Rule of three (aeronautics)
In the early days of aviation, few aircraft were pressurized. A pilot who descended rapidly would cause his passengers the discomfort of rapid pressure changes on their eardrums.
Transport pilots adopted this formula to assure a slow, steady and comfortable descent for their passengers. Many aircraft had a cruising speed between 100–120 miles per hour (160–190 km/h). 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. However, many pilots used a 300-feet-per-minute descent rate because doing so is almost unnoticed by passengers. A pilot cruising at 10,500 feet would calculate that in order to be at 1,000 feet at his destination, he would have to descend 9,500 feet. Dividing 9,500 feet by 300 feet per minute, that descent would require about 32 minutes. If his ground speed was 120 miles per hour (190 km/h), he would begin his descent about 64 miles (103 km) from his destination (traffic permitting).
The "rule of three" is used by pilots flying small, unpressurized aircraft as well as by those flying airliners and aircraft with pressurized cabins. These usually fly in a structured environment governed by instrument flight rules (IFR) and may not have the luxury of choosing a descent rate, instead observing the altitude constraints assigned to them by air traffic controllers. Although rate of cabin pressurization change may be a consideration, this is usually managed automatically and for an airliner a larger consideration is fuel economy. A 3:1 descent gives an efficient glide descent from cruising altitude, and aids in planning; the company standard descent speed will be close to (usually slightly higher than) the optimum Lift/Drag ratio speed, and will give a descent ratio close to 3:1.
An additional use of this rule-of-thumb is on final approach which generally has a 3 degree vertical angle. This corresponds to 3 nautical miles (nm) per 1000ft and again makes mental estimation simple.
- "Top 10 rules of thumb". Plan and Pilot Magazine. 2006-01-01. Retrieved 5 November 2012.