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:
- 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
- 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. [Figure 1]
|Figure 1. Angle-of-attack in degrees|
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 power-off 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 2] 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.
|Figure 2. Slow flight—low airspeed, high angle of attack, high power, and constant altitude|
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