[Patrick_Devine]

In the aviation world it's really mixed. ICAO uses imperial and metric measurements for different things. Feet are used for altitude and knots for distance, but temperature is always measured in centigrade.

It's convenient to use feet because planes are stacked in 500' increments and not 152.4m increments. VFR (visual flight rules) flights are usually on the 500's (eg. 3500', 4500', etc. depending on heading) whereas IFR (instrument flight rules) flights are on the 000's (4000', 5000', etc.).

Knots are convenient because 1 knot is equal to 1 minute of 1 degree of arc on a great circle. If you're flying anywhere far away this ends up being important as a great circle is the shortest route between any two places.

Oddly enough, the metric system is useful with temperature because the standard lapse rate is 2 degrees per 1000' of altitude. So if you had to climb from 6000' to 8000' on an IFR flight plan, you would usually drop 4 degrees centigrade, which might be significant if it was raining out and it dropped below freezing. Having water on your wings and climbing up to an altitude where it's freezing is going to make you have a really bad day.

[stepstep]

> a great circle is the shortest route between any two places

Wouldn't it be faster to adjust the heading to point directly to the destination and then fly in a straight line?

[Patrick_Devine]

Remember that we live on a sphere (or something approximating one -- it's a sphere that bulges). A great circle route actually is a straight line, it's just that on a 2D map it looks like you're constantly turning.