Contaminated Runways — Page 232, Lesson 339

I want to walk you through the topic of contaminated runways. This is a critical area for every pilot because the state of the runway directly affects your take-off and landing performance calculations. Let me start with the core definitions you need to know. First, what exactly is a contaminated runway? A runway is said to be contaminated if more than 25% of its surface area — whether that contamination is in isolated patches or spread out — is covered by any of the following. The first is surface water more than 3 millimetres deep, which is 0.125 inches, or by slush or loose snow that is equivalent to 3 millimetres of water. The second is snow that has been compressed into a solid mass that resists further compression and will hold together or break into lumps if you pick it up — that is called compacted snow. The third is ice, including wet ice. So if more than a quarter of the runway has any of those conditions, you are legally and operationally on a contaminated runway. Now, let's distinguish that from other runway states. A damp runway is when the surface is not dry, but the moisture on it does not give a shiny appearance. So it's moist, but not reflective. A wet runway is different. A runway is considered wet when it is covered with water, or an equivalent contaminant, but less than the amounts specified for a contaminated runway — so less than 3 millimetres depth and less than 25% coverage. Alternatively, it is also wet when there is sufficient moisture on the runway surface to cause it to appear reflective, but without significant areas of standing water. So the key difference between wet and contaminated is the depth and the percentage of coverage. A dry runway is one which is neither wet nor contaminated. Importantly, this includes paved areas that have been specially prepared with grooves or porous pavement and maintained to retain effectively dry braking action even when moisture is present. So a grooved or porous runway can still be classified as dry for performance purposes, as long as it is maintained to that standard. Now, let's talk about contaminant depth limitations. Operations from contaminated runways are sometimes unavoidable, but there are maximum depths above which you should not attempt a take-off. For dry snow, the limit is a depth greater than 60 millimetres, and for very dry snow it is 80 millimetres. For water, slush, or wet snow, the limit is a depth greater than 15 millimetres. These are hard limits — if the contaminant exceeds these depths, you do not take off. Finally, I need to cover aquaplaning, also called hydroplaning. During take-off and landing operations from contaminated runways — specifically when there is 3 millimetres or more of water — aquaplaning is a hazard you must consider. Here is how it works. During the take-off run, as the tyre displaces water or any other liquid contaminant, 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 that bow wave. When that happens, friction between the tyre and the runway will rapidly reduce. That is aquaplaning — you lose braking and directional control because the tyre is no longer in contact with the runway surface.