Where to start down
A comfortable descent starts early enough to arrive at pattern or approach altitude without steep dives or last-minute power reductions. The top of descent comes from three numbers:
- Altitude to lose: cruise altitude minus the target, such as pattern altitude.
- Time: altitude to lose divided by the planned descent rate.
- Distance: time multiplied by the ground speed during the descent.
For the worked example, descending from 8,500 to 1,000 feet at 500 feet per minute takes 15 minutes; at a ground speed of 120 knots, start down 30 nautical miles out. The resulting path is 250 feet per nautical mile, about 2.4 degrees.
Descent rate for a distance
The second calculator answers the reverse question: you are 25 nautical miles out with 7,500 feet to lose at 120 knots. The time available is 12.5 minutes, so the required descent rate is 600 feet per minute, a 2.8-degree path.
Rules of thumb
- 3 to 1: 3 nautical miles per 1,000 feet, roughly a 3-degree path. The example needs 22.5 miles by the rule and 30 by the calculation: at 500 feet per minute and 120 knots the descent is shallower than 3 degrees.
- Ground speed × 5: a 3-degree path needs about 5.3 × ground speed in feet per minute, 637 feet per minute at 120 knots. The result lists it every time.
- Half the ground speed × 10: another common approximation of the same rate (600 feet per minute at 120 knots).
Planning notes
Use the ground speed you expect during the descent, not the cruise figure: a descent into a headwind covers less ground per minute. Many piston pilots plan 500 feet per minute for passenger comfort, and airspace floors or terrain may push the top of descent even earlier. Put the result into your nav log as a checkpoint.
A worked crossing restriction
Suppose you are at 9,500 feet and need to cross a point 20 nautical miles ahead at 4,500 feet, with a ground speed of 140 knots. You have 5,000 feet to lose in 20 ÷ 140 × 60 = 8.6 minutes, so you need about 583 feet per minute if you start now. If you would rather hold 500 feet per minute, the first calculator says to start down 23.3 miles before the fix, so you are already a little late. Enter both and compare; the second calculator shows the rate you need from where you are.
Speed during the descent
Many pilots hold cruise power and let the airspeed build a little during a descent, which raises the ground speed and moves the top of descent farther out. Use the ground speed you actually expect in the descent, and recheck it against a timed checkpoint on the way down.
Questions pilots ask
How do you calculate top of descent?
Divide the altitude to lose by the descent rate to get minutes, then multiply by the ground speed in miles per minute. Losing 7,500 feet at 500 feet per minute takes 15 minutes; at 120 knots (2 miles a minute) that is 30 nautical miles.
What is the 3-to-1 rule?
Allow 3 nautical miles for every 1,000 feet to lose. It approximates a 3-degree descent path. At 500 feet per minute and 120 knots the path is shallower than 3 degrees, so you need more distance than the rule suggests.
What descent rate gives a 3-degree path?
About 5.3 times your ground speed in knots, in feet per minute: 637 feet per minute at 120 knots. A 3-degree path loses about 318 feet per nautical mile.
Should I use true airspeed or ground speed?
Ground speed. Distance covered over the ground during the descent depends on the wind, so a tailwind moves the top of descent farther out.