Top of Descent Planner

How to use it

1. Type the Altitude to lose in feet and your Ground speed in knots.
2. Choose Descent path given as: an angle such as 3 degrees, or a gradient in feet per NM.
3. Read Start the descent this far out and Vertical speed needed, then compare them with the 3 to 1 rule under them.
4. Open Working to see each step, and the Comparison of descent paths table for 2.5, 3.0 and 3.5 degrees.

How the top of descent is found

A descent path has a fixed gradient: the feet of descent for each nautical mile flown. A three degree path is about 318 ft per NM, which the FAA Instrument Procedures Handbook gives for a STAR as about 318 ft/NM, or about three degrees. The distance to start is the altitude to lose divided by the gradient.

The vertical speed follows from the gradient and the ground speed: at 120 kt a three degree path needs about 637 ft per minute, because 318 feet per NM at 2 NM per minute is 637 feet per minute.

gradient = tan(angle) × 6,076.1 ft per NM
distance = altitude to lose ÷ gradient
time = distance ÷ ground speed × 60
vertical speed = gradient × ground speed ÷ 60

• A descent angle of 3 degrees gives 318.4 ft per NM, and the gradient in feet per NM gives the angle back with atan.
• The check of the IPH for an approximate three degree glidepath is 300 feet to 1 NM: at 10 NM the height is 3,000 ft above the touchdown zone.

The 3 to 1 rule

The IPH gives a rule of thumb for initial IFR descent planning in jets: it takes 3 NM to descend 1,000 feet. Multiply the altitude to lose in thousands of feet by 3 to get the distance to start. The rule is a planning check: it gives a gradient of about 333 ft per NM, a little steeper than the three degree path.

distance (NM) = 3 × altitude to lose (ft) ÷ 1,000

• The IPH example: from FL 310 to an approach gate at 6,000 ft is 25,000 ft of descent, so the descent starts 75 NM out.
• The IPH adds 2 NM for each 10 knots of tailwind and takes 2 NM off for each 10 knots of headwind, for jets at idle thrust and a speed of Mach 0.74 to 0.78.

What this planner does not cover

• Wind along the route, which changes the ground speed and the distance to start.
• The aircraft's own descent profile, its speed limits and the idle-thrust performance in the flight manual.
• The altitude restrictions and the descent gradient that a published arrival sets. Those replace the figures here.
• This page is for training and planning practice, not for real flight.

Frequently asked questions