Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Slow Flight and Stalling - Theory Part 1:

Understanding and Recognising Stalls

CASA Recreational Pilot License (Aeroplane) — Lesson 5, Pre-flight theory

All text and presenter notes in this briefing are licensed under Creative Commons BY-SA 4.0. More info

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What is a stall?

  • What experience do you have with "stalls" outside of aviation?
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What is a stall?

  • What experience do you have with "stalls" outside of aviation?
  • How would you define a stall in that situation?
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What is a stall?

  • What experience do you have with "stalls" outside of aviation?
  • How would you define a stall in that situation?
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

How does an aeroplane stall differ?

  • How might an aeroplane stall be different from your own experience outside of aviation?
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

How does an aeroplane stall differ?

  • How might an aeroplane stall be different from your own experience outside of aviation?

Try searching for "aeroplane stall test" on YouTube:

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Theory Lesson Overview — Part 1

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Learning Objectives for this lesson

By the end of this session, our aim is to be able to:

  • Explain what causes a stall — in terms of angle of attack, not airspeed
  • Describe why the lift force is reduced during a stall
  • Outline how we can avoid stalls in the first place
  • Describe the symptoms of an imminent and a fully developed stall, including
    any stall warning devices on board
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Waypoint 1 — Stall Aerodynamics

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Review: how is lift produced?

right

Lift is created by the smooth airflow over the wing deflecting the air downwards.

This downward deflection of air by the wing has an equal and opposite force upwards on the wing - the lift force.

(Images CC-BY-SA 3.0 sourced from Wikimedia)

right

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Review: Lift is one of the four forces

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Review - Angle of Attack

right

The angle of attack is the angle between the chord line of the wing and the relative airflow (the direction the air is coming from).

  • As we increase back pressure on the elevator, we increase the AoA
  • As we decrease back pressure (or push forward), we reduce the AoA
  • The critical AoA for most light training aircraft is approximately 16° (see next slide)

This is why the first action in every stall recovery is: reduce the angle of attack — ease forward on the control column.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

How much lift? Putting names to the factors

With a hand out the car window in Lesson 2, you worked out the four things that change the size of the lift force. The formula just gives them names:

Term What changes it in flight
Lift coefficient — set by the wing's shape and its angle of attack The control column, and flap
Air density Height, temperature, humidity
Airspeed, squared Power and attitude
Wing area Nothing — it's fixed

Lift must balance weight. So as falls, something has to grow — and the only one you can grow quickly is , by increasing the angle of attack.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Angle of Attack and the Lift Coefficient

right
The graph shows the Lift Coefficient from our formula on the vertical axis — for a given wing, it is set by the Angle of Attack on the horizontal axis:

  • increases at a constant rate until the angle of attack approaches the critical angle of 16
  • After the critical angle, it falls rapidly — and with nothing else in the formula changing, so does the lift
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What actually causes a wing to stop producing lift?

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What actually causes a wing to stop producing lift?

right A 1905 photograph from a wind tunnel

  • A separation of the airflow from the wing after the critical angle of attack

Image originally from the German Aerospace Centre (DLR - Deutsches Zentrum für Luft- und Raumfahrt) but available on the Wikipedia page for Stall and Flow Separation and licensed by DLR under a Creative Commons By Attribution License 3.0

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What actually causes a wing to stop producing lift?

Notice where the stall starts on the wing surface!

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What is a Stall?

right medium A 1905 photograph from a wind tunnel

A stall is not about airspeed.

A stall occurs when the angle of attack (AoA) exceeds the critical angle of attack — the point at which the smooth airflow over the wing separates, and lift collapses.

  • Below critical AoA: airflow stays attached, lift is generated
  • At critical AoA: airflow begins to separate from the upper wing surface and we start losing lift, resulting turbulent air buffeting tailplane
  • Above critical AoA: airflow fully separates, lift drops dramatically

A wing can stall at any airspeed — even at cruise speed if the AoA is high enough (e.g. a steep turn with back pressure).

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Why Does Lift Collapse?

right medium A 1905 photograph from a wind tunnel

As AoA increases beyond the critical angle:

  1. Airflow can no longer follow the curvature of the upper wing surface
  2. The boundary layer separates from the wing, creating turbulent eddies
  3. The low pressure region that was producing lift near the leading edge of the wing collapses, causing the centre of pressure to move rearwards — behind the centre of gravity
  4. Drag increases sharply at the point of stall

The result: rapid loss of lift, combined with a pitch-down tendency — with the centre of pressure now behind the centre of gravity, what lift remains pitches the nose down.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Waypoint 2 — Stall Avoidance

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Stall Avoidance

The best way to handle a stall is to not enter one in the first place.

