Descent Calculator

Down to the pattern from 7,000 ft. This is the FAA Instrument Procedures Handbook’s own example: cruising at 7,000 ft and descending to a 1,000 ft pattern altitude means losing 6,000 ft. A 3° path loses tan 3° × 6,076.115 = 318.4 ft per nautical mile, so the descent starts 6,000 ÷ 318.4 = 18.84 NM out. The handbook’s rule, dividing by 300, says 20 NM, and the 3-to-1 rule says 18 NM. At 120 kt the 3° path needs 120 × 318.4 ÷ 60 = 637 ft/min, for 9.42 minutes; the “groundspeed × 5” rule gives 600 ft/min.

A jet from FL310. Losing 25,000 ft to an approach fix at 6,000 ft on a 3° path starts 78.51 NM out and, at 420 kt over the ground, needs 2,229 ft/min. The handbook’s 3-to-1 formula gives 25 × 3 = 75 NM. If you fly the groundspeed × 5 rate of 2,100 ft/min instead, you lose only 23,553 ft in the 11.2 minutes the 78.51 NM takes, and arrive about 1,447 ft high.

The common mistake: using airspeed instead of groundspeed. The descent path is over the ground. At 120 kt true airspeed with a 30 kt tailwind, the groundspeed is 150 kt, and a 3° path needs 150 × 318.4 ÷ 60 = 796 ft/min, not the 637 ft/min the airspeed suggests. Flown at 637 ft/min, the aircraft drifts above the path all the way down.

The FAA’s Instrument Procedures Handbook (FAA-H-8083-16B) says standard terminal arrivals typically include a descent gradient of about 318 ft per nautical mile, or about three degrees, and that deceleration typically adds 1 NM of distance for each 10 kt of speed to lose. The design guidance for new or revised arrivals is FAA Order 8260.3, published in March 2016, which asks for a shallower gradient on the last part of the arrival.

The handbook gives the quick versions pilots use in the cockpit. For descent planning it divides the altitude to lose by 300 and multiplies the groundspeed by 5 for the rate, checking that 20 NM at 120 kt (2 NM a minute) takes 10 minutes, and 10 minutes at 600 ft/min loses the 6,000 ft. For jets it gives the 3-to-1 formula, 3 NM per 1,000 ft, based on idle thrust at Mach 0.74 to 0.78 and 1,800 to 2,200 ft/min, adding 2 NM for each 10 kt of tailwind.

On non-precision approach charts the FAA publishes a vertical descent angle from the final approach fix, and the Instrument Flying Handbook (FAA-H-8083-15B) explains that pilots use that angle and their groundspeed with the rate of descent table in the back of the Terminal Procedures Publication to find a target rate. This calculator does the same sum for any angle and speed.

This calculator works out the top of descent from the altitude to lose and either a descent angle (in degrees, feet per nautical mile or percent) or a descent rate and groundspeed; the descent rate for any angle and groundspeed; and the angle and gradient of a planned descent over a known distance. Altitudes can be in feet, metres or flight levels, distances in nautical miles, kilometres or statute miles, and speeds in knots, mph or km/h. Everything runs in your browser.

Every answer is exact trigonometry over the ground, shown next to the FAA rules of thumb (divide by 300, 3 to 1, groundspeed × 5) so you can see how far the mental shortcuts are off. It does not model the aircraft: deceleration, configuration changes, idle-thrust performance and wind changing with height are up to you, so plan to arrive early rather than late.

Related tools: Airspeed Calculator, Altitude Converter and the Heading Calculator.

For planning and training only — always use your aircraft’s POH/AFM and official weather and charts.