Thunderstorms — Page 249, Lesson 208

I want to walk you through a very important safety document that the UK Civil Aviation Authority publishes — an Aeronautical Information Circular, or AIC. This one is number P 056/2010, and its full title is "The Effect of Thunderstorms and Associated Turbulence on Aircraft Operations." It was issued by NATS, which is the UK's air navigation service provider, and it replaces an earlier circular, P 019/2010. The purpose of this document is to give every pilot a proper understanding of the hazard that thunderstorms and their associated effects can pose to all aircraft operations. It's published for the information and safety of all pilots. Now, this circular was written with two-pilot operation of larger aircraft in mind — so think of a typical airline flight deck. However, any text that is highlighted by the use of capital letters is of particular relevance to pilots of all aircraft, whether you're flying a single-engine light aircraft or a wide-body jet. The overarching advice, the single most important message in this entire circular, is that flight through thunderstorms should be avoided. That is the fundamental rule. Let's move into the section on thunderstorm warnings. The organisations responsible for issuing these warnings are called Meteorological Watch Offices, abbreviated as MWO. These offices issue a specific type of warning called a SIGMET, which stands for Significant Meteorology. A SIGMET warning for thunderstorms is issued when two conditions are met: first, significant cumulonimbus clouds likely to produce thunderstorms are forecast, and second, those thunderstorms are expected to be difficult to detect visually by a pilot. So the warning is triggered not just by the presence of thunderstorms, but by the fact that a pilot might not see them coming. The circular then gives you the standard abbreviations you will see in these SIGMET texts. A thunderstorm that is obscured is abbreviated OBSC TS — that means it's hidden behind other weather, like haze or cloud. A thunderstorm that is embedded in other clouds is EMBD TS. If thunderstorms are frequent, you will see FRQ TS. And if they are organised along a line, that is SQL TS, which stands for squall line. The SIGMET warning includes information on the location, movement, and development of the thunderstorm areas. Now, here is an important point about what is not included in the SIGMET text. It is expected that all pilots will already be aware of the additional phenomena associated with thunderstorms — namely hail, severe icing, and severe turbulence. These are expanded on in the Annex to this circular, but they will not be written into the SIGMET text itself. There is one exception: heavy hail, abbreviated HVYGR, could be included in the SIGMET if it is expected. In addition to the MWO issuing warnings, aircraft commanders have a responsibility too. When conditions are encountered that are likely to affect the safety of the aircraft, the commander is required to send a Special Aircraft Observation. That report can then trigger a SIGMET warning to be issued for other aircraft in the area. Here is a critical point to remember: MWOs do not issue SIGMET warnings for isolated or scattered thunderstorms that are not embedded in cloud layers or concealed by haze — unless they are prompted by a Special Aircraft Observation. So the absence of a SIGMET warning does not necessarily indicate the absence of thunderstorms. You cannot assume the sky is clear just because no warning has been issued. That is a key safety takeaway.