General Principles - Landing — Page 289, Lesson 357

Right, let's pick this up with the landing performance chapter. We've already covered the dry runway landing distance, so now we're moving into the critical subject of runway contamination and its effect on the landing. First, I want to make a very clear distinction between three terms, because they are often confused: damp, wet, and contaminated. These are not interchangeable. A runway is considered damp when there is moisture present on the surface which changes its colour, but there is insufficient moisture to produce a reflective surface. So, the runway looks darker, but it doesn't shine. 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, it looks shiny, but the water isn't pooling deeper than 3 millimetres. And here's a key performance figure: wet runways can cause the average dry landing distance to increase by as much as 50%. That's a huge increase, and it's why we take this so seriously. Now, a contaminated runway is a different category altogether. It's defined as 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, it's not just about depth; it's about the extent of coverage and the density of the contaminant. The importance of runway contamination cannot be stressed enough, as many fatal accidents may have been avoided had due account been taken of the situation. So, as a pilot, it is vital that you are aware of the type of runway contamination, its depth, its extent, its effect on the braking, and of course its overall effect on the operation concerned—in this case, the landing. Now, how do you get this information? The information on runway contamination and braking effect can be given 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 state of the runway, and the flight manual or SOP tells you how to handle it. Let's look at the figure here, which shows the various codes and terminology associated with runway contaminations. Now, let's move on to a phenomenon that is directly related to wet and contaminated runways: hydroplaning, which is also now more commonly known as aquaplaning. There are three principal types, and we'll start with the first one. 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 escaping from the footprint area, and the tyre is buoyed or held off the pavement by hydrodynamic force. Most people have experienced this type of hydroplaning when they have driven over a patch of water at high speed—the car feels like it's floating, and the steering goes light. That's the same principle, but on a much larger scale with an aircraft tyre. So, to summarise what we've covered: we have the three distinct runway states—damp, wet, and contaminated—each with its own definition and performance implications. We have the reporting systems—SNOWTAM, PIREPs, and ATC—that give you the braking action or coefficient. And we've started on the three types of hydroplaning, beginning with dynamic hydroplaning, which requires speed and at least 3 mm of standing water.