Aeronautical Information Service (AIS) — Page 364, Lesson 487

Let’s pick this up right where the runway surface descriptions leave off, because that table is the key to everything that follows. I want to walk you through the interpretation of those runway surface states, because this is where the book turns a simple description into an operational decision. When a runway is reported as ‘DAMP’ or ‘WET’, you as a pilot may assume that an acceptable level of runway wheel braking friction is available. That’s the critical phrase — the authority is telling you that a damp or wet runway still gives you a usable, predictable braking surface. But the moment the report says ‘WATER PATCHES’ or ‘FLOODED’, that assumption changes completely. Braking may be affected by aquaplaning — that’s the condition where your tyres ride up on a film of water and lose contact with the runway surface — and appropriate operational adjustments should be considered. So the distinction is not cosmetic; it’s a direct signal about whether you can rely on your brakes. Now, there’s a precise threshold built into this. The term ‘WATER PATCHES’ will be used if at least 25% of the runway length is covered with standing water. That’s a hard number you need to hold onto — 25% of the runway length. Below that, it’s just ‘WET’; at or above that, it becomes ‘WATER PATCHES’ and the aquaplaning warning kicks in. There’s also a special rule for runways that are notified as liable to be slippery when wet. In that case, take-offs or landings in wet conditions should only be considered when the distances available equal or exceed those required for a very slippery or icy runway, as determined from information in the aeroplane’s Flight Manual. So you’re not using the normal wet-runway performance figures — you’re stepping up to the worst-case, very slippery or icy runway distances from the Flight Manual, and you need the actual runway length to be at least that long before you even consider operating. One more wrinkle: at military aerodromes in the UK, runway surface conditions will be described in plain language, not in those five standard terms. And where a braking action measuring device has been used, braking action will be described as good, medium, or poor. So you have two different reporting systems depending on where you are — civil aerodromes use the standard descriptions, UK military aerodromes use plain language plus a braking action rating. Now let’s move to the ASHTAM, which is a completely different kind of notice. The purpose is straightforward: volcanic ash cloud presents a significant hazard to turbine engine aeroplanes. That’s the core reason this whole system exists — ash can damage or destroy turbine engines. Timely warning of the presence of ash clouds, or the possibility of an ash cloud existing, is vital to safe operations in areas where volcanic activity is common. So the ASHTAM is the mechanism for that warning. Information concerning an operationally significant change in volcanic activity, a volcanic eruption, and/or a volcanic ash cloud is reported by means of an ASHTAM. Let me break down what the ASHTAM actually contains. It provides information on the status of activity of a volcano when a change in its activity is, or is expected to be, of operational significance. That status is given using the volcano level of alert colour code — so the colour code is the shorthand for how dangerous the volcano currently is. In the event of a volcanic eruption producing an ash cloud of operational significance, the ASHTAM also provides information on the location, extent, and movement of the ash cloud, and the air routes and flight levels affected. So you get the geometry of the hazard — where it is, how big it is, which way it’s drifting — and you get the operational impact — which routes and which flight levels are affected. Two hard limits on the ASHTAM: the maximum period of validity is 24 hours. And a new ASHTAM must be issued whenever there is a change in the alert level. So it’s a living document — it expires after a day, and any change in the volcano’s alert status forces a fresh issue. Now, the colour code is worth a closer look, because it’s the visual shorthand you’ll actually use in flight planning. The ASHTAM colour code runs through a sequence of alert levels tied to the volcano’s activity. Let me walk you through the standard progression, because this is the operational spine of the whole system. The lowest level is GREEN — the volcano is in a normal, non-eruptive state. Next is YELLOW — the volcano is showing signs of elevated unrest above known background levels. Then ORANGE — the volcano is exhibiting heightened unrest with increased likelihood of eruption. And the highest is RED — a volcanic eruption is underway or is imminent, with significant emission of ash into the atmosphere likely. That’s the escalation ladder you’ll see in the ASHTAM, and it directly drives the operational response — the higher the colour, the more restrictive the airspace becomes. So to tie it all together: the runway surface descriptions give you a standardised language for braking performance, with a hard 25% threshold for water patches and a special slippery-when-wet rule that forces you to use very slippery or icy runway distances. And the ASHTAM gives you a standardised, colour-coded warning system for volcanic ash, valid for 24 hours, re-issued on any alert level change, telling you where the ash is, how it’s moving, and which routes and flight levels are affected. Both are about giving you, the pilot, the precise information you need to make a safe operational decision before you commit to a take-off or landing.