Slow Flight

From Airplane Flying Handbook (FAA-H-8083-3B) Chapter 4 March 2020.

Slow flight is when the airplane AOA is just under the AOA
which will cause an aerodynamic buffet or a warning from a
stall warning device if equipped with one. A small increase in
AOA may result in an impending stall, which increases the risk
of an actual stall. In most normal flight operations the airplane
would not be flown close to the stall-warning AOA or critical
AOA, but because the airplane is flown at higher AOAs, and
thus reduced speeds in the takeoff/departure and approach/
landing phases of flight, learning to fly at reduced airspeeds is
essential. In these phases of flight, the airplane’s close proximity
to the ground would make loss of control catastrophic;
therefore, the pilot must be proficient in slow flight.

The objective of maneuvering in slow flight is to understand
the flight characteristics and how the airplane’s flight controls
feel near its aerodynamic buffet or stall-warning. It also
helps to develop the pilot’s recognition of how the airplane
feels, sounds, and looks when a stall is impending. These
characteristics include, degraded response to control inputs
and difficulty maintaining altitude. Practicing slow flight will
help pilots recognize an imminent stall not only from the feel
of the controls, but also from visual cues, aural indications,
and instrument indications.

For pilot training and testing purposes, slow flight includes
two main elements:
1. Slowing to, maneuvering at, and recovering from
an airspeed at which the airplane is still capable of
maintaining controlled flight without activating the
stall warning—5 to 10 knots above the 1G stall speed
is a good target; and
2. Performing slow flight in configurations appropriate
to takeoffs, climbs, descents, approaches to landing,
and go-arounds.

Slow flight should be introduced with the airspeed
sufficiently above the stall to permit safe maneuvering, but
close enough to the stall warning for the pilot to experience
the characteristics of flight at a very low airspeed. One way
to determine the target airspeed is to slow the airplane to the
stall warning when in the desired slow flight configuration,
pitch the nose down slightly to eliminate the stall warning,
add power to maintain altitude and note the airspeed.

When practicing slow flight, a pilot learns to divide attention
between aircraft control and other demands. How the airplane feels at the slower airspeeds aids the pilot in learning that
as airspeed decreases, control effectiveness decreases. For
instance, reducing airspeed from 30 knots to 20 knots above
the stalling speed will result in a certain loss of effectiveness
of flight control inputs because of less airflow over the
control surfaces. As airspeed is further reduced, the control
effectiveness is further reduced and the reduced airflow over
the control surfaces results in larger control movements
being required to create the same response. Pilots sometimes
refer to the feel of this reduced effectiveness as “sloppy” or
“mushy” controls.

When flying above minimum drag speed (L/DMAX), even
a small increase in power will increase the speed of the
airplane. When flying at speeds below L/DMAX, also referred
to as flying on the back side of the power curve, larger
inputs in power or reducing the AOA will be required for
the airplane to be able to accelerate. Since slow flight will be
performed well below L/DMAX, the pilot must be aware that
large power inputs or a reduction in AOA will be required
to prevent the aircraft from decelerating. It is important to
note that when flying on the backside of the power curve,
as the AOA increases toward the critical AOA and the
airplane’s speed continues to decrease, small changes in
the pitch control result in disproportionally large changes in
induced drag and therefore changes in airspeed. As a result,
pitch becomes a more effective control of airspeed when
flying below L/DMAX and power is an effective control of
the altitude profile (i.e., climbs, descents, or level flight)

It is also important to note that an airplane flying below
L/DMAX, exhibits a characteristic known as “speed instability”
and the airspeed will continue to decay without appropriate
pilot action. For example, if the airplane is disturbed by
turbulence and the airspeed decreases, the airspeed may
continue to decrease without the appropriate pilot action of
reducing the AOA or adding power.

