Flying and the Metric System

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.

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7 Responses to Flying and the Metric System

  1. Jim Howard says:

    I think that the old Soviet empire and China fully use the metric system in aviation including altitude and airspeed.

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  3. Hamish Reid says:

    I grew up with the metric system in Australia, but years of flying in California have made me all-too-comfortable with the US system. So it was very odd flying GA back in Australia a few years ago on a short visit — altitudes in feet based on millibars, distances in nautical miles from runways whose lengths are measured in metres, airplane engines measured in horsepower but consuming litres of fuel, etc. In some ways the worst of both worlds….

  4. I can imagine that if anything will ever enable American pilots to make the leap to metric measurements, it will be the proliferation of “glass” cockpits. Selecting “Metric” in a system settings menu is a lot easier than replacing a panel’s worth of steam gauges. That said, instrumentation markings are clearly only one very small piece of the puzzle.

  5. Olli Vainio says:

    In Europe, all aviation releated generally is in nautical units, so feet for altitude, knots for speed, nautical miles for distance etc. Altimeter setting is in hPa. Visibility in METARs are in meters. Gliders, at least in Finland, use metric system for altitude and speed as well, also they set their altimeters for QFE(altimeter showing 0 on the ground) and not to QNH..

    Russia and much of ex-Soviet Union uses metric system and QFE all the way, so does China.

  6. david says:

    Gliding is popular in North America as well, of course, but it seems to be huge in continental Europe. I guess that makes sense, given the airspace restrictions and high costs of powered flight there.

  7. Michael Poulos says:

    Obviously, a plane with CRT (or LCD) panels, metricating is easy. Just load in new firmware. The backup gauges have to be changed though like the altimeter. But older planes will be a pain. And pilots will have to learn the new panel. The hazard is an emergency and the pilot gets confused in the heat of the moment.

    Like the Mars mission, I wonder how many plane wrecks would occur during a changeover. Hopefully none, but I’m sure a few will happen, of course. Having a switch for American/metric would help, but potentially cause confusion is multiple pilots share a plane. I guess you have to add “check units selector” as part of the pre-flight checklist. (Maybe Microsoft can add that button to Flight Sim)

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