, but the good thing is that at altitude, that's not a problem.

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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 3:44 am

Quoting saab2000 (Reply 5):
Business jets go to the upper 40s and even FL510 on a regular basis. I can't even imagine!


Quoting flyingturtle (Reply 6):
There, the engines overheated and quitted.

Not quite.
 
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 4:31 am

Quoting Roseflyer (Reply 9):
High altitude stalls are very dangerous.

Not knowing much about anything in this area, I'm curious, is that what happened to Chuck Yeager many a year ago when he crashed a test plane going after some kind of altitude record? I know the movie "The Right Stuff" played with the details a little bit for dramatic effect, but I'm pretty sure the basics of the event were true.
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 5:19 am

Quoting homsar (Reply 12):
I'm curious, is that what happened to Chuck Yeager many a year ago when he crashed a test plane going after some kind of altitude record? I know the movie "The Right Stuff" played with the details a little bit for dramatic effect, but I'm pretty sure the basics of the event were true.


Well, the scene is based on an actual event. But about the only things they got right was that:
- The aircraft was an F-104 (though they used a plain vanilla one).
- It was piloted by Yeager.
- He lost control.
- The aircraft crashed.
- His helmet lining was burning (ouch!!!).

The test flight series was flown to altitudes above most of the atmosphere. Partly given the addition of a liquid fueled rocket engine and partly given the low air pressure, the aircraft tended to pitch up in a stall. The trick was to descend nose down so you got control authority and air into the engine so it would reignite. On this flight Yeager could not get the nose down and the aircraft entered an unrecoverable flat spin.

Well recounted here by the man himself: http://www.check-six.com/Crash_Sites/NF104-YeagerInterview.htm

I guess the F-104 has little wing to start with so this was dicey stuff. Yeager is very aw-shucks about it but even a normal flight in the program was pretty dangerous.

[Edited 2012-02-26 21:21:33]
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JETPILOT
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 9:44 am

Quoting Roseflyer (Reply 9):
A 747 will not reach the coffin corner.

How can that be a true statement? As fuel burns off the plane gets lighter. If you don't pull back the power as the plane gets lighter it will accelerate approaching the high speed buffet assuming your flying closerto your critical mach number in the first place. You have a performance chart to prove this?
 
tdscanuck
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 3:02 pm

Quoting jetpilot (Reply 14):
Quoting Roseflyer (Reply 9):
A 747 will not reach the coffin corner.

How can that be a true statement?

Most commercial aircraft can't accelerate to Mmo (let alone Md) in level flight at maximum altitude. They don't have enough thrust. To reach the coffin corner you need to be able to reach C_l_max on the wing (i.e. be at stall buffet) and have enough excess thrust to get beyond Md (i.e. Mach buffet). Most commercial jets can't do that. At C_l_max the induced drag is huge so you're typically going to run out of thrust to climb higher before you reach the point where you're at Mach buffet. Airliners can usually only reach Md (at high altitude), in a dive, so they can't do it in level flight or while gaining altitude.

Tom.
 
vikkyvik
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 5:10 pm

Quoting jetpilot (Reply 14):
How can that be a true statement? As fuel burns off the plane gets lighter. If you don't pull back the power as the plane gets lighter it will accelerate approaching the high speed buffet assuming your flying closerto your critical mach number in the first place. You have a performance chart to prove this?

Just because an airplane approaches Mach buffet doesn't mean it's at the coffin corner. It could still have some margin between buffet and stall.
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 6:27 pm

Quoting homsar (Reply 12):
I know the movie "The Right Stuff" played with the details a little bit for dramatic effect, but

Some. For example the alitmeter they kept showing was, in fact a clock made to look like an altimeter. You can buy one similar here:
http://www.mypilotstore.com/mypilotstore/sep/685

Altimeters have 9 to the left of TDC and 1 to the right. It had 11 to the left. So the longer hand would have to climb 1200 feet per thousand.

Quoting Starlionblue (Reply 13):
- The aircraft was an F-104

Actually the NF-104

Quoting Starlionblue (Reply 13):
- His helmet lining was burning (ouch!!!).

My understanding is that he collided with the seat after separation - propellant to face shield. The face shield of his helmet shattered and liquified solid rocket fuel from the seat hit him in the face. The neck seal on his helmet then burned or melted allowing the oxygen rich pressure contained therein feed the fire on his face. Yes, rather unpleasant.
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Faro
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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 8:03 pm

Quoting tdscanuck (Reply 8):
But, provided you don't cross the mach buffet or stall lines first, the aiplane will level off where the engine thrust degrades to the point that it equals drag.

And presumably follow the gentle ups and downs of the isobaric pressure surface at which it reached level-off equilibrium.

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RE: Climbing Too High, What Happens?

Mon Feb 27, 2012 8:27 pm

Quoting Roseflyer (Reply 9):
Pinnacle 3701 stalled and had an engine flame out.

As I recall, it was a double flameout (both engines), although the flight crew only reported a single flameout to ATC (the career alert was probably sounding in their heads...). There was an added complication, too: altough GE (the maker of the CF-34) had experienced a phenomena called core lock during testing, they had never documented it for anyone outside GE. When the crew couldn't air start the engines during the forced descent, it was apparent that they had experienced core lock. GE wasn't counting on someone reproducing conditions that would lead to core lock on an actual flight  
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Starlionblue
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RE: Climbing Too High, What Happens?

Tue Feb 28, 2012 3:03 am

Quoting SlamClick (Reply 17):
Quoting Starlionblue (Reply 13):
- His helmet lining was burning (ouch!!!).

My understanding is that he collided with the seat after separation - propellant to face shield. The face shield of his helmet shattered and liquified solid rocket fuel from the seat hit him in the face. The neck seal on his helmet then burned or melted allowing the oxygen rich pressure contained therein feed the fire on his face. Yes, rather unpleasant.

According to his autobiography, the recovery was no picnic either, as they had to debride his skin over and over and over and over.
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tdscanuck
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RE: Climbing Too High, What Happens?

Tue Feb 28, 2012 4:21 am

Quoting KELPkid (Reply 19):
GE wasn't counting on someone reproducing conditions that would lead to core lock on an actual flight  

Since then, testing for core lock has been a certification requirement. It's a fun test.

Tom.
 
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RE: Climbing Too High, What Happens?

Tue Feb 28, 2012 8:54 am

Quoting Starlionblue (Reply 13):
Well recounted here by the man himself: http://www.check-six.com/Crash_Sites/NF104-YeagerInterview.htm

How much did he charge for this interview!?
 
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RE: Climbing Too High, What Happens?

Tue Feb 28, 2012 8:52 pm

Quoting tdscanuck (Reply 21):
Since then, testing for core lock has been a certification requirement. It's a fun test.

Tom.

What do you do, bump the thrust to 100% and try to flameout the engines near the service ceiling by inducing compressor stalls? And then try to restart them on the way down?   
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RE: Climbing Too High, What Happens?

Tue Feb 28, 2012 10:29 pm

Quoting VC10er (Reply 7):
Also, in some airline magazines which feature their fleet and add spec like max speed and max cruising height (36,000 ft) is that height well below the ac's real altitude the ac is capable of?

The DC-8-43 that broke the sounded barrier in a dive on a test flight prior to delivery to Canadian Pacific in 1961 climbed to 52,000 ft. before beginning the supersonic dive. The DC-8's maximum certificated altitude is 42,000 ft.
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