“An aircraft stalls when it flies too slowly.” Almost every student pilot believes this at some point, and it is exactly the idea that exam questions are designed to catch. A stall is about angle of attack, not airspeed, and load factor explains why an aircraft can stall at a speed well above the one marked on the airspeed indicator. In the EASA PPL theory exam, these two ideas sit at the heart of the Principles of Flight paper, and they come back whenever turns, mass or spins are involved. This guide gives you the mechanisms, the useful formulas, worked examples and the classic exam traps.
A wing stalls when it exceeds its critical angle of attack, at any airspeed. The load factor n = lift ÷ weight equals 1 / cos(bank angle) in a level turn: 2 at 60°. The stall speed is multiplied by √n: +41% at 60°. It also rises with mass (as √mass) and with a forward CG, and it falls with flaps. To recover from a stall, reduce the angle of attack first. A spin is a stall plus yaw.
What Is a Stall? Angle of Attack, Not Airspeed
The angle of attack is the angle between the wing’s chord line and the relative airflow, the direction the air meets the wing from. It is not the pitch attitude (the angle between the aircraft’s longitudinal axis and the horizon) and not the flight path angle. An aircraft can have its nose below the horizon and a high angle of attack, for example during a sharp pull-up out of a dive.
As the angle of attack increases, lift increases too, up to a maximum. Beyond the critical angle of attack, the airflow can no longer follow the upper surface: it separates from the wing, lift drops sharply and drag rises steeply. That is the stall. For a typical light aircraft wing, the critical angle is roughly 15 to 18°, and it changes with flap setting and with the condition of the wing.
The Lift Equation
One formula explains everything:
Lift = ½ × ρ × V² × S × CL
where ρ is air density, V the true airspeed, S the wing area and CL the lift coefficient, which depends on angle of attack. CL has a maximum value, CL max, reached just before the critical angle.
In level flight, lift must equal weight. If airspeed falls, CL must rise, so the angle of attack must increase to compensate. At the lowest possible speed, the wing is flying at CL max: that is the stall speed. But if you ask the wing for more lift than the weight (in a turn or a pull-up), CL max is reached at a higher speed.
Signs of an Approaching Stall
| Sign | What you notice |
|---|---|
| 🔊 Stall warning | A horn or light that triggers a few knots before the stall |
| 🫨 Buffet | Vibration as the separated airflow hits the tail |
| 🕹️ Sloppy controls | The control surfaces, especially the ailerons, lose effectiveness |
| 📐 High nose attitude | The nose is high, yet the aircraft is not climbing |
| ⬇️ Sink | The vertical speed indicator shows a descent despite back pressure |
| ↪️ Nose drop or wing drop | The nose falls, sometimes with one wing going down |
On aircraft certified under the legacy CS-23 standards, the stall warning must begin at least 5 knots above the stall speed. It is a safety net, not a substitute for recognising the other signs: it can be badly adjusted, and it becomes less reliable when the wing is contaminated.
Load Factor: Definition and the Turn Formula
The load factor (n) is the ratio of lift to weight:
n = lift ÷ weight
It is expressed in “g”. In straight and level flight, n = 1. In a level turn, the wing must both support the weight and provide the force that turns the aircraft: the tilted lift vector must therefore be larger than the weight. The relationship is simple:
n = 1 / cos(bank angle)
| Bank angle | Load factor | Increase in stall speed (√n) |
|---|---|---|
| 0° | 1.00 | 0% |
| 15° | 1.04 | +2% |
| 30° | 1.15 | +7% |
| 45° | 1.41 | +19% |
| 60° | 2.00 | +41% |
| 70° | 2.92 | +71% |
| 75° | 3.86 | +97% |
Two important points:
- The relationship does not depend on aircraft type or airspeed: in a 60° level turn, a glider, a trainer and a fighter jet all pull 2 g.
- Load factor climbs very quickly beyond 45°. Between 60° and 75° it goes from 2 to almost 4, so steep level turns are soon limited by the structure and by the angle of attack.
Load factor is not only about turns. In a pull-up out of a dive it exceeds 1; when you push forward over the top of a climb it drops below 1 and can even go negative. Gusts and turbulence also change the load factor, sometimes abruptly.