Situations that increase the angle of attack and therefore increase stall risk:

  • Pulling back on the controls — the most direct way to increase AoA
  • Slow flight — maintaining altitude at low speed requires increased AoA
  • Steep turns — increased load factor raises the stall speed

Always maintain awareness of airspeed and pitch attitude together. Neither alone is enough.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Stall Avoidance

Other situations that can also increase the risk of stalling:

  • Flap retraction — sudden loss of lift at low speed requires prompt power response
  • Turbulence or gusts — can produce sudden AoA changes
  • Uncoordinated flight
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

The Importance of Coordinated Flight

Uncoordinated flight is particularly dangerous near the stall:

  • In a skid (too much inside rudder compared to aileron), the lower wing can reach its critical AoA while the upper wing is still flying — causing a wing drop
  • A wing drop at low altitude (e.g. during a base-to-final turn) leaves almost no height to recover
  • Always use balanced (coordinated) flight, especially at low speeds

right small Bold Method article
Read the Bold Method article "Why skids are more dangerous than slips" for more detail about why a skidding (adding too much inside rudder for the turn) during slow flight is dangerous when low.

Keep the balance ball centred. At slow speeds, every input matters.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Induced Drag at Slow Speed

As we fly slower to maintain lift, we must increase AoA. This creates:

  • Induced drag — the drag component that results from generating lift
  • Induced drag increases rapidly at high AoA / slow speed (it increases with the square of AoA)
  • This is why the power required to maintain altitude increases dramatically at slow speed

The aircraft is working harder for less result — we are "behind the power curve."

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

What is "behind the curve"?

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Waypoint 3 — Recognising the Stall

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Symptoms of an Imminent Stall due to slow speed

As we approach the stall, the aeroplane provides several warnings:

  • Reduced airspeed — ASI needle is low and falling
  • High nose attitude — nose is pitched well above the horizon
  • Mushy or spongy controls — reduced response to inputs, that is, Reduced control effectiveness
  • Buffeting — turbulent airflow from the stalling wing root disturbs the tailplane
  • Stall warning device — horn, light, or stick shaker activates

The imminent stall is the moment to act — recovery is easiest here, with minimum height loss.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Buffeting demo

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Control Effectiveness at Slow Speed

Control surfaces work by deflecting airflow. As airspeed decreases:

  • Less airflow passes over each control surface
  • The same deflection produces a weaker force
  • Controls feel mushy or unresponsive — larger inputs are needed for the same effect
  • At very slow speeds, controls may feel spongy or almost useless

This is the beginning of the degradation that leads to the stall.

This is why you feel the controls before you experience the full stall — the controls are telling you something important.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

The Stall Warning Device

right A stall warning vane on a PA28

Most training aircraft have an automatic stall warning device — typically a horn or buzzer:

  • Triggered by a vane or slot on the leading edge of the wing that detects AoA
  • Activates at approximately 5–10 kt above the stall speed (depending on configuration)
  • The warning sounds before the full stall — it is an early warning system

The warning horn is your cue to act immediately.

Do not wait for the full stall to develop. The moment the horn sounds, reduce AoA.

Image licensed CC-BY-SA 3.0 Frank Murrman

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Symptoms of a Fully Developed Stall

If the imminent stall warnings are ignored, the full stall develops:

  • Sudden, significant loss of lift — the aircraft is descending, possibly rapidly
  • Loss of control effectiveness — controls may feel reversed or ineffective
  • Wing drop — if any yaw or asymmetric lift is present, one wing may stall first and drop - keep ailerons neutral!
  • Nose pitches down — the natural self-recovery tendency of most training aircraft

The fully developed stall requires prompt recovery action — especially at low altitude.

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Waypoint 4 — Recap

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Summary — Part 1

What we covered:

Topic Key point
Stall aerodynamics When does a stall occur? Why?
Stall avoidance What types of flight can we avoid to avoid stalls?
Imminent stall symptoms What are the symptoms of an imminent stall
Fully developed stall What are the effects of a fully developed stall
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Summary — Part 1

What we covered:

Topic Key point
Stall aerodynamics A stall occurs when AoA exceeds the critical angle — not necessarily when airspeed is "too low" (though that is the most common scenario)
Stall avoidance Slow flight, steep turns, uncoordinated flight all increase stall risk
Imminent stall symptoms Low speed, high nose, control effectiveness, buffet, stall horn — act immediately
Fully developed stall Lift collapses, nose drops, possible wing drop — requires prompt recovery
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Objectives Check — Part 1

Can you:

  • Explain what causes a stall (hint: it's not airspeed alone)?
  • Describe what happens to lift when the critical AoA is exceeded?
  • List the symptoms that warn of an imminent stall?
  • Explain what the stall warning device detects and when it activates?
  • Describe what happens when the full stall develops?
Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Arrival

Slow Flight and Stalling — Theory Part 1: Understanding and Recognising Stalls

Questions?

