Chapter

9

Airframe Contamination

Introduction		233
Types of Contamination . . . . . . . .	. . . . . . . . . . . . . . . . . .	233
Effect of Frost and Ice on the Aircraft		233
Effect on Instruments		234
Effect on Controls		234
Water Contamination		234
Airframe Aging . . . . . . . . . . . .	. . . . . . . . . . . . . . . . .	234
Questions . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . .	235
Answers . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . .	238

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9		Airframe Contamination

Contamination Airframe 9

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Airframe Contamination		9

Introduction

The airframe may become contaminated by ice, frost or water either whilst it is in flight or when standing on the ground.

The meteorological conditions that cause ice and frost to form are dealt with elsewhere, but the effect is an accumulation of ice or frost on the surface of the aircraft which will affect its performance and handling.

Types of Contamination

a)	Frost
	Frost can form on the surface of the aircraft either when it is standing on the ground
	when the temperature falls below 0°C, or in flight, if the aircraft, after flying in a region	9
b)	where the temperature is below 0°C, moves into a warmer layer of air. It consists of a	Contamination
	Ice
	fairly thin coating of crystalline ice.
	The main forms of icing are clear ice, rime ice and rain ice. Clear ice (glaze ice) is a	Airframe
	translucent layer of ice with a smooth surface, caused by large supercooled water
	droplets striking the leading edges of the airframe. As there is some delay in freezing,
	there is some flow back along the surface behind the leading edge.
	Rime ice forms when small supercooled water droplets strike the leading edges and
	freeze almost immediately so that there is no flow back. It is a white opaque formation.
	Rain ice is caused by rain which becomes supercooled by falling from an inversion into
	air which is below 0°C. It does not freeze immediately and forms considerable flow
	back, and it builds up very quickly.

Effect of Frost and Ice on the Aircraft

The formation of ice and frost on the airframe will:

•modify the profile of the aerofoil.

•increase the roughness of the aircraft surface.

•increase the weight of the aircraft.

The main effect of frost will be to increase the surface roughness and this will increase the energy loss in the boundary layer. The skin friction drag will increase and the boundary layer

will have an earlier separation, giving a reduced CLMAX. Take-off with frost on the wings could result in a stall after lift-off if the normal take-off speed is used.

Tests have shown that frost, ice or snow with the thickness and surface roughness of medium or coarse sandpaper reduces lift by as much as 30% and increases drag by 40%.

Ice will normally form on and behind the leading edges of wings and tailplane and can result in severe distortion of the leading edge profile. This will give a large increase in drag and a substantial decrease in CLMAX.

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Contamination Airframe 9

The reduced CLMAX of the wing will give a higher stalling speed and the decreased CLMAX of the tailplane could cause it to stall when the aircraft is flying at low speed, particularly if the wing

downwash is increased as a result of flap extension.

Tailplane stall will result in loss of longitudinal control. Clear ice and rain ice especially can add considerable weight to the airframe, and this will in turn give a higher stalling speed, as well as increased induced drag. The margin of thrust to drag will be decreased, reducing the ability to climb. Increased power will be required to maintain height, resulting in increased fuel consumption.

Ice formation on propeller blades can upset the balance of the propeller and cause severe vibration, particularly if pieces of ice break off from one blade. Pieces of ice shed from propellers can also cause damage to the fuselage.

Effect on Instruments

Formation of ice on static vents and pitot heads could cause errors in the readings of pressure instruments and, eventually, failure to show any reading.

Effect on Controls

Any moveable surface could become jammed by ice forming in the gaps around the control or by pieces of ice breaking off and becoming jammed in the control gaps. The controls could become difficult to operate or immovable.

Water Contamination

If the wings are contaminated with water due to heavy rain, the boundary layer may become turbulent further forward on the wing, particularly if the section is of the laminar flow type. This will cause increased drag and may disrupt the boundary layer resulting in a significantly higher stall speed.

Adjustments to operational speed should be made in accordance with the recommendations of the aircraft manufacturer or aircraft operator when taking off and landing in heavy rain.

Airframe Aging

Over a period of years the condition of the airframe will deteriorate due to small scratches, minor damage, repairs and general accumulation of dirt and grease.

The overall effect of this will be to increase the drag of the aircraft (mainly skin friction drag) with a consequent increase in fuel consumption. The cost of operating the aircraft will therefore increase with the age of the airframe. The normal deterioration of the airframe is allowed for in the performance charts of the aeroplane.

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