BUFFET BOUNDARIES- COFFIN'S CORNER

Stall speed is defined as the minimum airspeed required to maintain 1g level flight. Any further reduction in speed will result in the lift produced by the wings to be less than the weight of the aircraft and leads to a loss of altitude. The increase in angle of attack will in turn cause flow separation from the upper surface of the wing . In a swept back high speed aerofoil, this flow separation and associated pitch down will not be a marked phenomenon. Instead the aircraft enters into a descent. The descent rate further tilts the relative airflow downwards and leads to an increase in angle of attack further driving the aircraft into the stall regime. Any attempt by the pilot to raise the attitude by aft pressure on the elevator will cause a further increase in angle of attack and further loss of altitude.

The lift, however, depends on both air density (kg/m³) and on the plane’s velocity, and air density decreases with altitude. So, the higher you go, the faster you have to fly to stay above the stall speed. As you go higher, temperature also decreases, at least in the troposphere were commercial planes are flying. As the temperature decreases, so does the speed of sound.

Similarly, the critical Mach number is the maximum speed at which the airflow can sustain over the wings without losing lift due to flow separation and shock waves,. Any increase in speed in will cause the airplane to encounter stall effects. When the critical Mach number is exceeded, there is an abrupt rise in drag rise as well as a pitch down due Mach tuck. This can result in aircraft upset, altitude loss and loss of control. As the aircraft descends, the airspeed increases. Excessive pull forces during recovery may lead to further loss of control or structural damage to the airplane.

Modern commercial jet aircraft may suffer both high and low speed stall buffet. The associated boundaries are depicted in the FCOM of the aircraft.  The high speed buffet is caused by flow separation from the wings as occurs behind a shockwave at high altitudes and/or Mach numbers. The low speed buffet is caused by the same airflow separation as the aircraft approaches the stall angle of attack. With stall speed increasing with altitude and sound speed decreasing, the velocity window in which an aircraft can operate becomes narrower and narrower.

Turning manoeuvres at these altitudes increase the angle of attack and results in stability deterioration with  a decrease in control effectiveness. The relationship of stall speeds to critical     Mach number (Mcrit) narrows to a point where sudden increase in angle of attack , roll rates and disturbances cause the limits of the airspeed to be exceeded.

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.

VMO is an aircraft’s indicated airspeed limit. Exceeding the Vmo may cause aerodynamic flutter and G load limitations to become critical during recovery. Structural design  integrity is also not predictable at airspeeds greater than Vmo.

A deeper understanding of the stall characteristics and recovery procedures are important proficiency issues.  When flying at high altitudes, the crew needs to be aware of the margins of safety available, especially when manoeuvring and while riding out turbulence.

To recover from a stall, the attitude needs to be decreased to reduce the angle of attack. The old maxim of Power for ROD or altitude control and Attitude for airspeed control holds good. A burst of power is not the solution for a stall recovery. In all cases, remember  “attitude before power” when you are in a stall.