Stalls and spins appear in almost every DGCA Technical General paper, and the questions often hide a simple idea behind tricky wording. This guide explains stall and spin for the DGCA exam in plain steps: what the critical angle of attack is, what makes stall speed go up or down, and the spin recovery technique you are expected to know.
What is a stall?
A stall happens when the wing exceeds its critical angle of attack and the airflow can no longer follow the upper surface smoothly. The flow separates, lift drops sharply and drag rises.
The angle of attack is the angle between the wing's chord line and the relative airflow. As it increases, the coefficient of lift rises, up to a maximum. Beyond that maximum, the boundary layer separates over much of the upper surface and lift falls away.
The key exam point: a wing stalls at an angle of attack, not at a speed. For a given wing and configuration, the critical angle stays the same. The speed at which you reach it changes with weight, load factor and other factors.
Signs of an approaching stall
- Buffet, as separated airflow shakes the tail or airframe
- A stall warning horn, light or stick shaker
- Controls that feel light and less effective
- A high nose attitude with decreasing airspeed in level flight
What changes stall speed?
The stall speed is the speed at which the wing reaches its critical angle of attack in a given condition. These factors change it:
- Weight: more weight needs more lift, so stall speed increases. Stall speed varies with the square root of weight.
- Load factor: in a turn or pull-up the wing must produce more lift than the weight, so stall speed rises with the square root of the load factor.
- Flaps: extending flaps increases the maximum coefficient of lift, so stall speed decreases.
- Centre of gravity (CG): a forward CG needs more tail down-force, so the wing carries more load and stall speed rises slightly. An aft CG lowers it but reduces stability.
- Power: with power on, the propeller slipstream and the upward part of thrust help, so the indicated stall speed is usually lower than with power off.
- Ice and contamination: even thin frost on the wing can disturb the airflow, lowering the critical angle and raising stall speed.
- Altitude: the indicated airspeed (IAS) at the stall stays roughly the same as altitude increases, but the true airspeed (TAS) at the stall is higher.
Stall speed in a turn: worked example
In a level turn, the load factor (n) equals 1 ÷ cos(bank angle). Stall speed increases by the square root of n.
Basic stall speed (wings level): 50 kt
45° bank: n = 1 ÷ cos 45° = 1.41
√1.41 = 1.19
Stall speed = 50 × 1.19 ≈ 59.5 kt
60° bank: n = 1 ÷ cos 60° = 2.0
√2 = 1.41
Stall speed = 50 × 1.41 ≈ 70.7 kt
So a 60° level turn raises stall speed by about 41%. The same square-root idea applies to weight: an aircraft 20% heavier has a stall speed about 9.5% higher, because √1.2 ≈ 1.095.
How wing design controls the stall
Designers want the wing root to stall before the tip. That keeps the ailerons, which sit near the tips, effective for longer and gives the pilot buffet as a warning.
- Washout: the wing is twisted so the tip meets the air at a smaller angle of attack than the root. The root reaches the critical angle first.
- Stall strips: small sharp strips on the leading edge near the root make the flow separate there first.
- Swept and tapered wings tend to stall at the tip first, so they need such design features to behave well.
Aircraft with a T-tail can meet a deep stall, where the wake from the stalled wing blankets the tailplane and the elevator loses the power to lower the nose. This is why some such aircraft carry a stick pusher.
How a spin develops
A spin is an aggravated stall with autorotation: the aircraft descends in a steep helical path while rotating about its vertical axis, with at least one wing stalled.
It starts when a stall happens with yaw present. The wing moving down meets a higher angle of attack and stalls more deeply, so it loses more lift and gains more drag. The wing moving up is less stalled and produces more lift. This difference keeps the roll and yaw going, which is autorotation.
A spin usually passes through three stages:
- Incipient spin: the first turns, as the stall and yaw develop into rotation.
- Developed spin: rotation rate, airspeed and descent rate settle into a steady pattern.
- Recovery: the rotation stops and the aircraft is flown out of the dive.
Spin recovery technique
The standard recovery taught for most light training aircraft is often remembered as PARE:
- Power to idle
- Ailerons neutral
- Rudder full opposite to the direction of rotation
- Elevator forward, enough to break the stall
When the rotation stops, centralise the rudder, level the wings and ease out of the dive without overstressing the aircraft. Each aircraft's flight manual gives its own approved procedure, and that procedure always comes first.
Common mistakes in DGCA stall and spin questions
- Saying a stall happens "at a certain speed". It happens at the critical angle of attack.
- Using aileron to pick up a dropping wing near the stall. This can deepen the stall on that wing; rudder is used to stop yaw instead.
- Confusing a spin with a spiral dive.
- Forgetting the square root when working out stall speed in a turn.
A wing stalls when it exceeds its critical angle of attack, whatever the airspeed, attitude or power setting.
Frequently asked questions
Can an aircraft stall at any airspeed?
Yes. A stall depends on the angle of attack, not the airspeed. If the pilot pulls hard enough, for example in a steep turn or a sharp pull-up, the wing can exceed its critical angle at a speed well above the published stall speed. This is called an accelerated stall and is a favourite exam question.
Why does stall speed increase in a turn?
In a level turn the wing must produce enough lift to support the weight and also turn the aircraft, so the load factor rises above one. To make that extra lift, the wing needs a higher angle of attack at any speed, so it reaches the critical angle sooner. Stall speed rises with the square root of the load factor.
What is the difference between a spin and a spiral dive?
In a spin, at least one wing is stalled, the airspeed stays low and fairly steady, and the aircraft rotates steeply. In a spiral dive, the wings are not stalled and the airspeed increases rapidly. Recovery is different: a spin needs opposite rudder and forward elevator, while a spiral dive needs power reduced and wings levelled.
Why is washout used on wings?
Washout twists the wing so the tip has a smaller angle of attack than the root. As a result, the root stalls first while the tips keep flying. This keeps the ailerons effective near the stall, gives the pilot buffet as a warning, and makes a sudden wing drop and spin entry less likely.
Practise stall and spin questions
Test what you have learned about stall and spin with free DGCA practice questions, and revisit how airspeed is measured in our guide to pitot static instruments. Fly with Sudharshan Aviation Academy (FSAA) teaches principles of flight, stalls and aircraft performance in depth in the DGCA Technical General course.