Introduction
Airplanes operate at low airspeeds and high AOA during the takeoff and landing phases of flight. Since the airplane is close to the ground, these phases of flight are critical, and a loss of control could be catastrophic.
This lesson introduces the procedural steps, control techniques, and aerodynamic principles for slow flight by practicing various maneuvers at a safe altitude. The purpose is to become familiar with the feel, sound, and visual cues of flight in this regime, where there is a degraded response to control inputs. This training helps pilots recognize an imminent stall.
Objectives
After this lesson, the learner will be able to:
- Describe the aerodynamics associated with slow flight in various aircraft configurations.
- Describe the flight characteristic known as speed instability.
- Accomplish coordinated turns, climbs, and descents at an airspeed just above stall.
- Recognize the cues of an imminent stall (e.g., sight, sound, and feel).
- Make appropriate corrections should a stall warning occur.
Lesson Briefing
Risk Management
- Inadvertent slow flight and flight with a stall warning, which could lead to loss of control
- Range and limitations of stall warning indicators (e.g., airplane buffet, stall horn, etc.)
- Failure to maintain coordinated flight
- Effect of environmental elements on airplane performance (e.g., turbulence, microbursts, and high-density altitude)
- Collision hazards, to include aircraft, terrain, obstacles, and wires
- Distractions, loss of situational awareness, or improper task management
Scenario
You are approaching to land at a busy, nontowered airport. You enter the downwind behind a slower, light-sport airplane. To avoid gaining on the other airplane, you need to reduce your speed and execute a series of s-turns.
Describe and demonstrate how to configure and maintain the slowest airspeed that can be flown safely.
Resources
- Airplane Flying Handbook (FAA-H-8083-3):
- Chapter 5, Maintaining Aircraft Control: Upset Prevention and Recovery Training
- AC 61-67: Stall and Spin Awareness Training
Schedule
- Lesson Briefing (0:20)
- Demonstrations and Practice (0:20)
- Lesson Debriefing (0:10)
Equipment
- Whiteboard, markers, and erasers
- Airplane models
- Airplane checklists
- Headsets and flight gear
Lesson Debriefing
This lesson concludes with a collaborative assessment and review of the main points and risk management items.
Additionally, the instructor ensures:
- All of the learner’s questions are resolved.
- The learner is made aware of his or her performance and progress.
Completion Standards
This lesson is complete when the lesson objectives are met and the learner’s knowledge, risk management, and skills are determined to be adequate for the stage of training. Ultimately, the learner must meet or exceed the Airman Certification Standards.
Lesson Content
What is Slow Flight?
Slow flight is when the airplane is flown at an angle of attack (AOA) just below the angle that causes an aerodynamic buffet or the activation of a stall warning device. A small increase in AOA may result in an impending stall, which increases the risk of an actual stall.
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 (5–10 knots above the 1G stall speed is a good target).
- Performing slow flight in configurations appropriate to takeoffs, climbs, descents, approaches, and go-arounds.
Special Emphasis Items
Left-Turning Tendencies: In propeller-driven airplanes, torque, slipstream effect, and P-factor produce strong left yaw, which requires right rudder input to maintain coordinated flight.
Reduced Control Effectiveness: The airplane is less responsive to flight control inputs at slower airspeeds. Pilots sometimes refer to it as “sloppy” or “mushy” controls.
Reduced Forward Visibility: The horizon may not be visible in front of the airplane in high-pitch attitudes. The “Lindbergh” visual reference can help maintain coordinated flight.
Power:
- A combination of higher power settings and reduced airflow into the cowling results in higher engine temperatures.
- In fixed-pitch propeller airplanes, a loss of RPMs may be evident when at high angles of attack.
Region of Reversed Command:
- The airplane exhibits a characteristic known as speed instability, in which lower airspeeds require higher power settings to hold altitude.
- Small pitch changes result in large airspeed changes due to variations in induced drag. As a result, pitch becomes more effective for controlling airspeed and power more effective for controlling altitude.
Bodily Sensations Experienced
Feel: Control pressures change as airspeed is reduced. Just before the stall occurs, buffeting, uncommanded rolling, or vibrations may begin to occur.
Vision: When a stall results from an unusual attitude, vision is useful for checking the pitch attitude. However, an airplane can also be stalled in a normal attitude, in which case vision cannot aid in detecting the stall.
Hearing: As airspeed decreases, a change in sound is made by the air flowing along the airplane structure. A loss of engine RPM is noticeable in an airplane with a fixed-pitch propeller.
Kinesthesia: Changes in direction or speed can be sensed by experienced pilots to warn of an impending stall.
Region of Reversed Command
The power required to achieve equilibrium in constant-altitude flight at various airspeeds is depicted on a power-required curve. The lowest point on the curve represents the speed at which the lowest brake horsepower sustains level flight. This is the best endurance airspeed.
Flight at speeds below the best endurance airspeed is known as “flying on the backside of the power curve” or “flying in the region of reversed command.”
In the region of reversed command:
- A higher airspeed requires a lower power setting to hold altitude.
- A lower airspeed requires a higher power setting to hold altitude.
- The aircraft likely exhibits no inherent tendency to maintain the trimmed speed.
Speed Instability
Speed instability is a condition in the region of reverse command that causes the aircraft to display negative longitudinal (pitch) stability because of the increased induced drag.
When a disturbance causes the airspeed to decrease, total drag increases, causing the airspeed to decrease further. Airspeed continues to decay without appropriate pilot action.
