Understanding an airplane stall requires more than knowing the airspeed at which a wing may stop producing enough lift for a particular flight condition. The real explanation lies in the way air flows around the wing and how that airflow changes as aerodynamic forces increase.
Why does a wing lose lift when its critical angle of attack is exceeded? Why does stall speed change with aircraft weight, center of gravity, or load factor? And how do flaps, wing design, and ice affect the aerodynamic characteristics of a stall?
This lesson explores the physics behind these questions. We will examine the relationship between angle of attack, airflow separation, lift, and stall speed, then study the aircraft and environmental factors that influence stall behavior.
Understand the aerodynamic mechanisms of an airplane stall and explain how wing design, aircraft loading, configuration, and atmospheric conditions influence stall characteristics.
Explore how airflow separation changes the lift coefficient and increases drag as the wing approaches and exceeds its critical angle of attack.
Discover why aircraft weight, center of gravity, and maneuvering loads influence stall speed and the aerodynamic forces acting on the wing.
Understand how wing shape, flaps, ice, frost, and atmospheric disturbances can modify stall characteristics and overall wing performance.
Why can the same airplane stall at different airspeeds even though its wing reaches essentially the same critical angle of attack in a given configuration?
What Happens to Airflow When a Wing Stalls?
An airplane stall is an aerodynamic condition that occurs when a wing exceeds its critical angle of attack. Airflow separation over the wing increases, causing its lift coefficient to decrease from its maximum value while aerodynamic drag increases. A stall can occur at different airspeeds, flight attitudes, and power settings.
A wing does not suddenly stop producing all lift when it stalls. Instead, it loses part of its ability to generate lift because the airflow around its surface has changed.
To understand this process, we need to examine the boundary layer and the mechanism that causes air to separate from the upper surface of the wing.
The Boundary Layer: Air Close to the Wing
As air moves around a wing, friction slows the air immediately next to its surface. This creates a thin region called the boundary layer.
Air farther away from the surface moves more freely, while the air inside the boundary layer has less energy and is more sensitive to pressure changes.
As long as this layer can follow the curved surface of the wing, the airflow remains largely attached. If it can no longer follow that surface, the airflow begins to separate.
Why Does the Airflow Separate?
As angle of attack increases, the airflow over the upper surface of the wing faces increasingly difficult conditions.
After passing the low-pressure region near the forward part of the wing, the air must generally move toward a region of increasing pressure. This is called an adverse pressure gradient.
The slower-moving air inside the boundary layer may not have enough energy to continue moving against this pressure increase. When it can no longer follow the wing's surface, the airflow separates.
As angle of attack continues to increase, the separated region can become larger. Once the wing exceeds its critical angle of attack, the loss of lifting capability becomes significant and the wing stalls.
Three Stages of Airflow Separation
At a moderate angle of attack, airflow remains largely attached to the wing. The pressure distribution around the airfoil allows the wing to generate lift.
As angle of attack increases, the boundary layer may begin separating from part of the upper surface. The wing approaches its maximum lift coefficient.
Beyond the critical angle of attack, separation becomes extensive. The lift coefficient decreases and aerodynamic drag increases.
How Does Airflow Separation Affect Lift and Drag?
As angle of attack increases, the wing's lift coefficient generally increases until it reaches a maximum value known as CLmax.
Beyond this point, a further increase in angle of attack causes more extensive airflow separation rather than an increase in lift coefficient.
The separated airflow changes the pressure distribution around the wing. Once CLmax has been exceeded, the wing becomes less effective at producing lift.
Separation produces a larger turbulent wake behind the wing. This increases pressure drag and reduces the aerodynamic efficiency of the airfoil.
Video — Understanding an Airplane Stall
Watch the video, then relate what you observe to the critical angle of attack, airflow separation, and changes in lift.
An aerodynamic stall occurs when a wing exceeds its critical angle of attack. The wing does not stop producing all lift. Instead, increased airflow separation reduces its lift coefficient and increases drag. The way a stall develops depends on the wing's design and aerodynamic condition.
How Wing Design Affects Stall Characteristics
Not all airplane wings stall in the same way. While every wing has an aerodynamic limit, its shape, construction, and design determine how airflow separation develops across the wing and how the airplane behaves as a stall progresses.