Stall Speed in a Turn and Under Load
Since the required lift is multiplied by n, and lift varies with the square of airspeed, the stall speed is multiplied by the square root of n:
Stall speed under load = level-flight stall speed × √n
Worked Example
Take a fictional trainer with a clean stall speed of 50 kt at maximum mass:
- 30° bank: n ≈ 1.15, √n ≈ 1.07, stall speed ≈ 54 kt.
- 45° bank: n ≈ 1.41, √n ≈ 1.19, stall speed ≈ 59 kt.
- 60° bank: n = 2, √n ≈ 1.41, stall speed ≈ 71 kt.
Now picture an over-tight base-to-final turn at 60 kt with 45° of bank: you are barely above the stall speed (about 59 kt). Pull a little harder to tighten the turn, and you are there. That is precisely the classic base-to-final stall accident scenario.
The Accelerated Stall
An aircraft can therefore stall at high speed if the angle of attack is increased abruptly, for instance by pulling hard out of a dive. This is an accelerated stall, also called a high-speed or dynamic stall. It is often more violent than a level-flight stall, and a wing drop is more likely.
Conversely, under a load factor below 1 (pushing forward), the stall speed falls. At 0 g the wing produces no lift at all and cannot stall, even at very low speed.
What About Altitude?
In indicated airspeed, the stall speed stays roughly the same at any altitude, because the airspeed indicator measures dynamic pressure, the same pressure that produces lift. In true airspeed, however, it increases with altitude. This is a favourite exam question: remember that the indicated stall speed hardly changes with altitude.
Mass, CG, Flaps and Power: What Changes the Stall Speed
Load factor is not the only parameter. These are the main ones to know for the exam:
| Factor | Effect on stall speed | Why |
|---|---|---|
| ⚖️ Higher mass | ⬆️ Increases (as √mass) | More lift is needed |
| ↩️ Higher load factor | ⬆️ Increases (as √n) | More lift is needed |
| ⏩ Forward CG | ⬆️ Increases slightly | The tailplane pushes down harder, the wing must compensate |
| ⏪ Aft CG | ⬇️ Decreases slightly | Less tail download, but reduced stability |
| 🛬 Flaps extended | ⬇️ Decreases | CL max increases |
| 🔥 Power on | ⬇️ Decreases a little | Propeller slipstream and the vertical part of thrust help |
| 🧊 Frost, ice, insects | ⬆️ Increases markedly | CL max drops and the critical angle falls |
| 🏔️ Altitude | ➡️ Unchanged in IAS | Lift depends on dynamic pressure |
Mass: a Simple Calculation
Stall speed varies with the square root of mass. With our fictional aircraft (50 kt at 1,000 kg), at 850 kg:
Stall speed ≈ 50 × √(850 ÷ 1,000) ≈ 50 × 0.92 ≈ 46 kt.
Remember the order of magnitude: 10% more mass gives about 5% more stall speed.
Centre of Gravity: the Link with Mass and Balance
With a forward CG, the horizontal tail must produce more downward force to balance the aircraft. The wing therefore has to carry the weight plus that download, and the stall speed rises a little. With an aft CG, the opposite happens, but the aircraft becomes less stable in pitch, more sensitive to control inputs, and recovery from a stall or spin can become difficult, or even impossible outside the limits. That is one reason the CG must stay inside the flight manual envelope: our guide to mass and balance step by step explains the calculation.
Airspeed Indicator Markings
The airspeed indicator arcs translate these ideas, and exam questions often mix Principles of Flight with Aircraft General Knowledge:
- Bottom of the white arc: stall speed in landing configuration (VS0), flaps down, at maximum mass.
- Bottom of the green arc: clean stall speed (VS1), at maximum mass.
- Top of the white arc: maximum flap extended speed (VFE).
- Top of the green arc: maximum structural cruising speed (VNO). The yellow arc, between VNO and VNE, is for smooth air only.
- Red line: never exceed speed (VNE).
Careful: these marks apply to level flight, wings level, at maximum mass. In a turn, the aircraft can stall in the middle of the green arc.
The Flight Envelope: Limit Load Factors and Manoeuvring Speed
An aircraft’s structure is designed for limit load factors. For light aeroplanes certified under the legacy CS-23 standards, the usual values are:
| Category | Positive limit load factor | Negative limit load factor |
|---|---|---|
| Normal | +3.8 g | -1.52 g |
| Utility | +4.4 g | -1.76 g |
| Aerobatic | +6.0 g | -3.0 g |
Beyond the limit load, the structure may suffer permanent deformation. A safety factor of 1.5 separates the limit load from the ultimate load, beyond which failure becomes possible. The exact values for your aircraft are in its flight manual, and they can depend on mass and configuration (with flaps down, the limits are lower).
