PPL Skills - Stalls and Slow Flight


Introduction

Slow flight and stalls are primarily in the context of dealing with stalls during the take-off / climb-out, or landing / approach phases of flight. During these phases, the pilot will be flying the aircraft near its stall speeds and it is therefore important to recognize the characteristics of the aircraft near stall speeds, as well as know the most effective way of recovering from them. Practicing maneuvering during slow flight not only builds up the pilot's general flying skill, but also develops his competency at maneuvering during these critical phases of flight. To practice slow flight, it makes sense that one first learns about stalls and how to recover from them.

Two types of stalls are practiced: power-on and power-off stalls. Practicing power-on stalls help prepare the pilot for stalls encountered during take-off and climb, while power-off stalls help prepare pilots for stalls encountered during descent and landing. Both can be either wings-level or turning stalls. Practicing stalls is important because it allows pilots to recognize the symptoms of a stall developing before it actually occurs, and if it does occur, to recover promptly, effectively and confidently.


Introduction to Stalling
  • Occurs when the critical angle of attack (AOA) is exceeded
    • Recall that AOA is defined as the angle at which the chord line (the imaginary line joining the leading edge of the wing to the trailing edge) meets the relative wind
    • Causes disruption of smooth airflow over the top of the wings - causes a loss of lift
  • Can occur at any speed, altitude and power setting
  • How does stall depend on the AOA and not the airspeed?
    • tl;dr AOA is relative to oncoming wind and not the horizon - an aircraft climbing at 1000fpm (relative wind comes from above) will have a lower AOA compared to the same aircraft maintaining altitude at the same nose-up pitch attitude
  • Pilots should remember that the stall speed is only valid for:
    • 1G, unaccelerated flight
    • Coordinated flight
    • Specific weight
    • Specific centre of gravity
  • Impending stall
    • Before the critical AOA is reached
    • Buffeting (vibrations as a result of disrupted airflow over the wings), stall warnings and stick shakers, uncommanded rolls. 
    • Control surfaces have less resistance and less effectiveness, larger deflections required to be effective
  • Full stall: 
    • The actual stall, after critical AOA
    • Usually an uncommanded nose-down pitch that can't be countered, may be accompanied by a roll. 
    • Stick pushers, if equipped, will be activated
  • Recovery from stalls will involve a loss in altitude. It goes without saying that practicing intentional stalls should be performed at altitude - estimated recovery altitude at least 1500 ft AGL (ie. throughout the whole maneuver, you should not drop below 1500ft)
  • Most training aircraft are actually designed such that the wing root stalls before the wing tip
    • The wings are curved such that the AOA of the wing root is greater than the wing tip
    • This means the pilot will still have some aileron control if the stall has not "spread" to the whole wing
    • However, care must be taken because aileron input before an acceptable AOA is restored might exacerbate the situation
    • For example, recovering from a left roll during an impending stall requires right aileron. This means the left wing's AOA increases, causing it to stall and worsen the left roll
Stall Recovery
  • Most important is to reduce AOA, and not attempt to maintain altitude, which is often the first instinct
    • This involves disabling the autopilot / wing leveller on complex aircraft as soon as possible for the pilots to take manual control
    • Exerting forward pressure isn't usually necessary, just release the back pressure and the plane should naturally restore itself to normal flight
    • If elevator input is insufficient, adjusting pitch trim might be necessary
  • Only after normal AOA is restored, roll wings level, using rudder to keep the aircraft coordinated
  • If rolling is attempted while above critical AOA, the stall might progress into a spin
  • Once control effectiveness has been regained, increase power promptly but smoothly, resisting any resulting changes such as left-turning tendencies
  • Retract speed brakes and spoilers (but not flaps until positive rate of climb is established)
  • Restore Vy attitude / airspeed
    • If performing a turning stall, roll wings to level before attempting to pitch up, which might result in excessive wing loading
    • Increase AOA smoothly and not dramatically or a secondary stall might occur
  • Return to designated altitude
  • Summary: autopilot, AOA, trim, roll, power, return

 

Power-On Stalls

  • Practiced in case of stall on takeoff or climb
  • Remember to clear the area before proceeding with the maneuver
  • Maintain heading - pick a reference point before beginning
    • Attempt to maintain the heading throughout the maneuver (but only as far as safely possible - do not use aileron input when you're not supposed to, such as before a normal AOA is achieved)
  • Aircraft to be in a clean configuration
  • If practicing a turning stall, maintain bank angle to a tolerance of 10° prior to the stall
  • Slow the aircraft to rotation speed, balancing pitch and power to maintain altitude, use trim as necessary
  • As aircraft approaches stall speed, stall horn activates and engine and wind noise decrease, buffeting will be felt
  • Then increase the power to takeoff power - use rudder to offset left-turning tendencies
    • This low speed, high power configuration simulates the take-off
    • If rudder control not maintained, the aircraft may enter a spin
  • As takeoff power is added, increase nose up pitch dramatically (stall occurs at about 20°)
  • As the aircraft stalls, execute steps as listed above under Stall Recovery. Attempt to lose as little altitude as possible
  • Return to designated altitude and resume cruise