Some wings experience a gradual loss of lift, while others can stall more abruptly. Aircraft designers therefore consider not only how much lift a wing can produce, but also where the stall begins and how it spreads across the wingspan.
The wing's curvature, thickness, and leading-edge shape affect its maximum lift coefficient and the angle of attack at which airflow separation becomes extensive.
The shape of the wing when viewed from above affects how aerodynamic loading is distributed along the span. Wing taper and sweep can influence where a stall begins.
Designers can change wing geometry or aerodynamic characteristics along the span to influence which sections reach their critical angle of attack first.
Why Do Many Training Airplanes Stall at the Wing Root First?
On many conventional training airplanes, the wing is designed so that the wing root, the section closest to the fuselage, reaches its stall before the outer portions of the wing.
As airflow separation begins near the root, the outer wing can remain largely unstalled for a little longer. This is useful because the ailerons are normally located toward the wingtips.
Simplified top view of one wing on a training airplane
In this example, airflow separation becomes extensive near the fuselage first.
The outer wing remains unstalled longer, helping preserve some aileron effectiveness during the early stages of the stall.
Why Does the Stall Pattern Matter?
A root-first stall can produce more progressive stall characteristics because the loss of lift begins near the fuselage while airflow remains attached over more of the outer wing.
By contrast, significant separation near the wingtips can reduce aileron effectiveness earlier and may contribute to less predictable rolling behavior.
However, root-first stalling is a design objective, not a guarantee for every airplane. Stall progression varies with wing geometry, aerodynamic loading, and aircraft configuration.
How Do Designers Control Stall Progression?
Aircraft designers can use several methods to influence how airflow separation develops along the wingspan. Two common examples are wing washout and stall strips.
Wing Washout
Washout is a geometric twist built into a wing so that its outer sections have a lower local angle of incidence than the wing root.
During flight, this generally gives the wingtips a lower local angle of attack than the inner wing.
As the airplane's overall angle of attack increases, the root can reach its critical angle before the tips, helping produce a more progressive stall.
Stall Strips
Stall strips are small, specially shaped devices installed on selected sections of a wing's leading edge.
Their purpose is to encourage airflow separation at a chosen location before other sections of the wing stall.
On some airplanes, stall strips are placed near the wing root to help the inner wing stall before the tips.
Designing the wing to stall at the root first can help preserve some aileron effectiveness during the early stages of a stall. However, this does not mean that normal roll control is guaranteed once the wing is stalled. Control effectiveness and stall behavior remain aircraft-specific.
Wing design affects where a stall begins and how it progresses across the wingspan. Many training airplanes are designed to promote a root-first stall. Features such as washout and stall strips can help control this progression, while airfoil shape and wing planform influence the overall stall characteristics.
How Weight and Wing Loading Affect Stall Speed
An airplane does not always stall at the same airspeed. Even when its wing shape and configuration remain unchanged, aircraft weight affects the speed at which the wing reaches its aerodynamic limit.
Understanding this relationship requires one important concept: wing loading. It describes how much aircraft weight is supported in relation to the size of the wing.
In steady, unaccelerated flight, a heavier airplane requires more lift to support its weight. With the same wing and configuration, it must fly faster to produce that lift without exceeding the wing's critical angle of attack. Consequently, its stall speed is higher.
What Is Wing Loading?
Wing loading is the airplane's total weight divided by its wing area. In U.S. aviation, it is commonly expressed in pounds per square foot (lb/ft²).
A larger wing can support a given airplane weight with lower wing loading. Conversely, adding weight without changing the wing area increases wing loading.
Consider a hypothetical airplane with a wing area of 160 square feet. The two examples below use exactly the same wing. Only the airplane's total weight changes.
Lighter Airplane
Heavier Airplane
The second airplane is 20% heavier. Because the wing area is unchanged, its wing loading is also 20% higher.
Why Does More Weight Increase Stall Speed?
In unaccelerated flight, the wing must generate enough lift to support the airplane's weight. As aircraft weight increases, the aerodynamic demands on the wing also increase.
The wing must produce more lift to support the heavier airplane.
At a given airspeed, the heavier airplane requires a higher lift coefficient and generally a higher angle of attack.
The wing reaches its maximum lift coefficient at a higher airspeed than it would at a lighter weight.