The V-n Diagram
These limits are plotted on a speed versus load factor diagram (the V-n diagram). At low speed, the limit is aerodynamic: the aircraft stalls before it can reach the limit load factor. At high speed, the limit is structural: the wing could produce far more than the limit load, and it is the structure that is at risk.
The point where the two limits meet corresponds to the design manoeuvring speed (VA):
VA ≈ VS1 × √(limit load factor)
With our normal category aircraft (VS1 = 50 kt, n = 3.8): VA ≈ 50 × 1.95 ≈ 97 kt. Below VA, a single full and abrupt elevator input will stall the aircraft before it reaches the limit load. Above VA, that protection is no longer guaranteed.
Classic trap: VA decreases as mass decreases. A lighter aircraft stalls at a lower speed, but at a given angle of attack it experiences a higher load factor, so it can reach the limit load at a lower speed. At 850 kg, our VA drops to about 97 × 0.92 ≈ 89 kt. Also bear in mind that VA does not protect you against full alternating inputs on the same axis or combined inputs on several axes.
💡 Tip: learn the three square roots that keep coming back: √2 ≈ 1.41 (60° bank), √1.41 ≈ 1.19 (45° bank) and √3.8 ≈ 1.95 (manoeuvring speed, normal category). With those three values you can solve most calculation questions without a calculator.
Stall Recovery: the Method
The principle fits in one sentence: to recover from a stall, reduce the angle of attack. As long as the angle of attack stays beyond the critical angle, adding power or pulling back only makes things worse.
The sequence commonly taught, to be adapted to your flight manual and your instructor’s guidance:
- Move the control column forward to reduce the angle of attack, until the stall warning and the buffet stop.
- Apply power to minimise height loss, using rudder to counter the engine effects.
- Level the wings, countering a wing drop mainly with rudder, because large aileron inputs near the stall can deepen the stall on the lower wing.
- Return to a climb or level flight, without pulling too hard, to avoid a secondary (accelerated) stall.
Why Does a Wing Drop?
If one wing stalls before the other, the aircraft rolls towards the stalled wing. Designers shape the wing so that the root stalls before the tip (washout, different aerofoils, stall strips): the ailerons stay effective longer and the stall is gentler. A skidding or slipping turn at the moment of the stall greatly increases the risk of a wing drop.
The Spin: a Stall Plus Yaw
A spin is an autorotation: the aircraft is stalled and descends while rotating around a vertical axis. It starts from a stall combined with yaw, for example too much rudder in a skidding turn. The inner wing, more deeply stalled, produces less lift and more drag than the outer wing, which keeps the rotation going.
Spin or Spiral Dive?
| Criterion | Spin | Spiral dive |
|---|---|---|
| Wing stalled? | ✅ Yes | ❌ No |
| Indicated airspeed | Low and fairly steady | Increasing rapidly |
| Load factor | Moderate | High, and rising |
| Recovery | Stop the autorotation: opposite rudder, stick forward | Throttle closed, wings level, gentle pull-out |
The standard spin recovery, unless your flight manual specifies otherwise, follows this order (often remembered as PARE): Power to idle, Ailerons neutral, Rudder full opposite to the rotation, Elevator forward until the rotation stops, then rudder neutral and a smooth pull-out from the dive. Your aircraft’s flight manual procedure always takes precedence.
Many training aircraft are not approved for intentional spins: check your aircraft’s limitations.
Classic EASA PPL Exam Traps
- “The aircraft stalls at a given airspeed”: wrong, it stalls at a given angle of attack.
- Mixing up pitch attitude and angle of attack: an aircraft can stall nose down.
- Forgetting the square root: at 60° the load factor doubles, but the stall speed only rises by 41%.
- Thinking VA increases when the aircraft is lighter: it decreases.
- Reversing the CG effect: forward CG raises the stall speed; aft CG lowers it but degrades stability.
- Believing the indicated stall speed rises with altitude: it stays practically constant.