Performing the Slow Flight Maneuver
Slow flight should be practiced in straight-and-level
flight, straight-ahead climbs and climbing medium-banked
(approximately 20 degrees) turns, and straight-ahead poweroff gliding descents and descending turns to represent the
takeoff and landing phases of flight. Slow flight training
should include slowing the airplane smoothly and promptly
from cruising to approach speeds without changes in altitude
or heading, and understanding the required power and
trim settings to maintain slow flight. It should also include
configuration changes, such as extending the landing gear
and adding flaps, while maintaining heading and altitude.
Slow flight in a single-engine airplane should be conducted
so the maneuver can be completed no lower than 1,500 feet
AGL, or higher, if recommended by the manufacturer. In
all cases, practicing slow flight should be conducted at an
adequate height above the ground for recovery should the
airplane inadvertently stall.

To begin the slow flight maneuver, clear the area and
gradually reduce thrust from cruise power and adjust the
pitch to allow the airspeed to decrease while maintaining
altitude. As the speed of the airplane decreases, note a change
in the sound of the airflow around the airplane. As the speed
approaches the target slow flight speed, which is an airspeed
just above the stall warning in the desired configuration
(i.e., approximately 5–10 knots above the stall speed for
that flight condition), additional power will be required to
maintain altitude. During these changing flight conditions, it
is important to trim the airplane to compensate for changes in
control pressures. If the airplane remains trimmed for cruising
speed (a lower AOA), strong aft (back) control pressure is
needed on the elevator, which makes precise control difficult
unless the airplane is retrimmed.

Slow flight is typically performed and evaluated in the
landing configuration. Therefore, both the landing gear
and the flaps should be extended to the landing position.
It is recommended the prescribed before-landing checks
be completed to configure the airplane. The extension of
gear and flaps typically occurs once cruise power has been
reduced and at appropriate airspeeds to ensure limitations
for extending those devices are not exceeded. Practicing this
maneuver in other configurations, such as a clean or takeoff
configuration, is also good training and may be evaluated
on the practical test.

With an AOA just under the AOA which may cause an
aerodynamic buffet or stall warning, the flight controls
are less effective. [Figure 4-3] The elevator control is less
responsive and larger control movements are necessary to
retain control of the airplane. In propeller-driven airplanes,
torque, slipstream effect, and P-factor may produce a strong left yaw, which requires right rudder input to maintain
coordinated flight. The closer the airplane is to the 1G stall,
the greater the amount of right rudder pressure required.

Maneuvering in Slow Flight
When the desired pitch attitude and airspeed have been
established in straight-and-level slow flight, the pilot must
maintain awareness of outside references and continually
cross-check the airplane’s instruments to maintain control.
The pilot should note the feel of the flight controls, especially
the airspeed changes caused by small pitch adjustments,
and the altitude changes caused by power changes. The
pilot should practice turns to determine the airplane’s
controllability characteristics at this low speed. During the
turns, it will be necessary to increase power to maintain
altitude. Abrupt or rough control movements during slow
flight may result in a stall. For instance, abruptly raising the
flaps while in slow flight can cause the plane to stall.

The pilot should also practice climbs and descents by
adjusting the power when stabilized in straight-and-level
slow flight. The pilot should note the increased yawing
tendency at high power settings and counter it with rudder
input as needed.

To exit the slow flight maneuver, follow the same procedure
as for recovery from a stall: apply forward control pressure
to reduce the AOA, maintain coordinated flight and level the
wings, and apply power as necessary to return to the desired
flightpath. As airspeed increases, clean up the airplane by
retracting flaps and landing gear if they were extended. A
pilot should anticipate the changes to the AOA as the landing
gear and flaps are retracted to avoid a stall.

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
• Insufficient right rudder to compensate for left yaw
• Fixation on the flight instruments
• Failure to anticipate changes in AOA as flaps are
extended or retracted
• Inadequate power management
• Inability to adequately divide attention between
airplane control and orientation
• Failure to properly trim the airplane
• Failure to respond to a stall warning