Importantly, increasing weight does not, by itself, change the wing's critical angle of attack when the configuration is unchanged. What changes is the airspeed at which the wing reaches that angle while supporting the airplane.
How Much Does Stall Speed Change With Weight?
Stall speed does not increase in direct proportion to weight. For the same airplane, wing configuration, and flight conditions, it changes approximately with the square root of the weight ratio.
Suppose our hypothetical airplane stalls at 50 knots when it weighs 1,600 lb. If its weight increases to 1,920 lb, the new stall speed can be estimated using the following relationship:
Although weight increased by 20%, stall speed increased by approximately 9.5%. The increase in stall speed is therefore smaller than the percentage increase in weight.
These figures illustrate the aerodynamic relationship only. They are not performance data for a real airplane. Always use the applicable aircraft flight manual or pilot's operating handbook (AFM/POH).
Does Higher Wing Loading Always Mean a Higher Stall Speed?
When comparing the same airplane at different weights, higher wing loading generally means a higher stall speed. However, comparing two different aircraft requires additional information.
Stall speed also depends on the wing's maximum lift coefficient, which is affected by its airfoil design and configuration. Two airplanes with the same wing loading can therefore have different stall speeds.
Weight determines how much lift an airplane needs. Wing area determines the area available to generate that lift. Wing design and configuration influence the maximum lift the wing can produce before stalling.
Wing loading is useful for understanding stall-speed differences, but it does not provide a complete description of an airplane's stall characteristics.
As fuel is consumed during flight, the airplane becomes lighter. With other conditions unchanged, this generally reduces its stall speed. The effect may be modest, but it explains why an airplane's stall speed is not necessarily identical at every operating weight.
For the same airplane and wing configuration, greater weight means greater wing loading and a higher stall speed. The critical angle of attack does not increase simply because the airplane is heavier. Instead, the heavier wing must reach that aerodynamic limit at a higher airspeed.
How Center of Gravity Changes Stall Behavior
Aircraft weight is not the only loading factor that influences stall characteristics. Where that weight is located also matters. Moving the airplane's center of gravity (CG) forward or aft changes its pitch stability, the aerodynamic forces needed to keep it balanced, and its behavior near a stall.
Two airplanes can have the same total weight but different CG positions because their passengers, baggage, or fuel are distributed differently. Their stall characteristics may therefore differ, even though their total weight is identical.
Why Does Center of Gravity Affect the Wing and Tail?
The center of gravity is the point through which the airplane's total weight acts. Its position relative to the aerodynamic forces acting on the wings and tail determines how the airplane balances in pitch.
On many conventional training airplanes, the horizontal tail normally produces a downward aerodynamic force to help balance the airplane's nose-down pitching tendency.
Because this tail force acts downward, the wing must produce enough lift to support both the airplane's weight and the additional downward force.
Moving the CG changes the balancing force needed from the tail. This can change the lift required from the wing and, consequently, the stall speed.
Forward CG vs. Aft CG
A forward CG and an aft CG affect an airplane in different ways. The comparison below describes the typical effects on a conventional airplane with a horizontal tail.
More Stability, Higher Stall Speed
When the CG moves forward, the airplane generally becomes more stable in pitch. However, the horizontal tail must usually generate a greater downward force to balance the airplane.
The wing must then produce additional lift to overcome that tail force. As a result, the airplane generally reaches its critical angle of attack at a higher airspeed.
- Greater longitudinal stability.
- Greater nose-up elevator effort may be required.
- Higher stall speed, with other conditions unchanged.
Less Stability, More Difficult Stall Recovery
As the CG moves aft, the downward balancing force required from the horizontal tail generally decreases.
The wing may therefore need to produce slightly less lift, which can result in a lower stall speed.
However, moving the CG aft also reduces longitudinal stability. The airplane may become more sensitive to pitch changes, and its behavior near a stall can become less forgiving.
- Reduced longitudinal stability.
- Potentially lower stall speed.
- Reduced nose-down control authority if the CG moves excessively aft.
The two illustrations show extreme CG positions outside the airplane's allowable limits. They do not represent the normal handling characteristics of an airplane loaded within its approved CG range.
An excessively forward CG may leave insufficient nose-up elevator authority, especially during takeoff or landing. An excessively aft CG can seriously reduce pitch stability and may leave insufficient nose-down elevator authority for stall recovery.
