An airplane's indicated airspeed decreases in relation
to true airspeed as altitude increases. As the indicated
airspeed decreases with altitude, it progressively
merges with the low speed buffet boundary where prestall
buffet occurs for the airplane at a load factor of
1.0 G. The point where the high speed Mach indicated
airspeed and low speed buffet boundary indicated airspeed
merge is the airplane's absolute or aerodynamic
ceiling. Once an airplane has reached its aerodynamic
ceiling, which is higher than the altitude stipulated in
the FAA-approved Airplane Flight Manual, the airplane
can neither be made to go faster without activating
the design stick puller at Mach limit nor can it be
made to go slower without activating the stick shaker
or stick pusher. This critical area of the airplane's
flight envelope is known as "coffin corner."

Mach buffet occurs as a result of supersonic airflow on
the wing. Stall buffet occurs at angles of attack that
produce airflow disturbances (burbling) over the upper
surface of the wing which decreases lift. As density
altitude increases, the angle of attack that is required to
produce an airflow disturbance over the top of the wing
is reduced until the density altitude is reached where
Mach buffet and stall buffet converge (coffin corner).
When this phenomenon is encountered, serious consequences
may result causing loss of airplane control.

Increasing either gross weight or load factor (G factor)
will increase the low speed buffet and decrease Mach
buffet speeds. A typical jet airplane flying at 51,000
feet altitude at 1.0 G may encounter Mach buffet

slightly above the airplane's Mmo (.82 Mach) and low
speed buffet at .60 Mach. However, only 1.4 G (an
increase of only 0.4 G) may bring on buffet at the optimum
speed of .73 Mach and any change in airspeed,
bank angle, or gust loading may reduce this straight and
level flight 1.4 G protection to no protection at all.
Consequently, a maximum cruising flight altitude must
be selected which will allow sufficient buffet margin
for necessary maneuvering and for gust conditions
likely to be encountered. Therefore, it is important for
pilots to be familiar with the use of charts showing
cruise maneuver and buffet limits. [Figure 15-11]

The transitioning pilot must bear in mind that the
maneuverability of the jet airplane is particularly critical,
especially at the high altitudes. Some jet airplanes
have a very narrow span between the high and low
speed buffets. One airspeed that the pilot should have
firmly fixed in memory is the manufacturer's recommended
gust penetration speed for the particular make
and model airplane. This speed is normally the speed
that would give the greatest margin between the high
and low speed buffets, and may be considerably higher
than design maneuvering speed (VA). This means that,
unlike piston airplanes, there are times when a jet
airplane should be flown in excess of VA during
encounters with turbulence. Pilots operating airplanes
at high speeds must be adequately trained to operate
them safely. This training cannot be complete until
pilots are thoroughly educated in the critical aspects of
the aerodynamic factors pertinent to Mach flight at
high altitudes.

Figure 15-11. Mach buffet boundary chart.