“An airplane stalls when it flies too slowly.” Nearly every student believes this at some point, and it is exactly the idea that exam questions are built to catch. A stall is about angle of attack, not airspeed, and load factor explains why an airplane can stall at a speed well above the number printed in the handbook.
A wing stalls when it exceeds its critical angle of attack, at any airspeed. In a level turn the load factor is 1 ÷ cos(bank): 2 G at 60°. Stall speed rises with √(load factor): +41% at 60°. Stall speed also rises with weight, and Va drops when the airplane is lighter.
What a Stall Is
The angle of attack is the angle between the wing’s chord line and the relative wind. It is not the pitch attitude and not the flight path angle: an airplane can have its nose below the horizon and still be at a high angle of attack, for example in a steep turn or a recovery that pulls too hard.
As the angle of attack increases, lift increases, up to a maximum. Beyond the critical angle of attack, typically around 15 to 20 degrees for a light airplane, the airflow can no longer follow the top of the wing. It separates, lift falls sharply and drag rises. That is the stall.
The consequence is the key fact for the test: a wing can stall at any airspeed, in any attitude, and at any power setting. What stays the same is the angle of attack at which it stalls, and that angle does not depend on the airplane’s weight or on its altitude.
The Lift Equation, in Words
Lift depends on air density, the square of the speed, the wing area and the lift coefficient (which depends on the angle of attack). In level flight, lift must equal weight. If speed falls, the lift coefficient must rise to compensate, so the angle of attack increases. At the lowest speed the wing can fly at, it is at its maximum lift coefficient. That is the stall speed.
Now ask the wing for more lift than the weight: in a turn, in a pull-up, in turbulence. The same maximum lift coefficient needs more speed, so the airplane stalls at a higher airspeed.
Warning Signs
- A stall warning horn or light, usually a few knots before the stall
- A buffet, the shaking of the airframe as separated air hits the tail
- Mushy controls, especially the ailerons
- A high nose attitude without a corresponding climb
- A sink with back pressure on the controls
Load Factor
The load factor is the ratio of the lift the wing produces to the airplane’s weight, measured in G. In straight and level flight, it is 1 G. In a level, coordinated turn, the wing must support the weight and also produce the force that turns the airplane, so the load factor goes up with the bank angle:
load factor = 1 ÷ cos(bank angle)
| Bank angle | Load factor | Stall speed increase (√n) |
|---|---|---|
| 0° | 1.00 G | 0% |
| 30° | 1.15 G | +7% |
| 45° | 1.41 G | +19% |
| 60° | 2.00 G | +41% |
| 70° | 2.92 G | +71% |
| 75° | 3.86 G | +97% |
Notice how fast the numbers climb above 60°. A weight of 2,400 pounds becomes a 4,800-pound load on the wings at 60° of bank, and a stall speed of 50 knots becomes about 71 knots.
Turns are not the only way to load the wings. A pull-up, a recovery from a dive, or a gust can load the airplane in the same way. The test often asks which maneuver increases load factor compared with straight and level flight: turns, and abrupt pull-ups.
Stall Speed: What Moves It
| Factor | Effect on stall speed |
|---|---|
| Higher load factor (bank, pull-up) | Increases, with the square root of the load factor |
| More weight | Increases: about 5% more stall speed for 10% more weight |
| Forward center of gravity | Increases slightly, because the tail must push down harder |
| Flaps extended | Decreases |
| Ice or frost on the wings | Increases, and it can destroy the lift suddenly |
| Higher altitude | Indicated stall speed stays the same; the true airspeed is higher |
The last line is a favorite: the airspeed indicator reads the same stall speed at sea level and at 8,000 feet, because the indicator responds to dynamic pressure, which is exactly what the wing responds to.
Speeds on the Airspeed Indicator
| Marking | Meaning |
|---|---|
| Bottom of the white arc | Vs0: stall speed in the landing configuration (flaps and gear down, power off) |
| Bottom of the green arc | Vs1: stall speed in a specified configuration, usually clean |
| Top of the white arc | Vfe: maximum flap extended speed |
| Top of the green arc | Vno: maximum structural cruising speed |
| Yellow arc | Caution range: smooth air only |
| Red line | Vne: never exceed speed |
Maneuvering Speed (Va)
Va is the speed at or below which you can use a full, abrupt control input without overstressing the airplane: the wing stalls before the load gets high enough to cause damage. It is not marked on the airspeed indicator, and it depends on weight:
Va decreases when the airplane gets lighter, in proportion to the square root of the weight ratio. If Va is 100 knots at 2,550 pounds, then at 2,040 pounds (80% of max weight) it is about 100 × √0.8 ≈ 89 knots. The number in the handbook for maximum weight is therefore too high on a light day, so in turbulence, fly at or below the Va that fits your weight.