Does Center of Gravity Change the Critical Angle of Attack?
Changing the CG position does not, by itself, change the wing's basic critical angle of attack. Instead, it changes the forces required to balance the airplane and the way the airplane responds to pitch-control inputs.
A forward or aft CG can therefore affect the airspeed at which a stall occurs and the airplane's behavior around that stall, without changing the fundamental aerodynamic condition that causes the wing to stall.
An aft CG may reduce stall speed, but this does not automatically improve safety. The accompanying reduction in pitch stability and possible loss of nose-down control authority can make an excessive aft CG particularly dangerous.
Maintaining the CG within the approved limits is more important than obtaining a slightly lower stall speed.
Why CG Position Matters During Aircraft Loading
Adding baggage, changing passenger positions, or consuming fuel can change the airplane's CG. The amount and direction of that movement depend on where the weight is located.
The approved CG range may also vary with total aircraft weight. This is why an airplane must remain within both its weight limits and its CG limits, as specified in its AFM/POH.
Center of gravity affects both stall speed and the airplane's stability near a stall. A forward CG generally increases stability and stall speed. An aft CG generally reduces stability and may lower stall speed, but an excessively aft position can make stall recovery difficult or impossible. Always remain within the aircraft's approved weight-and-balance limits.
How Flaps and Aircraft Configuration Affect a Stall
An airplane's stall characteristics depend not only on its weight and center of gravity, but also on its configuration. Extending or retracting flaps changes the aerodynamic shape of the wing and can alter the airspeed at which a stall occurs.
This is why an airplane can have different stall speeds with flaps retracted and extended, even when its weight, altitude, and flight conditions remain the same.
How Do Trailing-Edge Flaps Change the Wing?
Trailing-edge flaps are movable surfaces located along the rear portion of the wing. When extended, they change the wing's curvature, also called camber. Some flap designs also increase the effective wing area.
Increasing camber generally allows the wing to produce more lift at a given angle of attack. Flap extension also typically increases the wing's maximum lift coefficient (CLmax), allowing the airplane to fly at a lower airspeed before reaching its aerodynamic limit.
Flaps Retracted
With flaps retracted, the wing retains its normal clean shape. It generally has less maximum lift capability than when equipped with appropriately extended flaps.
For the same airplane and weight, the clean configuration usually has a higher stall speed than the landing configuration.
Flaps Extended
Extending the trailing-edge flaps increases wing camber and generally increases the maximum lift coefficient.
With greater maximum lift available, the wing can support the same weight at a lower airspeed. Its stall speed usually decreases, although the amount depends on flap design and deflection.
Flaps allow the wing to generate more lift in a particular configuration, but the wing can still stall. Its maximum lift coefficient and stall angle may change with flap deflection. A stall still occurs when the wing exceeds the critical angle of attack for its current configuration.
Three Important Effects of Flap Extension
Flaps generally increase the wing's maximum lift coefficient, allowing the airplane to maintain flight at lower airspeeds.
Flap extension also increases drag. At larger flap deflections, the additional drag can become much more significant than the additional lift.
Flaps alter the distribution of aerodynamic forces and may change airflow over the horizontal tail. Depending on the aircraft, flap extension may cause a nose-up or nose-down pitching tendency, or only a small pitch change.
Why Does Flap Deflection Matter?
Different flap positions do not produce identical aerodynamic effects. Small flap deflections often provide a useful increase in lift with a relatively modest increase in drag.
Larger deflections usually create substantially more drag. They may provide additional maximum lift, but the lift increase does not necessarily grow at the same rate as the drag.
The exact relationship depends on the airplane's flap design. Plain, split, slotted, and Fowler flaps do not all produce the same changes in lift and drag.
What About Other Aircraft Configuration Changes?
Flaps are not the only movable components that can influence an airplane's low-speed aerodynamic characteristics. Their effects should be distinguished from those of other aircraft systems.
Leading-Edge Devices
Some airplanes use leading-edge slats or other high-lift devices. These can improve airflow behavior at high angles of attack and increase the wing's maximum lift capability.
Depending on their design, leading-edge devices may allow the wing to reach a higher angle of attack before stalling.
