Chapter

19

Contaminated Runways

Contaminated Runways . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	233
Contaminated Runway . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	233
Damp Runway							233
Wet Runway							233
Dry Runway . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	233
Contaminant Depth Limitations . . . . . . . . . . . .	. .	. .	. .	. .	. .		.233
Aquaplaning (Hydroplaning)							233
Braking Action . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.234
Coefficient of Friction							234
Performance Considerations . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	235
SNOWTAMs . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	235
Questions . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.237
Answers . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	240

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Contaminated Runways

The state of a runway for both take-off and landing are factors to be taken into account when calculating performance. In practical terms, pilots should be aware of the general implications of the state of the runway and meaning of the terminology used to describe the state of the runway.

Contaminated Runway

A runway is said to be contaminated if more than 25% of the surface area (whether in isolated patches or not) is covered by any of the following:

•Surface water more than 3 mm deep (0.125 in), or by slush or loose snow equivalent to 3 mm of water.

•Snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up (compacted snow).

•Ice, including wet ice.

Damp Runway

A runway is considered damp when the surface is not dry, but when the moisture on it does not give a shiny appearance.

Wet Runway

A runway is considered wet when the runway is covered with water, or equivalent, less than specified in contaminated runway above, or when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water.

Dry Runway

A dry runway is one which is neither wet nor contaminated, and includes those paved areas which have been specially prepared with grooves or porous pavement and maintained to retain effectively dry braking action even when moisture is present.

Contaminant Depth Limitations

It is inevitable that operations from contaminated runways will be required. In such cases, the following depths are quoted, above which take-offs should not be attempted:

•dry snow depth greater than 60 mm (very dry - 80 mm)

•water, slush or wet snow greater than 15 mm

Aquaplaning (Hydroplaning)

During take-off and landing operations from contaminated runways (3 mm or more water), aquaplaning (hydroplaning) is a hazard that must be considered. During take-off runs, as water (or any other liquid contaminant) is displaced by the tyres, a ‘bow wave’ effect is created in front of the tyre. By a factor of the specific gravity of the contaminant and the tyre pressure, a speed will exist at which the tyre will ride up over the ‘bow wave’ and friction with the runway will rapidly reduce.

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Similarly, during landing runs, where touch-down speed is above the speed at which aquaplaning is likely, any application of the brakes may result in severe loss of friction between the tyre and the runway surface, thus drastically reducing braking. In this specific case the effect of aquaplaning will stop the rotation of the tyres and this can lead to dissipation of momentum energy in the form of heat generated in the contaminant. The temperature reached may scald tyres and skidding may result when the aquaplaning effect breaks down. In any case, loss of braking action and directional control are the hazards of aquaplaning. The aquaplaning speed is given by the formula:

V = 9 √P

σ

where V is the ground speed in knots; P is the tyre pressure in lb/sq in and σ is the specific gravity of the precipitant (contaminant). This speed, however, assumes a rotating tyre (spin down speed). For a non-rotating tyre, as in the situation on initial touchdown (spin-up speed), the aquaplaning speed is 7.7 times the square root of the tyre pressure in psi. If the pressure is stated in bar, 1 bar is approximately equal to 14.5 psi.

Braking Action

From data collected from operations on compacted snow and ice, an assessment table has been produced to relate a measured braking coefficient to an estimated braking action and hence to a simple code for braking action. It must be borne in mind that the description “good” is a comparative value and is intended to mean that aeroplanes should not experience directional control or braking difficulties when landing, but conditions would not be as good as on a clean, dry, runway:

Measured Coefficient	Estimated Braking Action	Code
0.40 and above	Good	5
0.39 - 0.36	Medium to good	4
0.35 - 0.30	Medium	3
0.29 - 0.26	Medium to poor	2
0.25 and below	Poor	1
unreliable	UNRELIABLE	9

Figure 19.1: Braking action code

Coefficient of Friction

In terms of assessment of braking action and the exchange of information relating to it between ATC and the pilot, the coefficient of braking on a wet runway is a factor of the difference between the non-torque limited braking and the torque limited coefficient. In laymen’s terms the difference between the maximum braking effect of a dry runway, and the braking effect on a wet (contaminated) runway for the same aeroplane at the same speed and the same mass.

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