I have a question concerning the coffin or q corner:
I understand that the corner is located at an (aircraft specific?) altitude. Decreasing the stall speed, e.g. by lowering operating weight, opens this corner.
I also understand, that in modern transport aircraft, the corner is way above the usually used flight levels.
Under which conditions is the q corner altitude lower than under "normal" conditions? Can extreme weather conditions, e.g. massive tail wind or CAT, change this altitude (maybe only for fractions of seconds)?

Please don´t laugh at my question, I am an aviation layman!

Chris

Hi Chris,
this may help?

Coffin Corner ? Wikipedia (http://de.wikipedia.org/wiki/Coffin_Corner)

KR

AS

Hi Chris,

Far from laughing, I'm reminded of the old saying, "the only stupid question is the one that isn't asked!"

This is actually a really interesting subject and not at all easy to explain in simple terms, but I'll have a go....

Excuse me if I appear to be 'talking down' to you, but I'm not sure of your starting point vis-a-vis principles of flight.

An airliner in level flight has to produce enough lift to balance its weight.

Assuming a constant weight and normal cruise altitude, if the airliner wants to fly slower, the lift must be maintained at the correct value by raising the nose and flying at a higher 'angle of attack'. If the aircraft keeps decelerating, it will eventually reach a limiting angle of attack where the airflow over the wing starts to break up - this is known as pre-stall buffet and marks the lower limit as far as speed is concerned.

The upper speed limit is set by the maximum mach number that the aircraft is certified for. Flight at a higher speed would cause shock waves to upset the stability and handling characteristics.

On modern electronic instruments, these two limits are marked with yellow or red 'bugs' on the airspeed indicator, so it's easy for the pilot to judge his 'manoeuvre margin' between the two limits. This margin will typically be in the order of 30 or 40 knots at optimum altitude for the weight. The cruising mach number will put the airspeed roughly halfway between the two limits.

Now let's assume the aircraft needs to climb for some reason (again at the same constant weight and mach number). As altitude is gained, the same cruising mach number will result in a lower indicated airspeed. The upper speed limit bug will also move towards a lower speed. The lower speed limit bug will tend to move towards a slightly higher speed. The result is that the range of available speeds has now reduced from 30 or 40 knots to, say 20 knots.

As the aircraft keeps climbing, the gap between the upper and lower speed limit bugs will close and eventually they will be only 3 or 4 knots apart. The aircraft has reached maximum altitude or 'coffin corner'. If the airspeed reduces by just 5 knots, the pre-stall buffet will occur and if the speed fluctuates to a higher value then 'mach rumble' will make itself felt.

As long as the air is smooth, and there is no 'mountain wave', the need to maintain the speed this accurately poses few problems for a trained crew and a good autothrottle. If turbulence or strong vertical currents occur, the speed will vary by more than the 3 or 4 knots and the only recourse is to descend again, to widen the gap between the two bugs.

You are quite right when you state that lowering the weight opens the corner up. As the aircraft burns fuel, the maximum altitude that can be reached will increase (typical figures for a heavy jet are an increase of 700ft to 1000ft per hour.) This due primarily to the lower value of lift that needs to be produced to balance the lower weight. Alternatively, if the aircraft stayed at the same altitude as weight was reduced, the gap between the speed bugs would gradually widen.

You may be surprised to hear that modern airliners are quite capable of reaching 'coffin corner' due to the efficient wing design and high thrust available from the engines. The maximum altitude is normally only 3000ft to 4000ft above the optimum altitude.

As well as 747s, I fly small private jets and here the picture is different in that the engines are not powerful enough to take the aircraft up anywhere near 'coffin corner'. These aircraft are said to be 'thrust limited' and even at the maximum certified altitude there can be a 100kt margin between the minimum and maximum speeds.

Hope this helps!

Regards,

Eckhard:)

Thanks Air Soul.

Wikipedia is actually the source I got most of my coffin corner knowledge from.

But to make it a little more clear:
Wikipedia says,
Similarly, the specific altitudes at which the stall speed meets the critical Mach number will differ depending on the actual atmospheric temperature..

Does this mean, that getting suddenly into severe icing conditions, and flying in a narrow flight envelope due to high FL, will increase risk of stalling?

Chris

Thanks Eckhard,

your answer is excellent! I believe, this fully resolves my problems!

Chris

May I ask a supplementary question?

It has struck me when flying and watching those moving maps on the screen that tell you where you are and how fast you are flying that the speed of the aircraft can vary dramatically in a just a few seconds - a tailwind can suddenly become a headwind and so on. Doesn't that mean that the aircraft is always breaking these limits?