In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air ...
, the rule of three or "3:1 rule of descent" is a
rule of thumb
In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...
that of travel should be allowed for every of descent.
For example, a descent from
flight level
In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa ...
350 would require approximately 35x3=105 nautical miles. This would have to be adjusted for headwind or tailwind.
Alternatively,
David P. Davies gives the rule as 300 feet of descent required for each nautical mile of distance.
Large aircraft approaching to land normally use a 3 degree approach path.
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
References
{{DEFAULTSORT:Rule Of Three (Aviation)
Air navigation
Flight training