Landing Gear
Extending retractable landing gear primarily increases aerodynamic drag. It can also affect pitching moments and aircraft handling.
However, landing gear extension should not be assumed to lower stall speed in the same way that trailing-edge flaps often do. Any stall-speed effect is aircraft-specific.
Retracting flaps generally reduces the wing's maximum lift capability. If the airplane is already flying close to the minimum speed supported by the clean wing, removing that additional lift capability may cause an unexpected sink or stall.
Changes in flap configuration must therefore be understood in terms of the airplane's actual airspeed, available lift, and approved operating procedures.
Why Stall Speeds Depend on Configuration
Aircraft manuals may publish different stall speeds for different configurations. For example, the stall speed in a specified clean configuration is generally different from that in the full landing configuration.
This reflects the aerodynamic changes produced by flap position and, where applicable, other configuration differences. The exact speeds and limitations must always be taken from the aircraft's AFM/POH.
Aircraft configuration changes the wing's lift capability and can change its stall speed. Extending trailing-edge flaps generally increases maximum lift and reduces stall speed, while also increasing drag. The airplane can still stall in any flap configuration, and the exact behavior depends on its aerodynamic design.
Load Factor, Bank Angle, and Stall Speed
An airplane can stall at a higher airspeed when the wing is required to generate more lift than it normally produces in straight-and-level flight. This additional aerodynamic demand is described by the load factor.
Load factor is especially important when examining steep, coordinated turns in which the airplane maintains altitude. It explains why the wing can reach its aerodynamic limit at an airspeed higher than its normal 1-G stall speed.
What Is Load Factor?
Load factor is the ratio between the lift produced by the wings and the airplane's weight. It is commonly expressed in units of gravity, or G.
Straight-and-Level Flight
In steady, unaccelerated level flight, the wings produce lift equal to the airplane's weight.
The load factor is therefore 1 G.
Increased Aerodynamic Loading
During a maneuver that requires additional lift, the wings may produce a force greater than the airplane's weight.
The load factor then exceeds 1 G. This can increase the stall speed.
Why Does Bank Angle Increase Load Factor in a Level Turn?
When an airplane banks, the lift vector tilts with the wings. Only its vertical component supports the airplane's weight.
To maintain a constant altitude during a coordinated turn, total lift must increase enough for the vertical component to continue balancing weight.
The steeper the bank angle, the greater the total lift required. This increases the load factor.
How Does Load Factor Change Stall Speed?
When the wing must produce more lift at a given airspeed, a higher lift coefficient and angle of attack are required. The wing therefore moves closer to its maximum lift capability.
For the same airplane, weight, configuration, and atmospheric conditions, stall speed increases with the square root of load factor.
The following table shows how the relationship changes with bank angle during a coordinated level turn.
| Bank Angle | Load Factor | Stall Speed Multiplier |
|---|---|---|
| 0° | 1.00 G | 1.00 × |
| 30° | 1.15 G | 1.07 × |
| 45° | 1.41 G | 1.19 × |
| 60° | 2.00 G | 1.41 × |
What Is an Accelerated Stall?
An accelerated stall occurs when the wing reaches its critical angle of attack while the airplane is experiencing a load factor greater than 1 G.
Because the wing is supporting a greater aerodynamic load, an accelerated stall can occur at a higher airspeed than the normal 1-G stall speed.
Steep turns are one situation in which load factor can increase, but accelerated stalls are not limited to turning flight.
A strong pull-up can increase the load factor even when the wings are level. The important factor is the lift demanded from the wing, not the bank angle alone.
Does a Higher Load Factor Change the Critical Angle of Attack?
For the same wing and configuration, increasing load factor does not, by itself, change the fundamental critical angle of attack.
Instead, the wing must generate more lift, requiring a higher lift coefficient at a given airspeed. It can therefore reach its critical angle of attack sooner as aerodynamic loading increases.
The relationship between bank angle, load factor, and stall speed shown above applies to coordinated level turns.
An airplane can be banked while climbing, descending, or changing its flight path. In these situations, its actual load factor may differ from the value calculated for a level turn.
Load factor also affects structural loading, so a higher stall speed is not the only consequence of a demanding maneuver.
