General Principles - Landing — Page 289, Lesson 357

I want to walk you through the landing part of General Principles now, and the first thing I want to hammer home is the distinction between a damp, a wet, and a contaminated runway, because these three terms are defined very precisely and they are not interchangeable. Let's start with damp. A runway is considered damp when there is moisture present on the surface that changes its colour, but there is insufficient moisture to produce a reflective surface. So think of it as the runway looking darker, but you don't see a shine on it yet. Next is wet. A wet runway is one whose moisture level makes the runway appear reflective, but there are no areas of standing water in excess of 3 mm deep. So you get that shine, but no puddles deeper than 3 millimetres. And here's a number you need to hold onto: wet runways can cause the average dry landing distance to increase by as much as 50%. That's a huge performance hit just from a reflective surface. Now contaminated. A contaminated runway is one where more than 25% of the runway is covered in a layer of moisture whose specific gravity is equivalent to a depth of 3 mm or more of water. So the threshold is 25% coverage, and the depth is measured by specific gravity equivalent to 3 mm of water. I want to stress the importance of runway contamination because many fatal accidents may have been avoided had due account been taken of the situation. That's not a throwaway line — that's the reason this whole subject exists. So as a pilot, you must be aware of the type of runway contamination, its depth, its extent, its effect on the braking, and its overall effect on the operation concerned — in this case, the landing. That information comes to you through a report. These reports are either by SNOWTAM, runway state code PIREPS, or spoken by air traffic control, and they may include braking action or braking coefficient. You then use any runway reports together with your aeroplane flight manual or standard operating procedure to best gauge the landing technique and landing performance. So the report gives you the raw data, and the flight manual or SOP tells you how to apply it. Now let's move to hydroplaning. There are three principal types of aquaplaning, or hydroplaning as it's now more commonly known. The first is dynamic hydroplaning. When an aircraft lands fast enough on a wet runway with at least 3 mm of standing water, inertial effects prevent water from escaping the footprint area, and the tyre is buoyed or held off the pavement by hydrodynamic force. Most people have experienced this when driving over a patch of water at high speed — the tyre literally rides up on the water film and loses contact with the surface. That figure shows you the various codes and terminology associated with runway contamination, so you can see how the reporting system ties together with what I just described.