9.2 Angle of Attack, Critical AoA, and Stalls

ACS PA.VII.A–C.K1 asks for the aerodynamics of slow flight and stalls in various configurations: the relationship among angle of attack, airspeed, load factor, power, weight and CG, attitude, and yaw. The highest-rate miss on the knowledge test is still the same folk theorem: “a stall is when you get too slow.”

A stall is an AOA event

PHAK Chapter 5: an aircraft stall is a rapid decrease in lift caused by separation of the airflow from the wing’s upper surface, brought on by exceeding the critical AOA. The wing does not stop producing lift. It cannot produce enough lift to sustain the flight condition you asked for — usually level flight.

CL rises with AOA up to CL-max. That peak is the critical (stalling) AOA. Any further increase in AOA and CL falls off sharply. For a typical straight-wing trainer PHAK places that angle in a band of about 16° to 20°. The exact number is a function of wing design, not of the airspeed you happened to be indicating. A given wing stalls at essentially the same AOA whether you are at gross weight or light, flaps up or down (the flaps change the shape, so they change the CL curve — they do not make “airspeed” the stall cause), climbing or diving, upright or in a steep turn.

Therefore you can stall:

• Nose-high and slow — the classic power-off approach stall.
• Nose-low and fast — an accelerated stall from an abrupt pull-up or a tight, level turn.
• In a climb, a descent, or inverted. Attitude is not AOA. AOA is chord versus relative wind.

Indicated stall speed in 1g, coordinated, flaps-up, max-gross, level flight is the number painted on the airspeed indicator as the bottom of the white or green arc. That number is a special case. It is not the definition of a stall.

What changes stall speed — and what does not

Rearrange the lift equation at CL-max and you see why stall speed moves around while critical AOA does not.

To hold L equal to the effective weight, Vs is proportional to the square root of (wing loading / (ρ × CL-max)).

Indicated stall speed in a given configuration is essentially independent of altitude. The airspeed indicator is already a dynamic-pressure instrument. True airspeed at the stall rises as density falls. Do not tell an oral that “you stall faster at altitude” without saying true versus indicated.

Washout, stall strips, and the stall warning

Most straight-wing trainers are built so the root stalls first and the stall progresses outboard. That leaves the ailerons in still-attached flow, so you can keep the wings level while you reduce AOA. PHAK lists two common design tools:

• Washout (geometric twist). The tip is built at a lower angle of incidence than the root, so the root reaches critical AOA first.
• Stall strips on roughly the first 20–25 percent of the leading edge, which trip the root boundary layer early.

A rectangular planform also tends to stall root-first. Highly tapered or swept wings can stall at the tip first — ailerons go soft and a wing drop is more likely. That is a design reason trainers look the way they do, not a claim that ailerons are a stall-recovery control.

The stall warning — a pneumatic reed in the leading edge, or an electric vane — is set to activate about 5 to 10 knots above the 1g stall in the certificated configuration. It is an AOA sensor, not an airspeed switch. It can stay silent in an accelerated stall until you are already at critical AOA, and it can lie if ice covers the slot or vane. The FAA has been pushing AOA indicators for exactly this reason: the 1g stall speed on the ASI does not display the stall that is available at 2g. Treat the horn as a gift. Treat reducing AOA as the recovery.

Load-factor preview (full treatment is Chapter 10)

In a level turn the wing must produce lift equal to weight × n, where n = 1 / cos(bank) for a coordinated level turn. Stall speed scales with √n:

Vs_accelerated = Vs_1g × √n

A 60° level bank is 2g. If the flaps-up 1g stall is 50 KIAS, the same wing stalls at 50 × √2 ≈ 71 KIAS in that turn. You did not “get slow.” You pulled the wing to critical AOA at a speed that still looks like cruise on a casual glance at the ASI. Bank-angle tables, VG diagrams, and Va belong in Chapter 10. The principle you need here is only this: load factor raises stall speed; it does not raise critical AOA.

Power-on versus power-off, conceptually

Power-off (approach) stalls are flown in a landing-like picture: low power, often landing flaps, a moderate pitch. Indicated stall speed is close to the AFM flaps-down number. Buffet and horn are usually honest. Yaw is milder because the propeller is not pulling hard.

Power-on (departure) stalls are flown in a takeoff-like picture: high power, takeoff flaps or clean, a much steeper pitch to get to critical AOA. Two things change the numbers you see:

• A vertical component of thrust supports part of the weight, so the wing can reach critical AOA at a lower indicated airspeed than in the power-off stall.
• The slipstream accelerates air over the inboard wing and the tail, which can delay the root stall and make the elevator more effective — until it isn’t.

Left-turning tendencies (torque, spiraling slipstream, P-factor) are strong in a power-on stall. Uncoordinated rudder plus a stalled wing is the spin entry that PA.VII.D is about. Keep the ball centered. If a wing drops, reduce AOA first, then pick the wing up with coordinated aileron and rudder — do not “lift the dead wing” with aileron while the nose is still parked at critical AOA.

Recover: reduce AOA first

Airplane Flying Handbook stall recovery is a sequence, not a simultaneous grab for everything. The wing is stalled because AOA is too high. The first action is forward elevator (or relaxed back-pressure) to put the chord back below critical. Then add power as appropriate, level the wings, and return to the desired flight path. Adding power without reducing AOA can raise the nose further and deepen the stall. Holding the stall and using aileron to pick up a wing is how a stall becomes a spin.

Most trainers are certificated so that with the CG in limits the CP remains aft of the CG and the nose drops at the break, reducing AOA for you. An aft-CG loading can take that gift away. PHAK: the most critical CG violation in a stall is exceeding the aft limit — the elevator may not be able to pitch the nose down.

Scenario: Maya’s base-to-final

Maya overshoots final, banks steeply, and pulls to keep the runway in the windscreen. Airspeed is 75 knots — “above stall speed” on the flaps-up 1g number she memorized. Bank is past 50°, load factor is climbing, and she is increasing AOA with the yoke. The stall horn, if it sounds at all, is late because this is not a 1g stall. The wing will stall when she reaches critical AOA, at an indicated speed well above 50 knots. Recovery is not “add power and hold the pitch.” Recovery is lower the AOA, then roll wings level, then fix the pattern. The 50-knot number was never a promise.