A jet does not stop in the air, so a descent that starts in the wrong place ends with either a long level slog or a rushed, steep dive onto the approach. The 3:1 rule is the mental arithmetic that puts the top of descent in the right place, and it is worth knowing whatever you fly.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
The 3:1 rule
The rule is simple: allow 3 nautical miles of track distance for every 1000 feet you need to lose. Multiply the height to lose, in thousands of feet, by three, and that is roughly how far out to begin the descent.
Three miles per thousand feet works out to a steady path of roughly 3 degrees, which is the same gentle angle used for a normal instrument approach. It is shallow enough to be comfortable and to keep the engines doing useful work, and steady enough that you can hold it without constant correction.
Where the rule comes from: 60-to-1
The 3:1 rule is a handy face of the 60-to-1 rule, the geometry the FAA Instrument Procedures Handbook leans on for vertical planning. At a distance of 60 nautical miles, an angle of 1 degree subtends about 1 nautical mile. Brought down to per-mile terms, 1 degree of slope is about 100 feet of height per nautical mile, so a 3 degree path loses about 300 feet per mile.
Turn that around and 300 feet per mile is close to 1000 feet every 3 miles, which is the 3:1 rule. The two are the same relationship seen from different ends, and the small rounding is why the rule is a planning aid, not a promise.
Finding the top of descent
To place the top of descent, work out the height to lose and apply the rule:
- Take your cruise altitude and subtract the altitude you need at the planned point.
- Multiply that height to lose, in thousands of feet, by three to get the distance in nautical miles.
- Add a margin of track miles to slow from cruise speed down to a clean approach speed, since deceleration also eats distance.
Measure that total back from the target point and you have the top of descent.
A worked example
Say you are cruising at FL350 and need to cross a fix at 5000 feet.
- The height to lose is 35,000 minus 5000, which is 30,000 feet, or 30 thousands.
- Distance to descend is 30 times 3, which is 90 nautical miles.
- Add roughly 10 nautical miles to decelerate from cruise toward approach speed, so plan the top of descent about 100 nautical miles before the fix.
To hold the path once you are descending, use the rate-of-descent shortcut: about five times your groundspeed in knots, in feet per minute. Early in the descent at 450 knots groundspeed that is about 2250 feet per minute; as you slow, the rate you need falls with the groundspeed. A good in-descent check is to divide the height still to lose by the miles still to run: if you have 10,000 feet to lose in 30 miles, you need about 333 feet per mile, which is right on a 3 degree profile.
Common pitfalls
- Forgetting the deceleration miles. The 3:1 distance gets you down, but slowing from cruise to approach speed needs extra track miles on top.
- Ignoring the wind. A strong tailwind raises your groundspeed, so you need a higher rate of descent and you cover the ground faster, both of which argue for starting down a little earlier.
- Letting the path go high. Once you are above profile it is hard to recover without speed brakes or a steep, uncomfortable rate, so it is better to start down on time and adjust gently, a point SKYbrary makes about managing energy on the way down. Plan to be stable and configured by the approach gates your operator uses.
In Pilot EFB
Pilot EFB includes top-of-descent and rate-of-descent calculators, with the working shown so you can see how the height to lose, the distance and the groundspeed feed the answer. The figures are for your own planning; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so fly the profile and rates approved for your aircraft and follow ATC.