Aeronautical Information Service (AIS) — Page 360, Lesson 485

Let's pick this up right where the runway condition reporting left off. We're deep inside the Aeronautical Information Service chapter, and we've just finished the SNOWTAM format — those lettered items J through T that describe snow banks, runway lights, clearance, taxiways, aprons, and observation times. Now I want to walk you through two more pieces of this reporting system. First, SNOWCLO. That's a specific term used in a VOLMET broadcast. VOLMET is the voice weather broadcast for aircraft in flight. When you hear "SNOWCLO" in that broadcast, it's telling you that an aerodrome is closed due to snow, or because snow clearance is in progress. So it's a single, standardized word that packs a lot of operational meaning — the field is not available for landing or takeoff, and the reason is snow-related. Then we move to something that affects every landing you'll ever make in wet conditions: Wheel Braking On Wet Runways. This is about the friction characteristics of a runway surface. Here's the key point: the inherent friction of a runway surface deteriorates only slowly over time. But in wet conditions, the friction — and therefore the braking action you get — can vary significantly over a short period. That variation depends on the actual depth of water sitting on the runway. There's also a longer-term factor. Seasonal variations in friction value can exist, on the order of six months. So the friction you measure in summer may not match what you'd measure in winter, even on the same runway. Now, here's the operational consequence, and I want you to hold onto this: because of that combination — rapid short-term variation with water depth, plus slow seasonal drift — there is no meaningful operational benefit to continually measuring the friction value of a wet runway. The number you'd get just isn't stable or useful enough to base decisions on. Let me be precise about what "wet runway" means in this context. It covers a range of conditions from Damp to Flooded. And critically, it does not include ice, and it does not include runways contaminated with snow, slush, or water associated with slush. Those are separate categories with their own reporting. Now, there's a design standard tied to this. Paved runways of 1200 metres and longer at civil aerodromes licensed for public use have been calibrated. That calibration ensures the friction characteristics of the runway surface are of a quality to provide good braking action in wet conditions. So the runway is built and maintained to a standard, but once water is actually present, the reporting shifts to a descriptive system. And that's where the excerpt cuts off — the presence of water on a runway will be reported on R/T, on radio telephony, using specific descriptions. Those descriptions are the surface friction reporting codes you see in Figure 18.1. That's the next piece we'll build on — how you actually phrase that report on the radio. So to tie it together: SNOWCLO is your voice-broadcast warning for a snow-closed field, and wet runway friction is something you don't measure continuously — you report it descriptively, because the numbers aren't operationally meaningful in the moment.