Increasing load factor increases stall speed, even when the airplane's weight and configuration stay the same. In a coordinated level turn, steeper bank angles require more lift. At 60° of bank, the load factor is 2 G and stall speed is approximately 41% higher than in 1-G flight. The wing still stalls when it exceeds its critical angle of attack.
How Ice, Frost, and Turbulence Affect Stall Aerodynamics
Weather can affect an airplane's stall characteristics in two different ways. Ice and frost change the condition of the wing itself, while turbulence can suddenly change the airflow approaching the wing. Both can reduce the margin between normal flight and a stall, but they do so through different aerodynamic mechanisms.
Ice and Frost
Contamination changes the wing's surface roughness or shape. This can reduce the maximum lift the wing can produce and cause airflow separation at a lower angle of attack.
Turbulence and Gusts
Rapid changes in the surrounding air can temporarily change the relative wind, angle of attack, and aerodynamic loads, even without a deliberate change in the aircraft's pitch attitude.
Why Frost and Ice Change the Stall
An airplane wing is designed to produce lift with a particular shape and surface condition. Even a thin layer of frost can make that surface rougher. Ice forming along the leading edge can also change the wing's contour.
These changes disturb the airflow and can make it separate from the upper surface sooner than it would on a clean wing. As a result, the wing may reach its maximum lift at a lower critical angle of attack. Because less maximum lift is available, the airplane's stall speed can also increase.
Upper-Surface Frost
Frost may look harmless because it can be very thin. However, its rough texture interferes with the airflow close to the wing's surface. This can reduce the maximum lift available before a stall.
Leading-Edge Ice
Ice near the leading edge can alter the airflow from the moment it meets the wing. Depending on the ice shape and location, separation can begin earlier, increasing drag and reducing lift.
How Ice Changes Airflow Over the Wing
On a clean wing, the airflow can remain attached over a relatively large range of angles of attack. Ice contamination makes this more difficult. As the airflow passes over the rough or distorted surface, it may separate earlier and become less effective at producing lift.
Important: Ice does more than add weight to the airplane. Its most serious aerodynamic effect can be a reduction in the wing's maximum lift and critical angle of attack. An ice-contaminated airplane may therefore stall at a higher airspeed than the same airplane with clean wings.
How Turbulence Can Bring a Wing Closer to a Stall
Turbulence affects stall behavior differently. It does not need to change the physical shape of the wing. Instead, moving air can change the direction and speed of the relative wind reaching it.
Consider an airplane flying through a sudden upward gust. The airflow approaching the wing changes direction, which can produce a rapid increase in the wing's effective angle of attack. If the increase is large enough, the wing may approach or exceed its critical angle of attack, even though the pilot has not deliberately raised the nose.
Gusts can also cause sudden changes in lift and load factor. These changes are especially significant when the airplane is already operating close to its aerodynamic limits.
Upward Gust
The relative wind shifts in a way that can increase the effective angle of attack. Lift and aerodynamic loading may rise suddenly, reducing the remaining margin before a stall.
Rapidly Changing Airflow
Successive gusts can produce changing angles of attack and wing loads. The aircraft may experience abrupt variations in lift, making its stall margin less predictable at any instant.
This does not mean that turbulence permanently increases the airplane's published stall speed. The important distinction is that gusts can temporarily change the aerodynamic conditions under which the wing is operating. A stall can occur if the critical angle of attack is exceeded during one of these changes.
Remember: Frost and ice reduce the wing's aerodynamic capability, while turbulence can rapidly change the angle of attack and loads acting on it. A clean wing and an adequate stall margin are essential to predictable aircraft behavior.
How Aircraft Stall Warning Systems Work
A stall warning system alerts the pilot when an airplane is approaching a stall. Its purpose is to provide a warning before the wing reaches the point where airflow separation causes a significant loss of lift.
Different aircraft use different warning devices. Some light airplanes have a simple mechanical or electrical stall warning horn. Others use electronic angle-of-attack sensors, visual indicators, or a stick shaker that vibrates the flight controls.
Key principle: A stall warning is not necessarily triggered at one fixed airspeed. Most systems detect a change in airflow or use information related to angle of attack to determine when the airplane is approaching a stall.
How a Stall Warning Horn Detects an Approaching Stall
On many light airplanes, the stall warning system uses a sensor near the leading edge of the wing. As the angle of attack increases, the airflow around this area changes. The system detects that change and activates an audible warning.
