Rule of three (aviation)
Encyclopedia
The rule of three in aviation
usage refers to the "3:1 rule of descent", in that 3
miles of travel should be allowed for every 1000 feet descent.
In the early days of aviation, few aircraft were pressurized. A pilot who waited until the last minute, then descended rapidly, would cause his passengers the discomfort of rapid pressure changes on their eardrums. He would not be very popular.
Transport pilots adopted this formula to assure a slow, steady, and comfortable descent profile for their passengers. Many aircraft had a cruise speed between 100 and 120 MPH in those days. Three miles would be traveled in about 1.5-1.8 minutes, resulting in a rate of descent of about 550–660 feet per minute. That was about as fast as passengers could comfortably adapt to the changing pressure on their eardrums. But many pilots would use a standard 300 feet per minute descent rate because doing so almost passes without notice among passengers. A pilot cruising at 10,500 feet would calculate that, in order to be at 1000' at his destination, he would have to lose 9500 feet. Dividing 9,500 feet by 300 feet per minute, that descent would require about 32 minutes. If his groundspeed was, say, 120 MPH, he would begin his descent about 64 miles from his destination, if traffic permitted.
Nowadays, this "rule of three" is employed by general aviation pilots who fly small, unpressurized aircraft. Large airliners and planes with pressurized cabins usually fly in a highly structured environment governed by the Instrument Flight Rules (IFR). In the more dense traffic structure of today, they do not often have the luxury of choosing to descend slowly, and must instead use the altitudes assigned to them by air traffic controllers. But, while a jet may descend at 3,000 feet per minute, the pilots can control the cabin pressure to provide a less uncomfortable rate of descent for the passengers. They use a cabin pressure descent rate of no more than about 600 feet per minute, which corresponds to the old "rule of three" descent rate.
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
usage refers to the "3:1 rule of descent", in that 3
miles of travel should be allowed for every 1000 feet descent.
In the early days of aviation, few aircraft were pressurized. A pilot who waited until the last minute, then descended rapidly, would cause his passengers the discomfort of rapid pressure changes on their eardrums. He would not be very popular.
Transport pilots adopted this formula to assure a slow, steady, and comfortable descent profile for their passengers. Many aircraft had a cruise speed between 100 and 120 MPH in those days. Three miles would be traveled in about 1.5-1.8 minutes, resulting in a rate of descent of about 550–660 feet per minute. That was about as fast as passengers could comfortably adapt to the changing pressure on their eardrums. But many pilots would use a standard 300 feet per minute descent rate because doing so almost passes without notice among passengers. A pilot cruising at 10,500 feet would calculate that, in order to be at 1000' at his destination, he would have to lose 9500 feet. Dividing 9,500 feet by 300 feet per minute, that descent would require about 32 minutes. If his groundspeed was, say, 120 MPH, he would begin his descent about 64 miles from his destination, if traffic permitted.
Nowadays, this "rule of three" is employed by general aviation pilots who fly small, unpressurized aircraft. Large airliners and planes with pressurized cabins usually fly in a highly structured environment governed by the Instrument Flight Rules (IFR). In the more dense traffic structure of today, they do not often have the luxury of choosing to descend slowly, and must instead use the altitudes assigned to them by air traffic controllers. But, while a jet may descend at 3,000 feet per minute, the pilots can control the cabin pressure to provide a less uncomfortable rate of descent for the passengers. They use a cabin pressure descent rate of no more than about 600 feet per minute, which corresponds to the old "rule of three" descent rate.