2
$\begingroup$

enter image description here

This picture is taken from Safety First (Airbus magazine). Could you please clarify some things:

1). Is "Absolute aerodynamic ceiling" = "Coffin corner"?
2). Have I correctly understood that there is no "Absolute Ceiling" (ROC=0) in this diagram?

And a question that doesn't relate to this particular picture. At absolute ceiling we can fly at only one speed. This is the speed which corresponds to intersection of power available and power required curves? And what is that speed at which flight at Coffin Corner is only possible?

$\endgroup$
  • $\begingroup$ The coffin corner is where the lines for minimum speed (which increases with altitude) and maximum speed (which decreases with altitude) cross. Some of the other lines show safety margins designed to prevent aircraft from ever actually flying in the coffin corner because, as the name implies, it's quite dangerous. $\endgroup$ – StephenS Oct 2 '19 at 19:15
4
$\begingroup$

At absolute ceiling we can fly at only one speed. This is the speed which corresponds to intersection of power available and power required curves?

The "intersection of power available and power required" is anywhere along the left hand edge of the red cross-hatched area. The absolute ceiling is the intersection of the stall curve (left-most red line) with this power limit.

And what is that speed at which flight at Coffin Corner is only possible?

Coffin corner in that chart should be the intersection of the red stall line with the black MMO/VMO line.

I say 'should be' because the stall line is drawn with a maximum at the "absolute aerodynamic ceiling" i.e. the plane stalls if it goes any faster or slower. Stalling when going faster must be due to increased Mach number.

In other words, the right hand side of the stall line is actually the maximum Mach number, which would make the "absolute aerodynamic ceiling" 'coffin corner'.

I suspect the confusion is that they wanted to draw MMO as a (mostly) straight line, and didn't think it mattered because the plane doesn't have the power to reach that region.

| improve this answer | |
$\endgroup$
  • $\begingroup$ As you said: absolute ceiling is at the intersection of stall curve and red cross-hatched area. Coffin corner is at the intersection of red stall line with black MMO/VMO line. Both of them doesn't correspond to "Absolute aerodynamic ceiling" marked in the picture. What is then that "Absolute aerodynamic ceiling"? $\endgroup$ – Nikita Oct 3 '19 at 8:41
  • 1
    $\begingroup$ Good point, I've reconsidered and changed by answer. $\endgroup$ – Robin Bennett Oct 3 '19 at 10:13
  • $\begingroup$ Now I got it. One more question. Is it correct that GD and CI=0 must meet each other at "absolute aerodynamic ceiling"? $\endgroup$ – Nikita Oct 3 '19 at 10:33
  • $\begingroup$ Now you've got me - I have no idea what either of those lines mean! $\endgroup$ – Robin Bennett Oct 3 '19 at 10:40
2
$\begingroup$

This graph is illustrative for a single weight, with the exception of Vmo/Mmo and Vd/Md.

Is "Absolute aerodynamic ceiling" = "Coffin corner"?

For all intents and purposes, coffin corner is where the top barber pole and the bottom barber pole on the speed tapes collide. This occurs when the stall warning speed or buffet onset speed coincides with the maximum operating speed (i.e. Vmo/Mmo). So while the graph defines Absolute Aerodynamic Ceiling as Vs1g, it will be lower than that.

The "Aerodynamic Ceiling" as defined, on the other hand, is the minimum operating speed for the aircraft at altitude. It is the minimum speed at which you should fly at to have enough margin for maneuvering. If you fly below this speed, you will see the barber poles converge as you are making turns or pulling up.

Have I correctly understood that there is no "Absolute Ceiling" (ROC=0) in this diagram?

There is, and it's the "maxspeed in level flight". This would correspond to a rate of climb of 0 at maximum thrust.

At absolute ceiling we can fly at only one speed. This is the speed which corresponds to intersection of power available and power required curves? And what is that speed at which flight at Coffin Corner is only possible?

Not sure what you mean by the last part. The blue curve titled "Service Curve" is roughly the intersection of power available to power required, although with a climb margin of 300ft/min. This is a performance limitation and has technically nothing to do with coffin corner.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Service ceiling is usually described as maximum altitude at which ROC of 500 ft/min for jet aircraft can be obtained. Have I correctly understood that judging by this chart, Airbus reduced this value to 300 ft/min? $\endgroup$ – Nikita Oct 3 '19 at 9:13
  • $\begingroup$ @Nikita I wasn't aware that there is a strict threshold. I've heard of 100ft/min as well. $\endgroup$ – JZYL Oct 3 '19 at 10:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.