- Adding power before reducing the angle of attack: the priority is always to unload the wing.
- Confusing a spin with a spiral dive: look at the airspeed.
- Reading the airspeed indicator arcs as valid in a turn: they apply to level flight at maximum mass.
How to Study Stalls and Load Factor
A good progression:
- Understand before you memorise: go back over the lift equation until you can explain, without notes, why the stall speed rises in a turn.
- Turn the key values into flashcards: 1 / cos, the load factors at 30°, 45° and 60°, √n, the categories and their limit load factors, the airspeed indicator arcs, the spin recovery sequence. Spaced repetition brings each card back just before you would forget it.
- Practise exam-style questions: with AltiPrep, you work through Principles of Flight questions with an explanation after every answer, flashcards with spaced repetition and mock exams to track your progress subject by subject.
Each subject is a separate paper and you need at least 75% on each one, with no negative marking; the number of questions and the time allowed are set by your national aviation authority. To see where Principles of Flight fits among the nine papers, read our guide to the 9 subjects of the EASA PPL(A) theory exam, and for the overall rules, how to pass the EASA PPL theory exam first time.
Are You Ready on This Topic?
You have mastered this chapter if you can, without hesitating:
- define angle of attack and tell it apart from pitch attitude and flight path;
- explain why an aircraft stalls at an angle of attack rather than at an airspeed;
- calculate the load factor and the stall speed at 30°, 45° and 60° of bank;
- state the effect of mass, CG, flaps, power and ice on the stall speed;
- explain the V-n diagram and the role of manoeuvring speed;
- give the stall and spin recovery sequences;
- tell a spin from a spiral dive.
Key Takeaways
- A wing stalls when it exceeds its critical angle of attack, whatever the airspeed.
- In a level turn, n = 1 / cos(bank angle): 1.15 at 30°, 1.41 at 45°, 2 at 60°.
- The stall speed is multiplied by √n and varies with √mass; it rises with a forward CG and falls with flaps.
- Manoeuvring speed VA ≈ VS1 × √(limit load factor), and it decreases as mass decreases.
- To recover from a stall, reduce the angle of attack first; a spin is a stall plus yaw.
Further reading:
- The 9 Subjects of the EASA PPL(A) Theory Exam Explained
- Mass and Balance Explained: Step-by-Step EASA PPL Guide
- How to Pass the EASA PPL Theory Exam First Time
- Spaced Repetition: The Most Effective Way to Study for Your EASA PPL Exam
Happy studying, and always keep a healthy margin below the critical angle of attack!
Frequently asked questions
At what speed does an aircraft stall?
There is no single stall speed. A wing always stalls when it exceeds its critical angle of attack, whatever the airspeed. The stall speed published in the flight manual only applies in straight and level flight, at the stated mass and configuration: it rises in a turn, in a pull-up and with extra mass.
How do you calculate the load factor in a turn?
In a steady level turn, the load factor is n = 1 / cos(bank angle). It is about 1.15 at 30°, 1.41 at 45° and exactly 2 at 60°. At 60° of bank, the structure and the pilot carry twice their normal weight.
How much does the stall speed increase in a turn?
The stall speed is multiplied by the square root of the load factor. It rises by about 7% at 30° of bank, 19% at 45° and 41% at 60°. An aircraft that stalls at 50 kt in level flight therefore stalls at around 71 kt in a 60° level turn.
What is the difference between a spin and a spiral dive?
In a spin the aircraft is stalled: the indicated airspeed stays low and fairly steady while it rotates in autorotation. In a spiral dive the wing is not stalled and the airspeed builds up quickly. The recoveries differ: in a spin you stop the rotation with opposite rudder and forward stick; in a spiral dive you close the throttle, level the wings and then pull out gently.
Do mass and centre of gravity change the stall speed?
Yes. Stall speed varies with the square root of the mass: 10% more mass gives about 5% more stall speed. A forward centre of gravity also raises it slightly, because the tailplane must push down harder. An aft CG lowers it, but makes the aircraft less stable and spin recovery harder.
Are stalls and load factor tested in the EASA PPL theory exam?
Yes. Stalls, load factor, the flight envelope and spins are core topics of the Principles of Flight paper, and airspeed indicator markings also come up in Aircraft General Knowledge. You need at least 75% on each paper. The number of questions and the time allowed are set by your national aviation authority.
