COFFIN CORNER- When the aircraft’s stall speed meets the critical Mach number

In aviation, coffin corner (or Q corner) refers to the point at which the Flight Boundary is defined by when a high incidence stall intersects with that defined by the critical Mach number.

Before we dig further, what is stall and what exactly causes it?

STALL

Stall is defined as a sudden reduction in the lift generated by an aerofoil when the angle of attack of an aerofoil exceeds the value which creates maximum lift as a consequence of airflow across it.

The Angle of Attack of an aerofoil – the incidence of the wing to the incident airflow – is not the same as the pitch attitude of the aircraft as displayed on the corresponding primary flight instrument and many aircrafts do not have an instrument which displays angle of attack.

PHOTO : aviationchief.com

When flying straight and level with a particular aircraft mass and prevailing density altitude, for every wing angle of attack there is a corresponding indicated air speed.

PHOTO : aerospaceweb.org

This indicated speed provides a fundamental reference for all other AFM(Aircraft Flight Manual). Aeroplane performance calculations which involve indicated airspeed helps define the Flight Envelope for each aircraft type.

PHOTO : aeronewstv.com

So now, what exactly is coffin corner?

  • The Coffin corner or the Q corner is the altitude at or near which a high speed fixed wing aircraft’s stall speed is equal to the critical Mach number.  Coffin corner exists in the upper portion of the manoeuvring envelope of an aircraft, for a given gross weight and G – Force.
  • As an aircraft climbs towards the altitude that defines its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller until the Flight Envelope boundaries intersect. At this point, any change in speed would result in exceeding one or the other of the limits.
PHOTO : http://www.quora.com

IT CAN GET TRICKY BECAUSE…

PHOTO : simpleflying,com

If the plane slows down, it will stall as there isn’t enough air pressure to keep it aloft. If the plane speeds up to compensate, you will hit the subsonic Mach and the aircraft will pitch down and stall.

This margin of error is so small that a plane flying at 70,000 feet has a buffer of 5 knots between going too fast and going too slow.

This is because as a plane increases in altitude, the air gets thinner, thus the plane has to move faster to generate lift. When a plane reaches an altitude where it can’t go fast enough to have enough air passing over the wing to generate lift, it becomes difficult to control and could stall.

THE BEST YOU CAN DO IN SUCH A SCENARIO IS..

 To maintain the speed and reduce the altitude. If the speed changes in any way, then the aircraft will stall.

DO YOU HAVE TO WORRY WHEN FLYING A COMMERCIAL AIRLINER?

No,you don’t. Pilots today are well trained in all kinds of stall scenarios and how to escape them.In addition to that,we have a lot of systems onboard to warn the pilots of an imminent stall.


SOURCE(s) :

  • skybrary.aero
  • en.wikipedia,org
  • simpleflying.com
  • COVER PIC COURTESY : pilotmail.com