Stalls and Spins

4.stalls-and-spins. Stalls and Spins

A stall is an aerodynamic condition in which the smooth airflow over the wing's upper surface separates and becomes turbulent, resulting in a sudden loss of lift and an increase in drag. Contrary to popular belief, a stall is not caused by lack of airspeed — it is caused exclusively by exceeding the wing's critical angle of attack (AOA). For most general aviation airfoils, the critical AOA is approximately 15° to 20° relative to the relative wind. A wing can be stalled at any airspeed, in any attitude, and at any power setting if the pilot exceeds this angle.

Aerodynamic Cause

As AOA increases, the center of pressure moves forward and the coefficient of lift (CL) increases — until the critical AOA is reached. Beyond that point, the boundary layer separates from the upper wing surface, CL drops sharply, and induced drag rises rapidly. Recovery requires reducing the AOA, which is accomplished by relaxing back-pressure or applying forward elevator pressure.

Indications of an Impending Stall

Pilots are trained to recognize several cues prior to a full aerodynamic stall:

• Stall warning horn or light (required by 14 CFR 23 to activate 5–10 knots above stall)
• Aerodynamic buffet as turbulent air strikes the empennage
• Mushy, sloppy, or ineffective flight controls
• High pitch attitude combined with decreasing airspeed
• Reduced wind noise and sink rate increase

Factors Affecting Stall Speed

While critical AOA is fixed, the indicated airspeed at which a stall occurs varies with several factors:

• Weight — heavier aircraft stall at higher speeds. Stall speed varies with the square root of the weight ratio.
• Load factor (G-load) — stall speed increases with the square root of load factor. In a 60° level bank, load factor is 2 G, so Vs increases by √2 ≈ 1.41. A clean stall speed of 50 KIAS becomes about 71 KIAS.
• Configuration — extending flaps and gear lowers stall speed by increasing CLmax.
• Power — higher power lowers stall speed because of the vertical thrust component and propeller-induced airflow over the wing roots.
• Center of gravity — a forward CG increases stall speed (more tail-down force required, so wing carries more load); an aft CG lowers it but degrades stall recovery.
• Frost, ice, or contamination — disrupts laminar flow, can raise stall speed by 25–40% with little visible accumulation.
• Turbulence — a sudden vertical gust increases the effective AOA and can produce an accelerated stall.

Types of Stalls

• Power-off (approach-to-landing) stall — simulates an inadvertent stall on final approach.
• Power-on (departure) stall — simulates an inadvertent stall during takeoff or climbout.
• Accelerated stall — occurs above normal stall speed when load factor is increased, typically in a steep turn.
• Cross-control stall — occurs in a slipping or skidding turn, most dangerous in the base-to-final overshoot scenario, and is the classic precursor to a stall/spin accident.
• Elevator trim stall — caused by excessive nose-up trim during a go-around.
• Secondary stall — caused by premature recovery, where back-pressure is reapplied before the wing has fully regained smooth airflow.

Stall Recovery

The FAA-recommended recovery sequence (consistent with the Airplane Upset Prevention and Recovery Training, AC 120-109) is:

1. Disconnect autopilot and autothrottle.
2. Reduce AOA — pitch nose down (this is the only action that ends the stall).
3. Roll wings level using coordinated aileron and rudder.
4. Apply maximum allowable thrust as needed.
5. Return to desired flight path with minimum altitude loss.

Spins

A spin is an aggravated stall that results in autorotation — the aircraft descends in a corkscrew path while rolling, yawing, and pitching. Two conditions must exist simultaneously to enter a spin: the airplane must be stalled, and there must be yaw. The stalled wing on the inside of the rotation is more deeply stalled than the outside wing, producing asymmetric lift and drag that drive the autorotation.

The four phases of a spin are:

1. Entry — pilot or environment provides yaw at the stall.
2. Incipient — rotation accelerates, lasting about 4–6 seconds or 2 turns.
3. Developed — stabilized rotation, airspeed and vertical descent constant.
4. Recovery — application of anti-spin controls.

Standard Spin Recovery (PARE)

Unless the AFM/POH specifies otherwise, the generic recovery is:

• P — Power to idle
• A — Ailerons neutral
• R — Rudder full opposite to rotation
• E — Elevator briskly forward to break the stall

Once rotation stops, neutralize the rudder and recover smoothly from the dive, avoiding a secondary stall or exceeding Vne or load-factor limits.

Regulatory and Training Notes

Under 14 CFR 61.105 and 61.107, private pilot applicants must demonstrate stall recognition and recovery, but only flight instructor applicants are required to demonstrate spins (61.183(i)). All practice stalls and spins must be conducted at an altitude that allows recovery no lower than 1,500 feet AGL for private pilot training (per the ACS), and spins should generally be initiated above 3,500 feet AGL with parachutes if required by 91.307.