When an aircraft is established in steady, level flight in the region of reversed command:
- If airspeed is increased, an excess of power exists. The aircraft accelerates to an even higher speed.
- If airspeed is decreased, a deficiency of power exists. The aircraft continues to slow down.
Speed Stability During Visual Approaches
Speed instability may be encountered in slow-speed phases of flight, such as during takeoff and landing; however, this region is small and is below the normal approach speed (1.3 VSO) of most general aviation aircraft.
Most landing approaches are flown in an area of neutral speed stability, where small speed changes result in little or no change in drag or power required.
Speed Stability During Instrument Approaches [IFR]
Instrument approaches are generally flown in an area of positive speed stability (approximately 1.8 VSO) to reduce workload. Operating in this regime permits the pilot to make slight pitch changes without changing power settings. Minor speed changes are accepted, knowing that the speed returns to the original setting when the pitch is returned. The aircraft is slowed to a normal approach speed just before landing.
Target Airspeed for Slow Flight
Reference: SAFO 17009
Maneuvering the airplane in slow flight is practiced at an airspeed just above the speed that would result in a stall warning (e.g., aircraft buffet or stall horn). If a stall warning occurs, the expectation is the pilot will take the appropriate action to correct it.
One procedure to establish this airspeed is to:
- Slow the airplane to the stall warning in the desired configuration and note the airspeed.
- Reduce the pitch or angle of attack (AOA) slightly and eliminate the stall warning indication.
- Adjust power to maintain altitude, and note the airspeed required to perform the slow flight maneuver.
Minimum Controllable Airspeed (Historical)
Previously, pilots were trained to maneuver during slow flight at the minimum controllable airspeed, the airspeed at which any further increase in AOA, an increase in load factor, or reduction in power would result in an immediate stall. The FAA has since found that it is not appropriate for a pilot to fly with the stall warning present.
Angle of Attack Indicators
Reference: InFO 14010
AOA indicators measure several flight parameters to determine the current AOA and the proximity to the critical AOA. Their purpose is to provide pilots with better stall margin awareness.
AOA information is provided to the pilot as a visual display. Optionally, an aural warning can supplement the display.
How to Perform Slow Flight
Entry
- Turn carburetor heat ON if necessary.
- Gradually reduce thrust from cruise power and adjust the pitch to allow the airspeed to decrease while maintaining altitude.
- When the airspeed is below the maximum landing gear extension speed (VLE), extend the landing gear, if applicable.
- Extend the flaps in increments once the airspeed is at or below the maximum flap extension speed (VFE).
- As the airplane slows down:
- Increase the pitch attitude gradually until reaching the target airspeed.
- Add power to maintain altitude.
- Apply rudder pressure to maintain coordinated flight.
- Adjust the trim as necessary.
- Continually cross-check outside visual references and instrument references to maintain accurate control.
Execution
- Accomplish straight-and-level flight, turns, climbs, and descents as specified by the instructor or evaluator.
At All Times:
- Ensure the airplane remains trimmed for “hands-off” flight.
- Adjust pitch to maintain the airspeed and power to maintain altitude.
- Apply right rudder to correct for left-turning tendencies, which are more prominent at low airspeeds.
- Make smooth and coordinated flight control applications. Abrupt control movements may result in a stall.
Turns:
- Turns should be practiced at various bank angles up to approximately 20°.
- Apply adequate rudder pressure to maintain coordinated flight when starting and rolling out of a turn.
- Add power to maintain altitude.
Climbs and Descents:
- Before making any changes, note the power setting for later use.
- To initiate a climb or descent, make a power adjustment. Adjust the pitch attitude to control airspeed.
- Due to the additional drag created from the landing gear and flaps being extended, large power settings are required when executing climbs.
Exit
- Smoothly apply forward pressure to reduce the angle of attack (AOA).
- Maintain coordinated flight and level the wings if necessary.
- Apply power to return to the desired flightpath.
- As airspeed increases, retract the flaps in increments.
- Retract the landing gear, if applicable.
- Verify that carburetor heat is OFF, if equipped.
- Return to the normal cruise speed, trimming as necessary.
- Complete the cruise checklist.
Common Errors for Slow Flight
- Failure to scan for traffic before and during the maneuver
- 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 rudder pressure to correct for torque or adverse yaw
- Fixation on the flight instruments
- Failure to anticipate pitch changes 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
Airman Certification Standards for Slow Flight
References: SAFO 17009, FAA-S-8081-29, FAA-S-ACS-6, FAA-S-ACS-7
Note: Environmental factors (e.g., turbulence) may result in a momentary activation of stall warning indicators such as the stall horn. If the applicant recognizes the stall warning indication and promptly makes an appropriate correction, a momentary activation does not constitute unsatisfactory performance.
Target Speed:
- SPT: An airspeed at which any further increase in angle of attack, increases in load factor, or reduction in power, would result in an immediate stall (minimum controllable airspeed).
- PVT and COM: An airspeed at which any further increase in angle of attack, increase in load factor, or reduction in power, would result in a stall warning (e.g., aircraft buffet or stall horn).
Configuration: As specified by the evaluator.
Airspeed | Altitude | Bank | Heading |
---|---|---|---|
+10/-0 KIAS (SPT, PVT) +5/-0 KIAS (COM) No stall warning | ±100′ (SPT, PVT) ±50′ (COM) ≥ 1,500′ AGL (ASEL) ≥ 3,000′ AGL (AMEL) | ±10° (SPT, PVT) ±5° (COM) | ±10° |