The featured article of the day on Wikipedia for Saturday 29 October is Metrication, the process of converting a country to metric from various historical units of measure. Now that Ireland has switched, the only three countries left not officially using metric (or in the process of changing) are Liberia, Myanmar (Burma), and the United States, though many people still informally use older systems for some things — for example, I use metric for temperature and distance (on the ground) and for buying food, but not for weighing myself, measuring my height, or buying lumber.
Because Canada and Mexico are metric while the U.S. is not, we’ve come up with a funny mishmash for North American aviation. We use nautical miles for distance and knots for speed (even most Americans don’t know those); statute miles for visibility (or feet under conditions of very low visibility); feet for elevation, altitude, and runway dimensions; inches of mercury for air pressure; and Celsius for outside air temperature (but not for cylinder head or oil temperature). Got all that? That’s right, if you’re six miles from the airport and there’s six miles visibility, don’t expect to see the airport, because six statute miles of visibility is 9,656 meters, while six nautical miles of distance is 11,112 meters, about a kilometer and a half further. Even American pilots use Celsius for temperature: you can always tell which Americans visiting Canada are pilots, because they’re the only Americans who understand the temperature on the Canadian weather report.
In Europe and most of the rest of the world, I know that they give runway dimensions in meters and air pressure in hectopascals (millibars), but I’m not sure if they use kilometers for distance, and I’m pretty sure they don’t use meters for altitude (or else standard altitudes wouldn’t mesh up). If the U.S. were finally to give in and go metric, would we switch to metric for all of aviation? It would certainly make things simpler for someone building a new plane or learning to fly from scratch, but there would be a lot of gauges to recalibrate, a lot of weight-and-balance to recalculate, and probably a lot of accidents caused by unit confusion until we straightened everything out. Remember that the Gimli Glider was, mainly, a result of confusion during metrication at Air Canada.