How to use the top of descent calculator
- Enter both altitudes. Where you are now (flight level 350 is 35,000 ft) and where you want to be level: a fix on the arrival, the approach gate or the traffic pattern.
- Enter your ground speed for the descent. Ground speed already includes the wind, so a tailwind gives a longer descent and a headwind a shorter one. Use the average you expect on the way down.
- Choose how you'll descend. On a fixed path angle (3° unless you choose otherwise), the calculator gives the vertical speed that keeps you on it. At a fixed vertical speed, as with an autopilot's vertical speed mode, it gives the angle that works out to.
- Optional: the distance to go. Take it from the GPS or flight plan: the distance to the point where you want to reach the target altitude. The calculator then counts down to the top of descent, or, if you've passed it, gives the descent rate that still gets you there.
The FAA's rules of thumb
The FAA's Instrument Procedures Handbook gives three quick rules for planning a descent, and the calculator shows each one next to the exact answer:
- Divide the altitude to lose by 300 for the miles to start down. Its example: from 7,000 ft to a pattern altitude of 1,000 ft is 6,000 ft, so start 20 NM out, and leave some extra room.
- Multiply your ground speed by 5 for the descent rate on a 3° path. At 120 knots, that's 600 feet per minute.
- For jets, 3 NM for every 1,000 ft, assuming idle thrust, Mach 0.74 to 0.78 and 1,800 to 2,200 feet per minute. From flight level 310 to 6,000 ft, that's 75 NM. Add 2 NM for each 10 knots of tailwind and take off 2 NM for each 10 knots of headwind.
The rules round in your favor: a 3° path really drops about 318 ft per mile, not 300, and needs about 5.3 times the ground speed, not 5. The handbook's own example comes out at 18.8 NM and 640 feet per minute exactly, so the rule starts you down a little early.
Examples
Worked out with this calculator:
| Descent | Ground speed | How | Start down | Rate | Time |
|---|---|---|---|---|---|
| 7,000 → 1,000 ft | 120 kt | 3° path | 18.8 NM | 640 fpm | 9 min |
| 9,500 → 2,000 ft | 140 kt | 3° path | 23.6 NM | 740 fpm | 10 min |
| 25,000 → 3,000 ft | 250 kt | 3° path | 69.1 NM | 1,330 fpm | 17 min |
| FL350 → 3,000 ft | 300 kt | 3° path | 100.5 NM | 1,590 fpm | 20 min |
| FL350 → 3,000 ft | 300 kt | 2,000 fpm | 80 NM | 3.8° path | 16 min |
| FL350 → 3,000 ft | 300 kt | 2,500 fpm | 64 NM | 4.7° path | 13 min |
The last three rows show the trade: a faster descent rate means a steeper path and a later start, but the same height to lose.
Leave room to slow down
The top of descent only gets you down. You also have to slow down, and it is hard to lose height and speed at the same time. The FAA handbook's planning speeds for an arrival are 250 knots at 25 NM from the runway threshold, 200 knots at 20 NM and 150 knots at 15 NM, until it is time for the gear and flaps, and never below the approach speed.
So plan to reach the target altitude a few miles before you need it, especially in a fast aircraft. If you end up high, the speed brake helps if your aircraft has one; so does asking ATC for more distance.
Using it in a sim
Airliner add-ons with a flight management computer work out the top of descent for you; the FAA handbook notes that flight management systems compute it automatically. In aircraft without one, you plan it yourself, and that's where this calculator comes in.
Before you leave cruise, check the destination's weather and the runway in use, as the handbook advises, because the arrival route and the active runway can change the distance a lot. Then set the descent rate on the autopilot's vertical speed mode, or fly it by hand, and use the distance to go to check you're on the path as you come down.