Rule of three (aeronautics)

In aviation, the rule of three or "3:1 rule of descent" is a rule of thumb that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent.^[1][2] For example, a descent from flight level 350 to sea level would require approximately 35x3=105 nautical miles. This would have to be adjusted for headwind or tailwind,^[1] and also to allow for deceleration time.

Alternatively, David P. Davies gives the rule as 300 feet of descent required for each nautical mile of distance.^[3]: 176

Large aircraft approaching to land normally use a 3 degree approach path.^[4] This is equivalent to 3.14 nautical miles per 1000 ft of descent. If exactly 3 nmi are allowed per 1000 ft of descent, the glide path will be 3.14 degrees.

See also

• ICAO recommendations on use of the International System of Units