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Flight, Stalls, and Spins > Slow
Flight > Flight At Minimum Controllable Airspeed
This maneuver demonstrates the flight characteristics
and degree of controllability of the airplane at its minimum
flying speed. By definition, the term “flight at minimum
controllable airspeed” means a speed at which any further
increase in angle of attack or load factor, or reduction in
power will cause an immediate stall. Instruction in flight at
minimum controllable airspeed should be introduced at reduced
power settings, with the airspeed sufficiently above the stall
to permit maneuvering, but close enough to the stall to sense
the characteristics of flight at very low airspeed—which
are sloppy controls, ragged response to control inputs, and
difficulty maintaining altitude. Maneuvering at minimum controllable
airspeed should be performed using both instrument indications
and outside visual reference. It is important that pilots form
the habit of frequent reference to the flight instruments, especially
the airspeed indicator, while flying at very low airspeeds.
However, a “feel” for the airplane at very low airspeeds
must be developed to avoid inadvertent stalls and to operate
the airplane with precision.
To begin the maneuver, the throttle is gradually
reduced from cruising position. While the airspeed is decreasing,
the position of the nose in relation to the horizon should be
noted and should be raised as necessary to maintain altitude.
When the airspeed reaches the maximum allowable
for landing gear operation, the landing gear (if equipped with
retractable gear) should be extended and all gear down checks
performed. As the airspeed reaches the maximum allowable for
flap operation, full flaps
figure4-1. Slow flight—Low airspeed,
high angle of attack, high power, and constant altitude.
should be lowered and the pitch attitude adjusted
to maintain altitude. [figure4-1] Additional power will be
required as the speed further decreases to maintain the airspeed
just above a stall. As the speed decreases further, the pilot
should note the feel of the flight controls, especially the
elevator. The pilot should also note the sound of the airflow
as it falls off in tone level.
As airspeed is reduced, the flight controls
become less effective and the normal nosedown tendency is reduced.
The elevators become less responsive and coarse control movements
become necessary to retain control of the airplane. The slipstream
effect produces a strong yaw so the application of rudder is
required to maintain coordinated flight. The secondary effect
of applied rudder is to induce a roll, so aileron is required
to keep the wings level. This can result in flying with crossed
During these changing flight conditions, it
is important to retrim the airplane as often as necessary to
compensate for changes in control pressures. If the airplane
has been trimmed for cruising speed, heavy aft control pressure
will be needed on the elevators, making precise control impossible.
If too much speed is lost, or too little power is used, further
back pressure on the elevator control may result in a loss of
altitude or a stall. When the desired pitch attitude and
minimum control airspeed have been established, it is important
to continually cross-check the attitude indicator, altimeter,
and airspeed indicator, as well as outside references to ensure
that accurate control is being maintained.
The pilot should understand that when flying
more slowly than minimum drag speed (LD/MAX) the airplane will
exhibit a characteristic known as “speed instability.”
If the airplane is disturbed by even the slightest turbulence,
the airspeed will decrease. As airspeed decreases, the total
drag also increases resulting in a further loss in airspeed.
The total dragcontinues to rise and the speed continues to fall.
Unless more power is applied and/or the nose is lowered, the
speed will continue to decay right down to the stall. This is
an extremely important factor in the
performance of slow flight. The pilot must
understand that, at speed less than minimum drag speed, the
airspeed is unstable and will continue to decay if allowed to
When the attitude, airspeed, and power have
been stabilized in straight flight, turns should be practiced
to determine the airplane’s controllability characteristics
at this minimum speed. During the turns, power and pitch attitude
may need to be increased to maintain the airspeed and altitude.
The objective is to acquaint the pilot with the lack of maneuverability
at minimum speeds, the danger of incipient stalls, and the tendency
of the airplane to stall as the bank is increased. A stall may
also occur as a result of abrupt or rough control movements
when flying at this critical airspeed.
Abruptly raising the flaps while at minimum
controllable airspeed will result in lift suddenly being lost,
causing the airplane to lose altitude or perhaps stall.
Once flight at minimum controllable airspeed
is set up properly for level flight, a descent or climb at minimum
controllable airspeed can be established by adjusting the power
as necessary to establish the desired rate of descent or climb.
The beginning pilot should note the increased yawing tendency
at minimum control airspeed at high power settings with flaps
fully extended. In some airplanes, an attempt to climb at such
a slow airspeed may result in a loss of altitude, even with
maximum power applied
Common errors in the performance of slow flight
• Failure to adequately clear the area.
• Inadequate back-elevator pressure as power is reduced,
resulting in altitude loss.
• Excessive back-elevator pressure as power is reduced,
resulting in a climb, followed by a rapid reduction in airspeed
• Inadequate compensation for adverse yaw during turns.
• Fixation on the airspeed indicator.
• Failure to anticipate changes in lift as flaps are extended
• Inadequate power management.
• Inability to adequately divide attention between airplane
control and orientation.
figure4-2. Critical angle of attack