    Power-Off Stalls

    • Similar to power-on stalls except that power will be reduced to idle
    • Similar to power-on stalls, you may descend to no lower than 1500ft AGL at any point in time
      • Safe altitude to begin the maneuver would be 2500ft AGL
    • To begin the maneuver, reduce power to descent power
      • Maintain altitude while slowing down and adding flaps
      • Ensure that every time you lower the flaps, the airspeed is below the maximum flap extension speed
        • 110kts for 10°, below 80kts for everything else in the C172
      • Forward pressure required to maintain altitude initially
      • Established a stabilized approach at normal approach airspeed (65kts)
      • Descend no lower than 200ft from entry altitude
      • When ready / when instructor gives the cue, reduce power to idle and pitch to Vy pitch attitude until the stall occurs (for wings-level stall)
      • If turning stall, this is the point to begin banking into a 10°-20° bank turn
        • Hold Vy pitch attitude until the stall occurs
      • Power off stalls have less obvious cues (buffetting, vibration, etc)
      • Maintain heading
      • Execute stall recovery procedures
        • If turning stall, level the wings after re-gaining control effectiveness
    • After which, recover to Vy pitch attitude when airspeed has recovered sufficiently
      • Retract flaps to 20°
      • As the aircraft passes through 60kts, retract the flaps to 10° (take-off configuration)
      • Retract flaps fully when vertical speed is 0 or positive (climb)
      • (These speeds are applicable for the Cessna 172 - the basic idea is that flaps cannot be retracted too quickly at low airspeeds but best climb performance is usually achieved at medium flap settings and not with full flaps extended)
    • Remember to trim


    Slow Flight

    • When AOA just under the AOA which would cause an impending stall
    • At 5-10kts above 1G stall speed
    • Observe this graph of the lift-drag ratio, plotted against the angle of attack
    • Slow flight occurs at a very high angle of attack, sometimes referred to the "back side of the curve"
      • This means during slow flight, more significant increases in power is needed to prevent deceleration (due to the higher drag relative to lift)
      • Pitch becomes a much more effective control of airspeed
      • Airspeed becomes more unstable
        • If careful watch over airspeed is not maintained, the airspeed may decay to stall speed without the pilot noticing
    • Select a safe altitude to perform the maneuver, similar to the altitudes at which you practice stalls
    • Begin slow flight by reducing power and increasing pitch, maintaining altitude and heading
    • Left turning tendencies are stronger during slow flight
    • Trimming is extremely important for making small, precise inputs necessary in slow flight
    • Abrupt movements may lead to a stall
    • During slow flight, practice straight and level, climbs, descents, climbing and descending turns, level turns
      • Typically done in a take-off or landing configuration (ie flaps, landing gear, etc.)
      • It is recommended to therefore complete the respective checklists as appropriate

    Spins

    • Watch this video for a demonstration
    • Develops from a full stall when the aircraft is yawed or in a slip
      • The yaw may be a result of improper rudder application, or adverse yaw as a result of aileron input before or during the full stall
      • Not allowing the nose to yaw before stall recovery is key to spin prevention - apply sufficient rudder pressure to prevent yaw
    • The aircraft corkscrews down a vertical axis
    • Airspeed is greater on one wing than the other, and so one wing is more stalled than the other
    Spin Recovery
    • Always follow any procedures outlined in the POH first, if not:
    • Retract flaps / landing gear
    • Apply full opposite rudder until rotation has stopped
    • Ailerons neutral (aileron in either direction may aggravate spin)
    • Power to idle (may aggravate spin)
    • Forward pressure on the stick until recovered from stall. Do not wait for rotation to stop
    • Return to original attitude / altitude
    • These steps should be performed almost simultaneously, if possible
      • Practice and visualize these in your head until you are familiar with them before attempting the maneuver
    Practicing Spins
    • Extremely important to first check if the aircraft is approved for spins
    • As always, begin the maneuver at a suitable altitude
    • Check POH for this as well as any other limitations you should be aware of, and if there are any spin procedures specific to the aircraft
    • Also check CG and weights are within limits, as this may have an adverse effect on spin characteristics
    • Also check that all aircraft parts are secure (anything coming lose may affect CG and stability which may cause the spin to become unrecoverable)
    • To begin the practice, similar to practicing stalls, increase AOA and while near the stalling speed apply full rudder
    • Apply full back pressure, maintain neutral aileron
    • The aircraft may take 2-4 rotations to properly establish the spin
    • Once established, the aircraft has sort of reached an "equilibrium" and each rotation is pretty similar in terms of radius, speed, angle etc
    • Begin recovery procedures

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