This rule is used in air navigation. It states that if a pilot has travelled sixty miles then an error in track of one mile is approximately a 1° error. In reality the error is 0.96° but this difference is trivial in air navigation. It is hard to fly more accurately than within about 2° of tolerance. The error increases with the angle but again remains within flying tolerances for any error that is likely to occur in the air. Because this rule is used by single pilots with many other tasks to perform, often in a basic aircraft without the aid of an autopilot, it must be a simple process that can be performed in their heads.

Examples

If a pilot is flying a leg of 120 miles and finds after travelling 60 miles that they are two miles to the right of track then a correction of 4° to the left (2° to fly parallel to the intended track and another 2° to bring them to their target) will bring them to their destination.

If a pilot is flying a 120 mile leg and finds after 30 miles that they are two miles left of track then they have flown 4° left of their intended track, i.e.

2 × 60/30

left of track. Changing the heading four degrees right will now bring them to parallel the intended track. At that point they still have 90 miles to their next waypoint. They are thus two miles to the left of that and thus the waypoint is 4/3 of a degree (2 × 60/90) to the right or approximately 1° right. The pilot then adds these two to get 5° and flies 5° right of their previous heading.