Maneuvering During Slow Flight

Introduction:

  • Slow flight develops the ability to recognize changes in aircraft flight characteristics and control effectiveness at critically slow airspeeds in various configurations
  • While pilots may perform slow flight to loiter over an area during cruise flight, it is most often performed incidental to takeoff and landing
  • With multi-engine aircraft the concept of minimum controllable airspeed comes into play
  • Speed Instability:
    • Flying slower than minimum drag speed (LD/max), more power will be required, due to total drag curve and slight disturbances will decrease airspeed
Instrument Flying Handbook. Figure 2-12, Vortex Generators
Instrument Flying Handbook, Vortex Generators

Slow Flight Background:

  • Anytime an aircraft is flying near the stalling speed or the region of reversed command, such as in final approach for a normal landing, the initial part of a go around, or maneuvering in slow flight, it is operating in what is called slow-speed flight
  • If the aircraft weighs 4,000 pounds, the lift produced by the aircraft must be 4,000 pounds
  • When lift is less than 4,000 pounds, the aircraft is no longer able to sustain level flight, and consequently descends
  • During intentional descents, this is an important factor and is used in the total control of the aircraft
  • However, because lift is required during low speed flight and is characterized by high angles of attack, flaps or other high lift devices are needed to either change the camber of the airfoil, or delay the boundary level separation
  • Plain and split flaps are most commonly used to change the camber of an airfoil
  • It should be noted that with the application of flaps, the aircraft will stall at a lower angle of attack
  • The basic wing stalls at 18° without flaps but with the application of the flaps extended (to CL-MAX position) the new angle of attack at which point the aircraft will stall is 15°
  • However, the value of lift (flaps extended to the CL-MAX position) produces more lift than lift at 18° on the basic wing
  • Delaying the boundary layer separation is another way to increase CL-MAX
  • Several methods are employed (such as suction and use of a blowing boundary layer control), but the most common device used on general aviation light aircraft is the vortex generator
  • Small strips of metal placed along the wing (usually in front of the control surfaces) create turbulence
  • The turbulence in turn mixes high energy air from outside the boundary layer with boundary layer air
  • The effect is similar to other boundary layer devices [Figure 2-12]

Region of Reverse Command:

  • The region of reverse command is a flight regime whereby drag and thrust requirements are inverted
    • This is sometimes called the "backside of the power curve," or "behind the power curve"
  • When flying within the region of reverse command, more power is needed as airspeed slows
  • If power required is not maintained, drag creep can be insidious leading to a stall
    • This is a contributor to the higher accident rate in the terminal environment
  • In cruise flight, pitch can control altitude and power can control airspeed
  • In slow flight, the reverse is true, due to AOA
    • In this case, the aircraft pitching up increases AOA and induced drag, resulting in airspeed slowing and altitude dropping (at an extreme its a stall
    • Pitching down increases airspeed but decreases altitude
  • Several articles including Pilot Workshop's - Pitch or Power? and Bold Method's - Why Do I Need To Demonstrate Slow Flight On My Check Ride? explain the region of reverse command, and how it differs from standard cruise flight

    • Small Airplane Specifics:

    • Most small airplanes maintain a speed well in excess of 1.3 times VSO on an instrument approach
    • An airplane with a stall speed of 50 knots (VSO) has a normal approach speed of 65 knots
    • However, this same airplane may maintain 90 knots (1.8 VSO) while on the final segment of an instrument approach
    • The landing gear will most likely be extended at the beginning of the descent to the minimum descent altitude, or upon intercepting the glide slope of the instrument landing system
    • The pilot may also select an intermediate flap setting for this phase of the approach
    • The airplane at this speed has good positive speed stability, as represented by point A on Figure 2-10
    • Flying in this regime permits the pilot to make slight pitch changes without changing power settings, and accept minor speed changes knowing that when the pitch is returned to the initial setting, the speed returns to the original setting
    • This reduces the pilot's workload
    • Aircraft are usually slowed to a normal landing speed when on the final approach just prior to landing
    • When slowed to 65 knots, (1.3 VSO), the airplane will be close to point C
    • [Figure 2-10] At this point, precise control of the pitch and power becomes more crucial for maintaining the correct speed
    • Pitch and power coordination is necessary because the speed stability is relatively neutral since the speed tends to remain at the new value and not return to the original setting
    • In addition to the need for more precise airspeed control, the pilot normally changes the aircraft's configuration by extending landing flaps
    • This configuration change means the pilot must be alert to unwanted pitch changes at a low altitude
    • If allowed to slow several knots, the airplane could enter the region of reversed command
    • At this point, the airplane could develop an unsafe sink rate and continue to lose speed unless the pilot takes a prompt corrective action
    • Proper pitch and power coordination is critical in this region due to speed instability and the tendency of increased divergence from the desired speed

    Large Airplane Specifics:

    • Pilots of larger airplanes with higher stall speeds may find the speed they maintain on the instrument approach is near 1.3 VSO, putting them near point C [Figure 2-10] the entire time the airplane is on the final approach segment
    • In this case, precise speed control is necessary throughout the approach
    • It may be necessary to temporarily select excessive, or deficient thrust in relation to the target thrust setting in order to quickly correct for airspeed deviations
    • For example, a pilot is on an instrument approach at 1.3 VSO, a speed near L/DMAX, and knows that a certain power setting maintains that speed
    • The airplane slows several knots below the desired speed because of a slight reduction in the power setting
    • The pilot increases the power slightly, and the airplane begins to accelerate, but at a slow rate
    • Because the airplane is still in the "flat part" of the drag curve, this slight increase in power will not cause a rapid return to the desired speed
    • The pilot may need to increase the power higher than normally needed to maintain the new speed, allow the airplane to accelerate, then reduce the power to the setting that maintains the desired speed

