Top of descent calculator
Starting down too late means either a rushed approach or arriving high. This works out how far from your target you need to begin, for whatever descent path you choose to fly.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The distance is how far out to start the descent. The rate of descent is what you need to hold to fly that path at your groundspeed.
You can describe the path as an angle, a gradient in percent, or feet per nautical mile, because all three are in common use. Three degrees is the standard instrument approach path and roughly what airliners use for a normal descent.
When this is useful
- Planning a descent from cruise, particularly from high level where the distances are long.
- Working out whether you can still make a crossing restriction from your current position.
- Understanding why a descent takes so much more distance than most people expect.
How it is calculated
The chosen path is converted into feet lost per nautical mile. Dividing the altitude to lose by that figure gives the distance, and dividing the distance by groundspeed gives the time. The rate of descent follows from the altitude and the time.
The common shortcut for a 3 degree path is to multiply the thousands of feet to lose by three to get miles, and to use half the groundspeed multiplied by ten for the rate. Both are good approximations and the calculator gives the exact figures.
distance = altitude to lose / (tan(angle) x 6076), rate of descent = altitude to lose / time
Worked example
- Current altitude
- 35,000 ft
- Target altitude
- 3,000 ft
- Descent path
- 3 degrees
- Groundspeed
- 450 kt
Start down about 100 NM out. The descent takes around 13 minutes and needs about 2,390 ft per minute.
Check: The three times table rule of thumb gives 32 thousands of feet multiplied by 3, which is 96 NM, close to the exact 100.5 NM. The rate of thumb rule of half the groundspeed multiplied by ten gives 2,250 ft per minute against the exact 2,388. Both approximations sit just below the exact answers, as expected.
Limitations
- This is pure geometry. It takes no account of slowing down, configuration changes, or any level segment ATC gives you, all of which lengthen the real descent.
- Groundspeed usually falls through a descent as speed reduces, which makes the actual descent slightly shorter than calculated.
- It assumes you are free to descend continuously, which is rarely true in busy airspace.
Common mistakes
- Forgetting that the target altitude is not usually the airfield elevation. Crossing restrictions and circuit heights are above it.
- Mixing up percent and degrees. A 3 degree path is about 5.2 percent, not 3 percent.
- Using true airspeed instead of groundspeed, which matters a great deal with a strong wind at altitude.
Sources
- Instrument Procedures Handbook (FAA-H-8083-16), arrivals , US Federal Aviation Administration
- Continuous Descent Final Approach (CDFA) , SKYbrary, EUROCONTROL