Slow Flight

Reference: The Airplane Flying Manual

Slow Flight: Flight at minimum controllable airspeed. A speed at which any further increase in angle of attack or load factor, or reduction in power, would cause an immediate stall.

Why Learn Slow Flight?

Slow flight increases stall awareness and allows the pilot to learn the aircraft’s flight characteristics and degree of controllability at less than cruise airspeeds.

During takeoff and landing, the aircraft is being operated at slower air speeds and is low to the ground. Learning how to operate the airplane safely is paramount. Practicing slow flight at higher altitudes first, creates a safer learning environment when the student begins operations which require slow flight low to the ground.

Slow flight can be broken down to two elements for pilot training:

  1. The establishment, maintenance & maneuvering of the aircraft at airspeeds and in configurations appropriate to take off, climbs, descents, approaches, landings, & go arounds
  2. Maneuvering at the slowest airspeed at which the airplane is capable of maintaining controlled flight without indications of a stall – typically 5-10 knots above stalling speeds

Characteristics of flight at minimum airspeed (5-10 knots above stall speed) are:

  • Sloppy controls
  • Ragged response to control inputs
  • Difficulty maintaining altitude

The Key Components of Slow Flight

Angle of Attack: 

  • Slow flight is when the airplane is just under the angle of attack at which an aerodynamic buffet would be felt or a stall alarm would go off. 
  • Operating at high AOA causes loss of airflow over the top of the wings, thus reducing the wing’s lift and the effectiveness of the control surfaces. 

 Power Setting:

  • When flying above minimum drag speed a.k.a L/Dmax, a small increase in power will increase the speed of the airplane. 
  • When flying below L/Dmax, known as 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. 
  • Slow flight is performed well below L/Dmax, therefore the pilot must be aware that large power inputs or a reduction in AOA will be required to prevent the airplane from deccelerting. 
  • When flying on the backside of the power curve, as the AOA increase toward the critical AOA and the airplane’s speed continues to decrease, small changes in the pitch control will result in disproportionately large changes in induced drag and, therefore, airspeed. 

Speed Instability:

  • Occurs when flying at airspeeds less than L/Dmax
  • Airspeed is unstable and will continue to decay if allowed to do so
  • If the airplane is disturbed by the slightest turbulence airspeed will decrease, increasing drag
  • In this condition, unless the nose is lowered or more power is applied the airspeed will decay into a stall condition
  • At reduced airspeeds a loss of control effectiveness is experienced because there is less airflow over the control surfaces; causing larger control movements to elicit the same response. 

Load Factors:

  • During slow flight, any increase in the load factor on the aircraft will cause the aircraft to stall. Large banks, abrupt control forces or turbulent conditions could impose even slight load factors which would cause a problem for a low and slow aircraft. 

Airplane Weight and CG:

  • An aircraft which is heavy will require more lift to sustain flight. Thus, accomplishing slow flight will require additional power at high AOA. 
  • A heavily loaded aircraft will stall at a high airspeed, thus making pushing slow flight to a higher margin of airspeed to hold before the aircraft stalls
  • An aft center of gravity will cause the nose to pitch up, making recovery from slow flight increasingly difficult. 

 Airplane Attitude:

  • During slow flight if power cannot be added to maintain forward movement and airspeed then the aircraft pitch attitude must be lowered. This will reduce the AOA and increase lift on the aircraft’s wings. 

Yaw (The Left Turning Tendency in Slow Flight):

  • In the configuration for slow flight, the airplane will exhibit strong left turning tendencies due increases in the torque and “P” factor effect
  • Operating at a high AOA, the descending blade (right on American-made airplanes) has a higher angle of attack, which means it creates more lift. And that yaws the aircraft to the left. P-factor is especially noticeable at high power settings with a high nose attitude.
  • To a pilot, torque is the force that causes an opposite rotation. As the propeller spins clockwise (as viewed from the pilot seat), the airplane experiences a rotating force in the opposite direction. Meaning the airplane wants to roll left, an effect that is dampened by the wings. Torque increases with an increase in power.

Elements of Performing Slow Flight

Throughout the maneuver both instrument indications and outside visual references are to be used.

  • Slow flight is to be practiced from: 
    • Straight & level flight
    • Straight glides
    • Level turns
    • Medium banked gliding turns
  • Slow flight should include:
    • Slowing the airplane smoothly & promptly from cruise power to approach speeds 
    • Without changes in altitude or heading
    • And determining & using the appropriate power & trim settings
    • Conducting configuration changes with the landing gear and flaps while maintaining altitude & heading

Practicing Slow Flight

Now, with a better understanding of slow flight, lets walk through how we are going to perform this maneuver in the air.  

From the Airplane Flying Handbook FAA

To Begin Slow Flight

  1. Gradually retard throttle from cruising position & Raise the nose as necessary to maintain altitude
  2. Extend the landing gear (if equipped) & Lower the flaps gradually 
  3. Apply rudder as necessary;  the slipstream effect will induce strong yaw
  4. Apply corrective aileron as necessary to maintain wings level; this will counteract the rolling effect induced by the application of the rudder
  5. Retrim the aircraft as necessary to relieve flight control pressure

Once Established

  1. Practice level turns (Power & pitch may need to be adjusted to hold altitude)
  2. Abrupt movements of the flight controls will cause a stall
  3. Practice descents and climbs 
  4. Adjust pitch and power appropriately
  5. Note the increased yaw tendency during slow flight with high power settings and the flaps fully extended

Recovering from Slow Flight

  1. Lower the nose, reducing the angle of attack and allow airspeed to build
  2. Gradually increase throttle settings
  3. Raise the flaps incrementally
  4. Raise the landing gear 
  5. Retrim for cruise configuration & run the cruise checklist

A Final Note

 

Slow flight is often introduced to the student pilot in the first few lessons of private pilot training. It is a fundamental maneuver to master in the journey of learning to fly. For those pilots who are already licensed, I would highly encourage you to practice slow flight with a qualified certified flight instructor on your next lesson or flight review. While the critical angle of attack should never be exceeded during slow flight, the aircraft is operating very close to a stalled condition. It is important to ensure that, should a stall occur, it is recoverable and is not allowed to develop into a spin.

The current ACS standards requires that the pilot establish slow flight 5-10 knots above stall speed, without any indication of a stall. Since most stall warning horns alarm at these air speeds, the FAA has published SAFO #16010 as guidance for how to perform the maneuver on the private pilot check ride. In the SAFO #16010 the FAA writes;

“One way to set up for the maneuver is to slow the airplane to the stall warning in the desired configuration and note the airspeed.

Next, pitch down slightly to eliminate the stall warning, adjust power to maintain altitude, and note the airspeed required to perform the slow flight maneuver in accordance with the standard. For example, the pilot may first note that the stall warning comes on at 50 knots. A slight pitch down to eliminate the warning, while adjusting the power to maintain altitude, might then cause the airspeed to increase to 52 knots.

That 52 knots would be the base airspeed to perform the slow flight maneuver. The pilot can adjust pitch and power as necessary during the maneuver to stay within the ACS airspeed standard of +10/-0 knots … without activating the stall warning.”

I use the method recommended to train student pilots for slow flight during their check ride. This information is direct from the FAA on how to set up the maneuver. Of course, as always, check with your CFI and your designated pilot examiner to ensure clarity on their expectations for your check ride.

As always, fly safe aviators! Thanks for joining in on the topic of slow flight.

*Consult your flight instructor for more information on slow flight. Please practice with a CFI and do not practice solo using this article as your predominant, reliant education on the maneuver.*

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