Question: True or false: Some airport signs are built to withstand the equivalent of an F5 tornado.
Answer: True, and not only that, but they must be able to do so for 10 full minutes, not only without damage, but without any distortion. That’s one tough sign, wouldn’t you say? In case you haven’t guessed though, this requirement exists for what are called Mode 3 signs, which are typically located in areas subjected to jet blast.
First, the FAA classifies signs according to type. (Examples of this would be the most familiar forms that we’re commonly called upon to distinguish and recognize, such as the ones indicating boundaries, destinations, directions, instructions, locations, distances remaining, etc.) Then comes size, style (relating to the type of power source for illuminated signs), class (ambient temperature), and lastly, mode, which has to do with wind loading. Many signs actually don’t have a mode designator, but in case you’re curious, Mode 1 signs must withstand 100 mph winds, and Mode 2 must endure 200 mph winds. An F5 tornado incidentally is the strongest tornado yet identified (which is done by analyzing damage after they pass by). Theoretically, there could be such a thing as an F6 tornado (up to an F12, which would be Mach 1 winds), but the damage evaluation hasn’t been described or standardized. In case you want to look up anything specific about runway and taxiway signs, it’s in Advisory Circular 150/5345-44G.