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.