Any questions before we move on to Part 2?

Part 2 covers: stall recovery technique, effect of power and flap, factors affecting stall speed, instrument indications, and the HASELL check.

This first theory session covers the aerodynamics of slow flight and stalling, how and why a stall happens, and how to recognise an impending stall. Part 2 covers recovery techniques, factors affecting stall speed, instrument indications, and the HASELL check. Together these prepare the student for what they will experience in the air.

Most people have stalled cars either at traffic lights or on a steep hill. The key points are that the engine doesn't have enough power to continue producing the force required to move forwards.

- We're used to stalling in a car - when the car doesn't have enough power to continue climbing a hill (until we change gears or apply more power). - Set 38% throttle for best stall. - How is a stall different for an aeroplane?

- This is where our analogy breaks down: it's quite different. - First, in a car, it's about not having enough force available to keep moving you forward (whether at the lights or on a hill), whereas in an aeroplane it's about not having enough force to hold you up in the air. - Second, in an aeroplane it's not necessarily related to power - you can stall a plane both when you're going slow or fast.

Part 1 builds the foundation: what a stall actually is aerodynamically, why it matters, how control effectiveness degrades before the stall, and the symptoms that warn us it's coming. Part 2 covers recovery and the factors that change stall speed.

Click Direct-To to advance to Stall Aerodynamics.

- Briefly review that the Lift force is just one of the four forces, balancing the weight. - The lift is split into the vertical component of lift and the horizontal component of lift when banking. **Worth a side note here: where those two arrows act.** Follow the grey lines through the lift and weight arrows. Weight acts at the **centre of gravity**; lift acts at the **centre of pressure** — the point through which all the pressure differences over the wing add up. The two are not in the same place, so lift and weight form a **couple**, and a second couple between thrust and drag opposes it. **Which way each couple acts depends on the aeroplane** — be careful not to teach the model's arrangement as a universal rule: - The **biplane in this model** has its centre of pressure a little **forward** of the centre of gravity, so its lift/weight couple is **nose-up**, opposed by a nose-down thrust/drag couple (the thrust line sits above the drag line of the wing cell and its rigging). - **Most conventional trainers are the other way round**, and this is the arrangement the textbooks draw: the centre of pressure sits **behind** the centre of gravity, giving a **nose-down** lift/weight couple, balanced by a **download on the tailplane** and by a nose-up thrust/drag couple. It is deliberate — lose the engine and the nose drops rather than rises, so the aeroplane keeps flying speed by itself. The arrows show *where* the forces act, not a pitching simulation — the model has no moment dynamics behind it. **What matters for stalling is the same either way.** As the airflow separates, the suction peak near the leading edge collapses, so the centre of pressure **moves rearward**. Wherever it started, it ends up further behind the centre of gravity, so the lift/weight moment shifts **nose-down**. The nose drops on its own, the angle of attack reduces, and the wing starts flying again — the self-recovery tendency most training aircraft are designed to have. Two consequences to draw out (they return in Part 2's table of factors affecting stall speed): - A **forward** centre of gravity gives a longer arm to that nose-down moment, so the nose drops more decisively — the stall is more pronounced but recovery is easier. It also means the tailplane must hold more download, so the wing carries more than just the weight and the stall speed rises. - An **aft** centre of gravity leaves the centre of pressure much less room to move behind, so the nose-down tendency is weak. The stall is gentler in feel but **recovery is harder** — one of the reasons the loading limits matter. If it helps, sketch it rather than relying on the model: a fulcrum at the CoG, the lift arrow near it, then slide the lift arrow well behind it as the wing stalls.

Use a physical aeroplane model to show, or have the student show, what happens as we increase back pressure or decrease back pressure. As the elevator changes, the angle of attack of the wing, the main lifting surface is changed.

