• A stall is an aerodynamic condition in which the Angle of Attack (AoA) becomes so steep that air can no longer flow smoothly over the airfoil
  • Said another way, a stall is a condition of flight in which an increase in AoA results in a decrease in lift
    • Angle of Attack (AoA/α): angle between the relative wind and the chord-line of an airfoil
    • Critical AoA: the angle of attack whereby any further increase will result in a separation of airflow which results in a stall
  • Upon airflow separation from the wing the airfoil will no longer produce lift
  • Angle of attacks are usually measured as arbitrary units
Figure 1: CL-Max


  • Pitch Attitude: angle between an airplane's longitudinal axis and the horizon
  • Flight Path: path described by its center of gravity as it moves through an air mass
  • Relative Wind: airflow the airplane experiences as it moves through the air
    • Equal in magnitude and opposite in direction to the flight path
  • Angle of Incidence: Chord line of the wing is angled up when attached to fuselage
  • Critical Angle of Attack: stalls occur at the same angle of attack regardless of airspeed, flight attitude, or weight

Flight path, relative wind, and angle of attack should never be inferred from pitch attitude

Figure 2: Wing Definitions

Boundary Layer:

  • That layer of airflow over a surface that demonstrates local airflow retarding due to viscosity (as it gives up kinetic energy to friction) is called the boundary layer
  • The air molecules in the boundary (surface) layer have zero velocity in relation to the surface; however, the layer just above moves over the stagnant molecules below because it is pulled along by a third layer close to the free stream of air
  • The velocities of the layers increase as the distance from the surface increases until free stream velocity is reached, but all are affected by the free stream
  • The distance (total) between the skin surface and where free stream velocity is reached is called the boundary layer
    • At subsonic levels the cumulative layers are about the thickness of a playing card, increasing in thickness as it moves aft
  • When air flows across any surface, friction develops
  • As a viscous fluid resists flow or shearing, the adjacent layer of air is also slowed
  • Succeeding streamlines are slowed less, until eventually, some outer streamline reaches the free air stream velocity
  • Laminar Flow:
    • The air moves smoothly along in streamline
  • Turbulent Flow:
    • Streamlines that break up causing the flow to be disorganized and irregular
    • Produces higher friction than laminar
    • Adheres better to the surface of the airflow, delaying separation
  • Favorable Pressure Gradient (FPG): assists the boundary layer in adhering to the surface by maintaining its high kinetic energy
    • As air flows aft from the point of maximum thickness toward the trailing edge (low to high static pressure), it encounters adverse pressure gradient
  • Adverse Pressure Gradient (APG): impedes the flow of the boundary layer
    • Strongest during high lift conditions and at high AoAs in particular
    • If the boundary layer does not have sufficient kinetic energy to overcome the APG, then the lower levels of the boundary layer will stagnate and separate as airflow is reversed
    • As separation moves forward, the net suction decreases and CL decreases, resulting in a stall
    • Even at low angles of attack, there will be a small APG behind the point of maximum thickness
      • As the separation moves forward with increasing AoA, eventually the air cannot conform to the sharp turn
  • To learn more about stalls and airflow check out NASA's FOILSIM III below


  • Rudder pedal shakers
  • Stick shakers
  • Horns
  • Buzzers
  • Warning lights

Stall Speed:

  • As AoA increases up to CLMAX AoA, TAS decreases to a point where it cannot be any slower (Vs)
  • Airspeeds may change based on weight and configuration, but units of AoA remains the same
    • You can stall at any airspeed
    • Going too slow causes high AoA while going too fast causes shock waves on aircraft not designed for supersonic or even transonic flight, causing the same disruption has high AoA
  • Weight:
    • As weight decreases, so does stall speed, due to less lift required
    • Dropping a bomb or just using fuel decreases stall speed and thus, approach speed (AoA approaches)
  • Altitude:
    • Higher altitude results in less air molecules, so a higher TAS is required however, IAS remains the same
    • Increase altitude results in increased stall speed
    Stall Strips
    Figure 3: Angle of Bank vs. Stall Speed
  • Angle of Bank:
    • As you increase your angle of bank, stall speed increases
    • See turns
  • Power Off/On:
    • Power on conditions will have lower stall speeds as the aircraft is supported partially by the vertical component of thrust
    • In addition, with power you will have induced airflow over the wings
  • Wing Tailoring:
    • Makes stalling characteristics more predictable by attempting to stall the root first
    • Power-on stalls may tend to stall at the tip first, due to induced lift
    • With the wings stalling at the root first, the aircraft maintains some aileron authority
  • Geometric Twist/Washout:
    • A decrease in angle of incidence from wing root to wingtip
      • The wing is gradually twisted downward, decreasing its AoA
    Geometric Twist/Washout
    Figure 4: Geometric Twist/Washout
  • Aerodynamic Twist (Section Variation):
    • Gradual change in airfoil shape accomplished by a decrease in camber from root to tip and/or reducing the chord
  • Stall Fences:
    • Redirect the airflow along the chord
    • Allows the wing to achieve a higher AoA without stalling (delaying tip stall)
  • Stall Strip:
    • Sharply angled piece of metal at the root section to induce a stall at the root
    • Stall Strips
      Figure 5: Stall Strips
    • Subsonic air cannot make sharp angles

  • Recognition:
    • Vision: noting the attitude of the airplane; however not conducive to recognizing approaching stalls
    • Hearing: RPM loss, more airflow noise around cabin
    • Kinesthesia: sensing in directions or speed of motion. Most important indicator
    • Feel: control pressures and pressures exerted

  • Stall Recovery:
    • Stall recoveries are fundamentally the same if you remember "Max - Relax - Level"
      1. Apply maximum power (increases lift)
      2. Relax the nose (decreases the AoA)
      3. Level the wings (reduces the stall velocity to allow all available lift to break the descent

Compressor Stalls:

  • Compressor stalls, while related in their cause, have nothing to do with the wing
  • To learn more visit the Powerplant page

Stall Avoidance

  • Avoid flying at minimum airspeeds
  • Remain in the normal flight envelope
  • Avoid abrupt maneuvers


  • There is no question why it is important to understand stalls
  • Stalls are dependent on AoA only and the only way to recover is to reduce the AoA
  • To learn more check out Stalls, Spins, and Safety