Why a wing stalls
A wing stalls when it exceeds its critical angle of attack (AoA), about 17° for most aerofoils (TP 1102). AoA is the only trigger. Airspeed and attitude do not cause it directly.
Mechanism (TP 1102 Ex.12 l.2734; PHAK):
- Low-to-moderate AoA: airflow smooth and attached (laminar) most of the way to the trailing edge.
- AoA rises: turbulent-flow (separation) point creeps forward from trailing edge toward leading edge.
- Critical AoA: airflow separates from upper surface entirely. Downwash and pressure differential collapse. Wing loses most of its lift (TP 1102, PHAK).
Lift curve:
- Coefficient of lift (CL) rises with AoA up to CL-MAX, reached exactly at critical AoA (AC 700-031 s.2.3(1)(f)).
- Beyond that, CL drops rapidly (PHAK).
AoA, not speed (TP 1102 Ex.12 l.2748; PHAK):
- Same critical AoA at any airspeed, weight, bank, or attitude.
- Airspeed and attitude only set the AoA flown.
- Within structural limits, wing can stall at any airspeed and any attitude, nose-level or nose-down included (TP 1102).
Critical AoA is a fixed aerofoil property:
- Unaffected by weight, load factor, bank angle, temperature, density altitude, or CG. Those change the speed at which the wing reaches critical AoA (see stall-speed and factors-affecting-stall-speed), not the angle itself.
- Surface contamination (ice, snow, frost): separation at a lower AoA (TP 10643 l.622; TP 13014 q23 key 23-(4); PHAK ch.5).
Stalls during turns (TP 1102):
| Turn | Wing that normally stalls first | Why | Result |
|---|---|---|---|
| Level | Inside | Inside wing travels slower, makes less lift, sinks, AoA rises | Aircraft rolls to the inside of the turn |
| Descending | Inside | Downward spiral path meets inside wing at steeper AoA | Aircraft rolls to the inside of the turn |
| Climbing | Outside (higher) | Upward spiral meets outside wing at steeper AoA | Outside wing drops abruptly |
Stall strips: fitted to leading edge, introduce an early stall at the root (TP 1102 Ex.12 l.2782; PHAK ch.5).
Self-correcting cycle (PHAK ch.5):
- Stall: wing lift and tailplane downward force both collapse.
- Unbalanced condition pitches aircraft down about its CG.
- AoA falls, airspeed rises, lift returns.
- Considerable altitude may be lost before the cycle completes.
Leading-edge shape sets abruptness (PHAK ch.5):
| Leading edge | Stall |
|---|---|
| Well-rounded, almost blunt: airflow stays attached to higher AoA; separation starts at trailing edge, creeps forward gradually | Smooth |
| Pointed (needed for high-speed flight) | Abrupt |
Trap: critical AoA (~17°, wing stalls) is not the best lift/drag AoA (a much lower angle); confusing them swaps "most efficient" for "about to stall."
References
- TP 1102 (Flight Training Manual), Basic Principles of Flight and Exercise 12 Stalls
- FAA-H-8083-25 PHAK ch.5 Aerodynamics of Flight
- AC 700-031 s.2.3(1)(f)
- TP 10643 l.622 (frost, ice and snow contamination)
- TP 13014 sample exam q23, key 23-(4)