What is the critical angle of attack?

It is the maximum angle at which smooth airflow is maintained over the wing; exceeding it causes a stall.

Can an aircraft stall at any airspeed?

Yes, a stall can occur at any airspeed if the critical angle of attack is exceeded.

How do flaps affect stall speed?

Extending flaps lowers stall speed by increasing the wing's maximum lift coefficient.

What is a deep stall?

A deep stall is a condition where recovery is difficult or impossible, typically in T-tail, swept-wing aircraft.

Why is stall recognition important for pilots?

Early recognition allows prompt recovery, preventing loss of control and potential accidents.

Stall

Easy4 min readPrinciple of Flight (A)

Moderately Examined

A stall occurs when the wing exceeds its critical angle of attack, causing a rapid loss of lift and a sharp increase in drag. Recognising and understanding stall behaviour is crucial for safe flight, as stalls can lead to loss of control if not managed properly.

In depth

Explanation

What is a Stall?

A stall happens when the airflow over the wing separates due to an excessive angle of attack (AoA). This separation causes lift to drop sharply and drag to rise, making the aircraft difficult to control. The critical angle of attack is a fixed value for a given wing design, and exceeding it always results in a stall, regardless of airspeed or attitude.

Indications and Aerodynamic Effects

Typical signs of an impending stall include buffeting, sluggish controls, and sometimes a nose-down pitch or uncommanded roll. At the stall, the centre of pressure moves rearward, often causing a natural nose-down moment that can help recovery. However, in some aircraft (especially those with swept wings and T-tails), a deep stall can develop, where recovery is extremely difficult due to the elevator being blanked by turbulent airflow.

Recovery Technique

Stall recovery requires reducing the angle of attack. This is achieved by moving the control column forward, lowering the nose, and restoring smooth airflow over the wing. Power application may assist recovery, but the primary action is always to decrease AoA.

Factors Affecting Stall

Flap Setting: Extending flaps lowers stall speed by increasing maximum lift, but retracting flaps too early or late can raise stall speed unexpectedly.
Spoilers: Deploying spoilers reduces lift and increases stall speed, shifting the CL-alpha curve.
Contamination: Ice or dirt on the wing reduces maximum lift, increases drag, raises stall speed, and degrades controllability.
High Altitude: At altitude, the margin between stall speed and maximum operating Mach number (MMO) narrows, increasing the risk of both low-speed stall and high-speed overspeed.

Special Stall Types

Deep Stall: Most common in aircraft with swept wings and T-tails, where elevator effectiveness is lost after the stall, making recovery nearly impossible.
Shock Stall: Occurs at high speed due to shock wave formation, not high AoA, and typically at a lower angle of attack than a classic stall.

The essentials

Key Points

A stall occurs when the wing exceeds its critical angle of attack.

Stall is identified by loss of lift, buffeting, and reduced control effectiveness.

Recovery requires reducing angle of attack by pitching nose down.

Flap and spoiler settings directly affect stall speed and margins.

Ice and contamination increase stall speed and reduce maximum lift.

Deep stalls are dangerous and often unrecoverable in T-tail, swept-wing aircraft.

At high altitude, stall speed and MMO margins are reduced, increasing risk.

Watch out

Common Exam Traps

Confusing stall speed with the speed at which the nose drops—stall is about angle of attack, not airspeed.

Assuming all stalls are recoverable—deep stall in T-tail aircraft may not be.

Believing that more elevator always improves climb—excessive back pressure can induce a stall.

Thinking spoilers decrease stall speed—they actually increase it by reducing lift.

Forgetting that ice or contamination raises stall speed and lowers maximum lift.

Test yourself

Example Exam Questions

Question 1Easy

What is the primary cause of an aerodynamic stall?

Question 2Medium

Which configuration is most susceptible to a deep stall?

Question 3Easy

How does ice contamination on the wing affect stall speed?

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