Load Limits and the V-g Diagram
Airplanes are certificated for a limit load factor. For the normal category, the positive limit is +3.8 G. For the utility category it is +4.4 G, and for the acrobatic category +6.0 G. The V-g diagram shows the combinations of airspeed and load factor the airplane can safely reach: the left boundary is the stall curve, the top is the limit load factor, and the right is the never-exceed speed.
Recovery
At the first indication of a stall, recover:
- Reduce the angle of attack: lower the nose (release back pressure and, if needed, push forward).
- Level the wings with coordinated aileron and rudder.
- Add power as needed.
- Return to the desired flight path, without pulling so hard that you stall again (a secondary stall).
Release the back pressure first. Adding power alone does not fix a stall, because the cause is the angle of attack.
Spins
A spin needs two ingredients: a stalled wing and yaw. One wing stalls deeper than the other and the airplane rotates in an autorotation while descending steeply. A common cause is skidding in a turn at a low speed, with too much rudder and too much back pressure.
The standard recovery acronym is PARE:
- Power to idle
- Ailerons neutral
- Rudder, full opposite to the direction of rotation
- Elevator, brisk forward movement to break the stall
After the rotation stops, neutralize the rudder and pull out of the dive smoothly, without exceeding the load limits. Always follow the procedure in your airplane’s handbook, because it takes precedence.
Common Mistakes
- Thinking of the stall as a speed. It is an angle of attack, and it can happen at any speed.
- Pulling back to stop a descent in a steep spiral. This tightens the turn and raises the load factor. Level the wings first.
- Forgetting that the stall speed rises in a turn. The wing needs more lift, and the maximum lift coefficient has not changed.
- Treating Va as a fixed number. It changes with weight.
- Confusing Vs0 and Vs1. The white arc starts at Vs0, the green arc at Vs1.
Practice It
AltiPrep FAA has aerodynamics questions on stalls, load factor, V-speeds and spins, each with an explanation that shows where the number comes from, so you can answer the variants the test uses.
This article is a study aid. Your airplane’s approved handbook gives the speeds and procedures that apply to it. Practice stalls and spin recovery only with a qualified flight instructor.
Keep reading: FAA Private Pilot Knowledge Test: Format, Subjects and How to Pass · How to Read a METAR and TAF · Weight and Balance Explained · Wind Correction and Heading Conversion · VFR Weather Minimums and Airspace Classes
Frequently asked questions
At what airspeed does an airplane stall?
There is no single stall speed. A wing stalls whenever it exceeds its critical angle of attack, at any airspeed and in any attitude. The published stall speed is only the speed at which that happens in straight and level flight at a given weight and configuration. In a turn or a pull-up it stalls at a higher speed.
How do you calculate the load factor in a turn?
In a coordinated, level turn, the load factor equals 1 divided by the cosine of the bank angle. It is about 1.15 at 30 degrees, 1.41 at 45 degrees and exactly 2.0 at 60 degrees. At 60 degrees of bank, the wings carry twice the airplane’s weight.
How much does the stall speed rise in a turn?
Stall speed increases with the square root of the load factor. It rises about 7 percent at 30 degrees of bank, 19 percent at 45 degrees and 41 percent at 60 degrees. An airplane that stalls at 50 knots in level flight stalls at about 71 knots in a level 60-degree bank.
What is maneuvering speed (Va), and does it change with weight?
Va is the speed below which a full, abrupt control input will stall the wing before the load becomes high enough to damage the structure. It decreases when the airplane is lighter, in proportion to the square root of the weight ratio, so the value printed for maximum weight is too high for a light airplane.
What is the correct first response to a stall?
Reduce the angle of attack by lowering the nose, then roll the wings level and add power as needed, and return to the desired flight path. Recover at the first indication of a stall, such as the stall warning or buffeting, and avoid pulling back too hard afterward, which can cause a secondary stall.