In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...
, the rule of three or "3:1 rule of descent" is a
rule of thumb
In English language, 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 associat ...
that of travel should be allowed for every of descent.
For example, a descent from
flight level
In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA sea level pressur ...
350 to sea level would require approximately 35x3=105 nautical miles. This would have to be adjusted for headwind or tailwind,
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.
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