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Accelerated Stalls

Introduction:

  • Stalls do NOT occur without warning
    • While flying along in cruise flight, a stall will not rip the airplane out of the sky and throw it uncontrollably to the ground to a big smoking crater
  • A stall above 1-g flight is termed an accelerated stall
    • For this reason, you may hear of an accelerated stall referred to as a "G-Stall"
  • At high load factors, the sudden loss of lift from one wing (whichever stalls first) creates a much large rolling moment than with a 1-g stall
    • The result is a violent departure that resembles a snap roll
  • During a rapid departure from controlled flight, the normal CLmax may be momentarily exceeded and as a result, the aircraft could be overstressed or damaged below the corner speed (maneuvering airspeed)
  • Accelerated stalls demonstrate stalls are a function of angle of attack and not airspeed
  • If the airplane is slipping toward the inside of the turn at the time of the stall, it tends to roll rapidly toward the outside of the turn as the nose pitches down, because the outside wing stalls before the inside wing
  • If the airplane is skidding toward the outside of the turn, it will have a tendency to roll to the inside of the turn, because the inside wing stalls first
  • If the coordination of the turn at the time of the stall is accurate, the airplane's nose will pitch away from the pilot just as it does in a straight flight stall
  • Stalling will occur at a higher airspeed and thus, a lower than expected pitch resulting in a quick, unexpected stall
  • If an uncoordinated turn is made, one wing may tend to drop suddenly, causing the airplane to roll in that direction; if that occurs, the excess back-elevator pressure must be released, power added, and the airplane returned to straight-and-level flight with coordinated control pressure

WARNING:
All procedures here are GENERALIZED for learning.
Fly the maneuver in accordance with the Pilot Operating Handbook (POH)
and/or current Standard Operating Procedures (SOPs)


Stall Avoidance

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

Load Factor:

  • Load factor is the weight the wings are supporting
  • Load factor is generally not calculated as part of preflight however, it has a close relation to stall speed, which is very important
    • As load factor increase, stall speed increases
  • In level flight, the load factor is the weight of the aircraft
    • This is due to the aircraft feeling "1-g" or 1 times the force of gravity
  • As you increase the angle of bank however, a pilot must pull back on the controls in order to avoid descending due to the loss of vertical lift, which then raises the load factor
  • Calculating Load Factor:

    • Load Factor Formula:

      Load Factor = 1 / cos(angle of bank)
    • Conditions:

      • Given an angle of Bank of 60°
    • Calculation:

      • Load Factor = 1 / cos(60)
      • Load Factor = 1 / 0.5
      • Load Factor = 2
    • Chart:

      [Figure 1]
      • Look at the 60° mark at the bottom of the chart and move up until you intercept the load factor reference line
      • Move over to the left and see the load factor imposed on the aircraft
        • You should come up to approximately 2
Crosswind Component Chart
Figure 1: Load Factor vs. Stall Speed
Crosswind Component Chart
Figure 1: Load Factor vs. Stall Speed

Stall Speed & Angle of Bank:

  • As mentioned above, stall speed increases with load factor due to a loss in the vertical component of lift
  • Calculating Stall Speed:

    • Load Factor Formula:

      Stall Speed Banked = [Stall Speed Level x Square Root of Load Factor]
    • Conditions:

      • Given an aircraft with a stall speed of 48 knots with a load factor of 2 (60° angle of bank):
    • Calculation:

      • Stall Speed Banked = [Stall Speed Level / Square Root of Load Factor]
      • Square Root of 2 = 1.41
      • Stall Speed Banked = 48 x 1.41
      • Stall Speed Banked = 68 KIAS
    • Chart:

      [Figure 1]
      • Look at the 60° mark at the bottom of the chart and move up until you intercept the stall speed increase reference line
      • Move over to the left and see the percent increase in stall speed
        • You should come up with an increase in stall speed of approximately 41%
          • Stall Speed Banked = [Stall Speed Level x percent increase in stall speed]
          • Stall Speed Banked = 48 x 1.41
          • Stall Speed Banked = 68 KIAS
Accelerated Stall Graphic
Figure 2: Accelerated Stall Graphic
Accelerated Stall Graphic
Figure 2: Accelerated Stall Graphic

Common Errors:

  • Failure to adequately clear the area
  • Failure to establish the specified landing gear and flap configuration prior to entry
  • Improper pitch, heading, and bank control during straight ahead stalls
  • Use outside and instrument references
  • Right rudder in nose-high power-on condition; release at break
  • Improper pitch and bank control during turning stalls
  • Rough or uncoordinated control technique
  • Failure to recognize the first indications of a stall
  • Failure to achieve a stall
  • Improper torque correction
  • Poor stall recognition and delayed recovery
  • Excessive altitude loss or excessive airspeed during recovery
  • Secondary stall during recovery

Airman Certification Standards:

  • Commercial Pilot Standards:

    • Clear the area
    • Select an entry altitude that will allow the Task to be completed no lower than 3,000 feet AGL
    • Establish the configuration as specified by the evaluator
    • Set power appropriate for the configuration, such that the airspeed does not exceed the maneuvering speed (Va), flap extension speed (VFE), landing gear extended speed (VLE), and any other POH/AFM limitation
    • Establish and maintain a coordinated turn in a 45° bank, increasing elevator back pressure smoothly and firmly until an impending stall is reached
    • Acknowledge the cues and recover promptly at the first indication of an impending stall (e.g., aircraft buffet, stall horn, etc.)
    • Execute a stall recovery in accordance with procedures set forth in the POH/AFM
    • Retract the flaps to the recommended setting, if applicable; retract the landing gear, if retractable, after a positive rate of climb is established
    • Accelerate to Vx or Vy speed before the final flap retraction; return to the altitude, heading, and airspeed specified by the evaluator

Conclusion:

  • It is important to note that individual aircraft may have stall characteristics unique to them due to bends/twists which develop in the airframe over time depending on their use

References: