Contaminated Runways — Page 232, Lesson 341

I want to walk you through the topic of contaminated runways, starting with a critical phenomenon called aquaplaning. During a landing roll, if your touchdown speed is above the speed at which aquaplaning is likely to occur, any application of the brakes may result in a severe loss of friction between the tyre and the runway surface. That drastically reduces your braking effectiveness. In this specific case, the effect of aquaplaning will actually stop the rotation of the tyres. This can lead to dissipation of momentum energy in the form of heat generated in the contaminant itself. The temperature reached may scald the tyres, and skidding may result when the aquaplaning effect breaks down. In any case, the two main hazards of aquaplaning are loss of braking action and loss of directional control. Now, there is a formula to calculate the aquaplaning speed. It is given by V equals 9 times the square root of P divided by sigma. Let me break that down: V is the ground speed in knots. P is the tyre pressure in pounds per square inch, or psi. Sigma is the specific gravity of the precipitant, which is the contaminant on the runway. This speed, however, assumes a rotating tyre — we call this the spin-down speed. For a non-rotating tyre, as in the situation on initial touchdown — that is the spin-up speed — the aquaplaning speed is 7.7 times the square root of the tyre pressure in psi. If the tyre pressure is stated in bar, remember that 1 bar is approximately equal to 14.5 psi. Let's move on to braking action. From data collected from operations on compacted snow and ice, an assessment table has been produced. This table relates a measured braking coefficient to an estimated braking action, and then to a simple code for braking action. You must bear in mind that the description "good" is a comparative value. It 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. Here is the table. A measured coefficient of 0.40 and above corresponds to an estimated braking action of "Good" and a code of 5. A coefficient between 0.39 and 0.36 gives "Medium to good" and code 4. Between 0.35 and 0.30 gives "Medium" and code 3. Between 0.29 and 0.26 gives "Medium to poor" and code 2. A coefficient of 0.25 and below gives "Poor" and code 1. And if the measurement is unreliable, the code is simply "UNRELIABLE" with code 9. Finally, let's talk about coefficient of friction in this context. 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 layman's terms, it is the difference between the maximum braking effect of a dry runway, and the braking effect on a wet or contaminated runway, for the same aeroplane at the same speed and the same mass.