There is no need to drill the algebra here — the point is that the formula holds no surprises. It is the same four factors the student reasoned out for themselves with a hand out the car window in Lesson 2, now with names attached. Work the level-flight argument through on the board, because it is the whole lesson in one line: lift has to equal weight, so flying slower forces a higher angle of attack. Keep slowing, and eventually the angle of attack needed to hold you up is more than the wing can give — that is the stall. **Where flap fits, if it comes up.** $S$ is a fixed reference area — it is a property of the aeroplane, not something the pilot changes, so treat it as a constant. Lowering flap increases the wing's **camber**, which raises $C_{L}$ at any given angle of attack; that is why flap lowers the stall speed. Some types (Fowler and semi-Fowler flaps) do also slide the flap rearward and add a little real area, but $S$ in the formula stays the fixed reference value, so even that shows up as a change in $C_{L}$. Everything flap does, it does through $C_{L}$. The punchline to land: **only $C_{L}$ has a ceiling.** Density, speed and area will keep giving you more lift for more of them, but $C_{L}$ stops climbing at the critical angle and then falls away. That is why we define the stall by angle of attack and not by a speed. Everything in the "factors affecting stall speed" table in Part 2 is this same equation rearranged. If a student wants it explicitly, the stall speed falls out as $V_{S} = \sqrt{2W / (\rho\,S\,C_{L_{max}})}$ — heavier or thinner air raises it; more wing area, or flap, lowers it. Offer that only if they ask for it.

Note the spirals of air, with some air going backwards - we'll see more of this later. - When this angle between the surrounding airflow and the wing passes a critical angle (around 16 degrees), the air breaks away from the wing as we'll see soon. - It's a good thing that aeroplanes are designed to pitch forward when they stall - it doesn't have to be that way, but it's designed in to help in a stall Lead in: But what does that look like outside of a wind tunnel? Is it something we can see? With some help of wool, yes!

One of the best *real* visualisations I've found for what's actually happening in a stall is achieved by taping pieces of cotten or wool to the wing of an aircraft.

The key misconception to address is that "stalling = going too slow." While slow speed is the most common trigger, the stall is always caused by exceeding the critical angle of attack. This is fundamental — return to it throughout the lesson.

Refer back to the four forces side note: lift acts at the centre of pressure, weight at the centre of gravity, and in normal flight the two sit close together — which side of the other varies with the type. As the wing stalls the centre of pressure moves aft, so wherever it started it ends up well behind the centre of gravity and the lift/weight moment becomes strongly nose-down. The pitch-down tendency is a useful natural self-recovery tendency in most light aircraft — the nose drops, AoA reduces, airflow reattaches, and lift returns. The danger is when we instinctively pull back to "stop the nose dropping."

Click Direct-To to advance to Stall Avoidance.

You might notice that pulling back on the controls and slow flight are risks that we're exposed to both during take-off and landing. There are videos on the internet showing stalls on take-off due to these two factors, such as: - https://www.youtube.com/shorts/4Px1f3ukBpo but I removed the slide with that from the deck since it's quite confronting for the first stalls lesson. Feedback was maybe introduce those for advanced stalling. Leaving the link here in the presenter notes in case. The only real problem in this video and many others like it, is that it happens so close to the ground. It's easy to recover from if you have sufficient height, but low to the ground you don't have that option. That's why the **H**eight, for sufficient height, is the first item is our **HASELL** check.

In what stage of a flight might you find yourself pulling back on the controls, slowing the aeroplane down and possibly turning steeply? The turning stall and base-to-final stall (with a low, slow, uncoordinated turn) is a leading cause of fatal general aviation accidents. Plant the seed now — we will explore this more in Lesson 16 (Advanced Stalling).

This connects to the secondary effects covered in Lesson 1 — rudder-alone use causes roll; aileron-alone causes slip. Combining them incorrectly near the stall is dangerous.

Keep this brief — induced drag is covered in the aerodynamics theory. The key takeaway is: at slow speed, you need more power to stay level, not less.

The power required as you slow down isn't shown so dramatically here, but it can still be seen: At the minimum power required, around V_x, as we slow down further, *more* power is required.

Click Direct-To to advance to Recognising the Stall.

We will practise recognising the imminent stall in the air and recovering the moment the warning sounds, before the full stall develops. This is the preferred recovery technique.

To see buffetting, use full power and 15 or 16 AoA 17 and 17.5 show lift dropping to nothing, but 18 collapses the simulation at full power.

Relate this to the hand-out-the-window analogy: at slow speed, very little force on the hand. The student will feel this in the air today.

The stall warning vane works by detecting the angle at which air is flowing into the leading edge. As AoA increases, the stagnation point moves down toward the lower leading edge, causing the vane to deflect and trigger the warning. Some aircraft use a heated pitot-type sensor instead.

Emphasise: in a fully developed stall, the instinct to pull back makes things worse. The control column must move forward to reduce AoA. This is counter-intuitive for new students and must be reinforced.

Click Direct-To to advance to the recap.