Coffin Corner (Thinking exercise)

[slima]

Hi fellow mates.,
this is my first question on PPRune ;)

I'm battling with a question that i have in my mind since i took the Ryanair interview and was dismissed !?

The issue is Coffin Corner. Hypothetically lets assume we have an 737 ;P with tanks full super awesome engines that can climb straight to the point of the coffin corner.
At this point, and for that weight, we're kinda screwed (if we got there) and we cant increase, decrease speed or climb because we'll stall.
I think i get it so far. (if anything is not accurate please let me know, i'm here to learn)

So the solution to this point is, from what i saw on other posts, to decrease altitude and maintain the airspeed, because it will take us out of this problem.

So my question is:
if the problem is the weight of the airplane, and we can't act on the thrust or lift forces, why can't we act on the weight force?
Is it possible to jetisson some fuel to take us out of this intricacy ? and thus with a slight bigger margin start to decrease altitude to get safer ?

This is just a thinking exercise (i like to use my brains:)), if you can share your thoughts that will bring more clarity to this will help me get closure with this question i get for months!

Cheers,
Sergio

[TopBunk]

slima

I'll start!

1. The 737's can't dump fuel, so that option is ruled out.
2. Why have you climbed there to start with?
3. You can safely descend at current speed to an appropriate level - the coffin corner gap on the speedtape will open quite rapidly
4. You won't stall - you may get a stall / overspeed warning if you speed up/down/climb - but you will still have the mandated 1.3g UK (1.2g in some states) margin

On a B747-400, the max alititude increases by about 1000ft per hour as fuel is burned, I can't remember what the B737 family numbers are.

[Dont Hang Up]

The bit that I think is wrong is...

...and we can't act on the thrust or lift forces...

The point is that you cannot change either of these individually. Assuming you have ventured beyond all safety margins and are literally on the cusp then any adjustment of thrust or any pitch change to affect lift will, if done alone, result in loss of control.

However, pitching down to decrease lift while decreasing thrust to stop the speed from increasing will work.

Of course this is all on a knife edge and has to be balanced perfectly. But then maintaining straight and level while waiting for some of the weight to burn off would be equally knife edge!

PS; There is an assumption here that we still have some pitch authority. Generally the tail plane operates at a lower angle of attack than the wings and should be able to oblige.

[phiggsbroadband]

Hi Guys, I think you have missed the first lesson of any flight training, that to descend or climb you just use the throttles. Changing the Pitch Attitude will increase or decrease the speed.


So you are in the apex of the coffin corner triangle, you can't go slower or faster, or you would go outside the triangle. The only way is towards the middle of the triangle. i.e. Keep your airspeed constant, and just descend.


The descent will happen if you just close the throttles, don't touch the Pitch Trim.
Have I got the job, mr Ryanair?

[de facto]

I'm battling with a question that i have in my mind since i took the Ryanair interview and was dismissed !?

The issue is Coffin Corner.
B737 have no issue of so called coffin corner...been shown to be handling well past its MMO by FAA required margin of M0.07.
So at M0.89 and 1.5 G,all works just fine...controlling wise.
Just dont forget to fill in that "excursion" into the flight log:}

[Dont Hang Up]

Hi Guys, I think you have missed the first lesson of any flight training, that to descend or climb you just use the throttles. Changing the Pitch Attitude will increase or decrease the speed.


So you are in the apex of the coffin corner triangle, you can't go slower or faster, or you would go outside the triangle. The only way is towards the middle of the triangle. i.e. Keep your airspeed constant, and just descend.


The descent will happen if you just close the throttles, don't touch the Pitch Trim.
Have I got the job, mr Ryanair?

I think the lesson you are referring to says adjust throttle to change height while maintaining airspeed with pitch control.

Any thrust change with no corresponding pitch input will initially create an airspeed change. In a well behaved aeroplane the speed change will result in a natural pitch change to counter the speed change. But at coffin corner that may be too late!

I have yet to meet an aeroplane so stable that it will maintain airspeed across the whole throttle range without any retrim. However I understand there are some aircraft that have an "IAS Hold" autopilot mode, which adjusts pitch for airspeed allowing thrust to be used for height change.

[oggers]

phiggs

Hi Guys, I think you have missed the first lesson of any flight training, that to descend or climb you just use the throttles. Changing the Pitch Attitude will increase or decrease the speed.

Well I think that is a simplification that may be useful for the first lesson but has no basis in aerodynamics.

The only way is towards the middle of the triangle. i.e. Keep your airspeed constant, and just descend.


True enough but to maintain the speed constant after reducing the throttle requires a pitch down, though not necessarily a trim change.

The descent will happen if you just close the throttles, don't touch the Pitch Trim.

Yes but in general the aircraft will pitch down nonetheless. Don't Hang Up explains it well.

[phiggsbroadband]

Maybe the initial question was specifically related to the 737 type of aircraft, ( vs. an axial thrust Eurofighter), Where the increased drag of a windmilling engine, on the end of a pylon, already causes a nose down pitch change.


So... ''True enough but to maintain the speed constant after reducing the throttle requires a pitch down, though not necessarily a trim change.'' Might be aircraft type specific only. It would all depend upon the position of the engine thrust line.

[keith williams]

I can think of two reasons why they would ask this type of question.

Reason 1. To see how you react to difficult situations. If your reaction is to say "well I am dead!" Then you will not get the job. They do not want people who cannot think when under pressure.

Reason 2. To test your knowledge of how different factors interact in the real world. They would be looking for something more than "just throttle back and you will descend at constant speed".

What answer did you give in the interview?

[slima]

Hi all,

Thanks for the inputs, this was just an exercise its not intended to be a real life situation (at least i do not have the intentions to go through such situation)

Ya i think the normal answer and the one i gave was to throttle down and pitch down (maintaining speed and descend) which is the normal answer. But then when asked if that was the only option i started thinking on another option, kinda backup plan. And i answered this, and i would like to know if it was correct or if it could be feasible. This is why i put the question here, because i didn't get a straight comment on my answer.

I also learned some things with you' all. The 737 does not dump fuel.
The speed tapes even if it says that you'r on the coffin corner you still have the 1.3g margin (which i was not thinking)

de facto says
B737 have no issue of so called coffin corner..
Is this true ? Can you give some more info on this ? I will google for it anyways :)

keith williams
yes i know that's the intention of the interview and these kind of questions .:)