Two common designs use different methods to detect the approaching stall.
Pneumatic Stall Warning
Some airplanes have a small opening near the wing's leading edge, connected to a reed inside the wing.
As the angle of attack increases, the stagnation point moves along the leading edge. Air flowing past the opening can create suction that causes the reed to vibrate.
The vibrating reed produces the familiar stall warning sound heard in the cockpit.
Electrical Stall Warning
Other airplanes use a small movable vane or tab near the wing's leading edge.
When the local airflow changes as the wing approaches a stall, the vane moves and activates an electrical switch.
This completes a circuit that operates a warning horn or, on some aircraft, an additional warning light.
Although these devices work differently, both are designed to respond to changes in airflow associated with an approaching stall. Their activation point is established for the particular aircraft and its approved operating conditions.
Angle-of-Attack Indicators and Stick Shakers
More advanced systems can provide information about how close the wing is to its aerodynamic limit. Rather than relying only on a simple warning switch, they may use sensors and electronic processing to monitor the angle of attack.
Angle-of-Attack Indicator
An angle-of-attack (AOA) indicator displays information about the wing's angle of attack, often using a colored scale or other visual indication.
A properly calibrated indicator can help the pilot understand the available margin before a stall, even when the aircraft's stall speed changes because of weight, configuration, or load factor.
An AOA indicator supplements the airspeed indicator and any installed stall warning system. It does not replace them.
Stick Shaker
A stick shaker is an artificial stall warning device that vibrates the control column or yoke when the airplane approaches a stall.
It is commonly associated with larger and more complex airplanes. The vibration provides a warning that the pilot can feel, even without looking at an instrument.
Some aircraft also have a separate stick pusher. Unlike a stick shaker, a stick pusher actively applies a forward force to the controls under specified conditions.
Natural Stall Warnings Versus Installed Warning Systems
Not every stall warning comes from a mechanical or electronic device. An airplane may also produce natural aerodynamic indications as it approaches a stall.
One example is pre-stall buffet, which occurs when disturbed airflow causes vibrations in the airframe or flight controls. The strength and timing of this buffet vary with aircraft design and flight conditions.
An installed stall warning horn or stick shaker is an artificial warning. It is designed to provide a recognizable alert with a margin before the stall under the conditions for which the system was designed.
Why Stall Warning Systems Have Limitations
A stall warning system is an important safety feature, but it cannot be assumed to provide the same warning margin under every condition.
For example, ice contamination can change the wing's stall characteristics. If the wing stalls at a lower angle of attack than expected, a warning system that does not account for that change may provide less warning than it would with a clean wing.
A sensor malfunction, incorrect calibration, or an aircraft operating outside its approved limitations can also affect the reliability of the warning.
Important distinction: A stall warning system does not prevent the wing from stalling. It only provides information or an alert about an approaching aerodynamic limit. The absence of a warning does not guarantee that the wing is safely below its critical angle of attack.
Remember: Stall horns, AOA indicators, and stick shakers use different methods to help identify an approaching stall. Their common purpose is to provide awareness of the wing's aerodynamic condition before the critical angle of attack is exceeded.
Conclusion: Understanding Airplane Stall Aerodynamics
Understanding airplane stalls begins with one essential principle: a wing stalls when it exceeds its critical angle of attack, causing airflow separation and a reduction in lift. A stall is therefore an aerodynamic event, not simply the result of flying below a particular airspeed.
Wing design, aircraft weight, center of gravity, flap configuration, and load factor all influence stall characteristics. Frost and ice can change the wing's aerodynamic behavior, while gusts can temporarily alter its effective angle of attack. Stall warning systems help identify an approaching stall, but they do not remove the aerodynamic limit.
The key takeaway: Stall speed can change with flight conditions, but the fundamental cause of a stall remains the same: the wing exceeds its critical angle of attack. Understanding what changes the available stall margin is essential to understanding how an airplane flies.
To explore stall recognition and recovery in practical flight training, see our Stall — Pre-Flight Lesson .
Typical Exam Questions
Test your understanding of airplane stall aerodynamics, including angle of attack, wing design, weight, center of gravity, flaps, load factor, icing, and stall warning systems. Choose one answer for each question. You will receive an explanation after every answer.
