Subsonic Aerodynamics | Aerodynamic Degradation | PPL(A) Principles of Flight

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What is Aerodynamic Degradation?

Aerodynamic degradation refers to any reduction in an aircraft’s aerodynamic performance caused by contamination of its surfaces or systems. Ice is the most significant and dangerous form of contamination — even a thin layer can dramatically alter the aircraft’s behaviour.

Light Frost is Not Harmless

Even light frost on the upper wing surface can reduce lift by approximately 30% and increase drag by approximately 40%. Never depart with any contamination on lifting surfaces.

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The Four Main Hazards of Icing

Ice on an aircraft creates hazards across four categories:

Aerodynamic

• Reduces lift and control surface effectiveness
• Increases drag, stall speed, and fuel consumption
• Even light frost can cut lift by ~30% and raise drag by ~40%
• Severe icing may prevent climb, prevent level flight, or prevent recovery from a stall
• High-lift devices (flaps, slats) may jam in or out of position
• Control surfaces may freeze, reducing or eliminating pilot authority

Weight

• Adds significant mass, particularly with clear (glaze) ice accumulation
• Raises stall speed — the aircraft must fly faster to generate the same lift
• Lowers climb performance — more thrust is consumed overcoming the added weight and drag
• Clear ice can shift the centre of gravity (CG) aft, reducing longitudinal stability

Instruments

• Ice on pitot tubes, static ports, or angle-of-attack sensors can block or corrupt data
• Airspeed, altitude, and vertical speed indications may become unreliable or freeze entirely
• Stall warning flapper switches can freeze, eliminating the cockpit stall warning

Other Effects

• Can obscure the forward view through the windscreen
• May disrupt radio and navigation equipment
• Can prevent landing gear retraction or extension

Stall Warning and Ice

Ice can freeze stall warning flapper switches, removing the cockpit alert entirely. Combined with a higher stall speed caused by the distorted wing profile and added weight, the aircraft may stall with no warning and at a significantly higher speed than placarded.

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How Icing Raises Stall Speed

Icing raises stall speed through three compounding mechanisms:

• Added weight — the aircraft must fly faster to generate sufficient lift at the critical angle of attack
• Distorted wing profile — ice changes the wing’s camber and surface finish, reducing the maximum lift coefficient (cLmax). A lower cLmax means the wing stalls at a lower angle of attack and a higher airspeed
• Reduced lift efficiency — disruption of the boundary layer by ice roughness increases turbulence over the upper surface, degrading the pressure differential that generates lift

Lower cLmax → higher stall speed → reduced margin above stall at any given airspeed

Stall Speed Increase with Ice

An iced wing may stall at a speed significantly above the published stall speed. The pilot has no reliable way to know the new stall speed in flight — making avoidance of icing conditions the only safe strategy.

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Types of Ice — Rime, Clear (Glaze), and Mixed

Three types of structural ice are relevant to PPL examinations. They form under different conditions and each presents distinct hazards.

Property	Rime Ice	Clear (Glaze) Ice	Mixed Ice
Droplet size	Small supercooled droplets	Large supercooled droplets	Both sizes present
Freezing behaviour	Freezes instantly on impact	Spreads along surface before freezing	Both behaviours simultaneously
Location of build-up	Stagnation point	Spreads along wing profile	Stagnation point and beyond
Shape	Rough, irregular, distorted	Smoother but heavier layer	Rough, irregular, and heavy
Appearance	White and cloudy	Transparent — harder to detect	White/opaque with embedded clear sections
Weight	Lighter	Heavier	Heaviest — combines both penalties
Profile distortion	High — rough surface disrupts airflow	Moderate — follows wing shape more closely	Highest — combines roughness and mass
Most dangerous?	No	Individually more dangerous than rime	Yes — worst combination of both types

Rime Ice

Rime ice forms from small supercooled water droplets that freeze instantly on contact with the aircraft surface. Because the droplets freeze immediately, they trap air between them, producing a rough, white, opaque deposit. This build-up concentrates at the stagnation point — the leading edge — and creates a distorted, irregular profile that disrupts the boundary layer and significantly increases drag.

Clear (Glaze) Ice

Clear ice forms from larger supercooled droplets that do not freeze instantly. Instead, they spread along the wing surface before freezing, following the wing’s contour more closely. This produces a smooth, transparent, dense layer that is harder to detect visually. Clear ice is generally heavier than rime ice for the same volume, making it more dangerous from a weight and CG perspective.

Detection Difficulty

Clear (glaze) ice is transparent, making it significantly harder to detect visually than rime ice, which appears white and cloudy. A pre-flight inspection in icing conditions must include tactile checks — do not rely on visual inspection alone.

Mixed Ice

Mixed ice forms when both small and large supercooled droplets are present simultaneously — typically in cloud layers where temperature and droplet size vary. The result combines the worst characteristics of both rime and clear ice:

• The large droplets spread and freeze as clear ice, adding significant weight
• The small droplets freeze instantly on top, creating a rough, irregular outer surface
• The combined deposit is heavier, rougher, and more aerodynamically disruptive than either type alone
• It builds rapidly and is difficult to remove with standard de-icing systems

Mixed ice is considered the most hazardous structural ice type because it simultaneously maximises weight penalty, profile distortion, surface roughness, and boundary layer disruption.

Mixed Ice — Exam Point

Mixed ice is the most dangerous of the three structural ice types. It combines the weight and density of clear ice with the surface roughness of rime ice — producing the greatest reduction in cLmax, the largest increase in drag, and the most severe stall speed increase. De-icing systems that handle rime or clear ice individually may be overwhelmed by mixed ice accumulation.

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Stall Implications — Cross-Reference

Icing directly affects stall behaviour in ways that go beyond the numbers. Because the wing profile is distorted and cLmax is reduced, the critical angle of attack is reached sooner — at a lower geometric angle than the pilot expects. Combined with the frozen stall warning switch hazard, this means:

• The stall may occur with no warning
• It may occur at a higher airspeed than placarded
• It may occur at a lower pitch attitude than expected
• Recovery may be harder if control surfaces are partially frozen

See Also — The Stall

For full coverage of stall theory, recovery technique, load factor effects, and spin entry, see the The Stall page.

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Summary — Icing Effects at a Glance

Effect	Mechanism	Result
Lift reduction	Distorted profile, reduced cLmax	Higher stall speed
Drag increase	Rough surface, disrupted boundary layer	Higher fuel burn, reduced performance
Weight increase	Ice mass accumulation	Higher stall speed, reduced climb
CG shift	Uneven ice distribution (especially clear ice aft)	Reduced stability
Instrument failure	Blocked pitot/static/AoA sensors	Unreliable flight data
Loss of stall warning	Frozen flapper switch	No cockpit alert before stall
Control restriction	Frozen control surfaces or high-lift devices	Reduced or lost control authority