Descent planner
AviationHow far out should I start descending?
Top of descent, vertical speed and time for a path angle or a descent rate, with the 3:1 rule beside the exact answer.
What you enter stays on your device while you use this tool. Opening a shared link sends the settings in its address to HowToolbox to prepare the page.
Where you want to be at the target point, such as circuit altitude or the field elevation.
Path, glide and units
3° is the usual descent and approach angle.
Optional: from the flight manual, e.g. 9 for 9:1. Gives the still-air glide range.
Start descending
25.1 NM
before the target: descend at 640 ft/min on a 3° path, for 12 mins, 34 secs.
The descent
- Height to lose
- 8,000 ft
- Vertical speed
- 640 ft/min
- Path angle
- 3°
- Height per nautical mile
- 318 ft/NM
- Time to the target
- 12 mins, 34 secs
What this means
- The 3:1 rule would start down 1.1 NM later, so you would need a steeper path or a higher rate.
3:1 rule of thumb (3° path)
- 3 NM per 1,000 ft
- 24 NM
- Ground speed × 5
- 600 ft/min
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Descent profile
The descent from 10,000 ft to 2,000 ft starts 25.1 NM before the target on a 3° path.
| Altitude on the path | Distance to the target | Time to the target |
|---|---|---|
| 10,000 ft | 25.1 NM | 12 mins, 34 secs |
| 8,000 ft | 18.8 NM | 9 mins, 25 secs |
| 6,000 ft | 12.6 NM | 6 mins, 17 secs |
| 4,000 ft | 6.3 NM | 3 mins, 8 secs |
| 2,000 ft | 0 NM | — |
Top of descent, vertical speed and time for a path angle or a descent rate, with the 3:1 rule beside the exact answer.
How to use
- Enter your current altitude, the altitude you want at the target point, and your ground speed in the descent.
- Choose to plan by path angle (3° is usual) or by the vertical speed you will hold.
- Read how far before the target to start down, the vertical speed and the time; the 3:1 rule is shown beside the exact answer.
- Optionally, add your aircraft's glide ratio to see how far it would glide without power from the top of descent.
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How it works
A descent is planned backwards from where you want to be: the height to lose, divided by the height lost per mile on your path, gives the distance from the target at which to start down, the top of descent. The vertical speed then follows from how fast you cover those miles.
On a path of angle θ an aircraft loses 6,076 × tan θ feet per nautical mile, about 318 ft per NM at 3°. The vertical speed in ft/min is the ground speed in knots × that gradient ÷ 60. Example: from 10,000 ft down to 2,000 ft is 8,000 ft; on a 3° path that takes 8,000 ÷ 318 ≈ 25.1 NM, and at 120 kt the vertical speed is 120 × 318 ÷ 60 ≈ 640 ft/min, for about 12½ minutes. The 3:1 rule of thumb says 3 NM per 1,000 ft (24 NM) and ground speed × 5 (600 ft/min). Planning by vertical speed works the same way in reverse. Source: FAA Instrument Procedures Handbook (FAA-H-8083-16B), chapter 4.
The glide range is the height to lose × the glide ratio, in still air: with a 9:1 ratio, 8,000 ft takes you about 12 NM. A headwind shortens it and a tailwind lengthens it, and the flight manual's ratio holds only at the best glide speed (FAA Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C, chapter 5).
The answer assumes a constant ground speed and path, but the wind changes with height and you usually slow down as you descend, so keep a margin. It is for learning and simulators, not for flight planning or navigation: follow the published procedures and air traffic control.
Frequently asked questions
What is the 3:1 rule?
A quick way to plan a 3° descent: allow 3 nautical miles for every 1,000 ft to lose, and descend at about five times your ground speed in ft/min. It is close to the exact 3° answer, a little early on distance and a little slow on rate.
Why use ground speed rather than airspeed?
The distance to the target is covered over the ground, so the wind matters: with a tailwind you cover the miles faster and need a higher vertical speed to stay on the same path.
What path angle should I use?
3° is the standard for instrument approaches and a comfortable descent for most aircraft. Some airports with obstacles nearby publish steeper approaches, and gliders and helicopters fly very different angles.
What does the glide ratio mean?
How far the aircraft glides forward for each unit of height it loses without engine power. A ratio of 9:1 means 9,000 ft forward for every 1,000 ft down, in still air at the best glide speed.
Should the target be an altitude or a height above the runway?
Use the same reference for the current and target altitudes. The calculator subtracts them; mixing sea-level altitude with height above the runway gives the wrong height to lose.
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