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
should be lowered and the pitch attitude adjusted to
maintain altitude.
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 controls.
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 drag
continues 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 do so.
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 are:
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 and "mushing."
Inadequate compensation for adverse yaw during
turns.
Fixation on the airspeed indicator.
Failure to anticipate changes in lift as flaps are
extended or retracted.
Inadequate power management.
Inability to adequately divide attention between
airplane control and orientation.
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