    Multi-Engine Considerations During Slow Flight:

    • By definition, the term "flight at minimum controllable airspeed" (MCA) means a speed at which any further increase in angle of attack or load factor, or reduction in power will cause an immediate stall
    • Minimum controllable airspeed (VMC) is the airspeed below which aircraft control cannot be maintained if the critical engine fails
    • MCA is dependent on gross weight, load factors, and existing density altitude

    Maneuvering During Slow Flight Procedure:

    WARNING:
    All procedures are GENERALIZED.
    Always fly per Pilot Operating Handbook procedures,
    observing any relevant Standard Operating Procedures (SOPs)


    1. Perform clearing turns
    2. Reduce power and adjust pitch to maintain altitude
      • Trim as necessary
      • Maintain heading
      • Because the controls will be less effective, more trim will be required to relieve back pressure
    3. If performing for the "dirty" configuration, perform the following:
      • Below VLO, extend the landing gear
      • Below VFE, extend the flaps to full
    4. Advance the propeller control to full forward (high rpm) as required
    5. When approximately 5 knots above target speed (MCA), increase power to maintain altitude
      • Trim as necessary
      • Remember speed instability
    6. Turn, climb, and descend as directed
      • Remember to be smooth with the controls as airfoils are less effective at slower speeds
    7. To recover smoothly and continuously, increase power to full, adjust pitch to maintain airspeed and constantly maintaining heading
      • Trim as necessary
    8. If performing for the "dirty" configuration, perform the following:
      • As airspeed increases, raise the flaps in increments, to 10 °:
        • Too abrupt of flap retraction will result in a dramatic loss of lift and possibly stall
      • As airspeed increases, but below VLO raise the landing gear
      • At or above Vx retract flaps to 0°
    9. As cruise airspeed is attained, set cruise power
    10. Re-trim as necessary
    11. Complete cruise checklist

    Manevuering During Slow Flight Common Errors:

    • 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

    Maneuvering During Slow Flight Airman Certification Standards:

      Maneuvering During Slow Flight:

      • To determine that the applicant exhibits satisfactory knowledge, risk management, and skills associated with maneuvering during slow flight
      • Note: See Appendix 6: Safety of Flight and Appendix 7: Aircraft, Equipment, and Operational Requirements & Limitations
      • References: FAA-H-8083-2, FAA-H-8083-3; POH/AFM

      Maneuvering During Slow Flight Knowledge:

      The applicant must demonstrate an understanding of:
      • PA.VII.A.K1:
        Aerodynamics associated with slow flight in various airplane configurations, to include the relationship between angle of attack, airspeed, load factor, power setting, airplane weight and center of gravity, airplane attitude, and yaw effects

      Maneuvering During Slow Flight Risk Management:

      The applicant demonstrates the ability to identify, assess, and mitigate risks, encompassing:
      • PA.VII.A.R1:
        Inadvertent slow flight and flight with a stall warning, which could lead to loss of control
      • PA.VII.A.R2:
        Range and limitations of stall warning indicators (e.g., airplane buffet, stall horn, etc.)
      • PA.VII.A.R3:
        Failure to maintain coordinated flight
      • PA.VII.A.R4:
        Effect of environmental elements on airplane performance (e.g., turbulence, microbursts, and high-density altitude)
      • PA.VII.A.R5:
        Collision hazards, to include aircraft, terrain, obstacles, and wires
      • PA.VII.A.R6:
        Distractions, loss of situational awareness, or improper task management.

      Maneuvering During Slow Flight Skills:

      The applicant demonstrates the ability to:
      • PA.VII.A.S1:
        Clear the area
      • PA.VII.A.S2:
        Select an entry altitude that will allow the Task to be completed no lower than 1,500 feet AGL (ASEL, ASES) or 3,000 feet AGL (AMEL, AMES)
      • PA.VII.A.S3:
        Establish and maintain 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., airplane buffet, stall horn, etc.)
      • PA.VII.A.S4:
        Accomplish coordinated straight-and-level flight, turns, climbs, and descents with the airplane configured as specified by the evaluator without a stall warning (e.g., airplane buffet, stall horn, etc.)
      • PA.VII.A.S5:
        Maintain the specified altitude, ±100 feet; specified heading, ±10°; airspeed, +10/-0 knots; and specified angle of bank, ±10°

    Maneuvering During Slow Flight Case Studies:

    Conclusion:

    • While there are several reasons to practice slow flight, the beginner pilot will find the most translation into flight within the traffic pattern
    • Some flight maneuvers do require flight at minimum controllable airspeed such as the Chandelle and Lazy Eights
    • Conventional training and evaluation used to require a stall warning buzzer to activate and be held
      • This is no longer the case, nor is it desirable, as this requires (encourages) intentional disregard of the warning
      • Airman Certification Standards therefore carefully word the task to "establish and maintain 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, stall horn, etc.)"
    • Coordination is key, as uncoordinated flight close to stall speed can induce a spin
    • Consider practicing maneuvers on a flight simulator to introduce yourself to maneuvers or knock off rust
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    References: