Top of Descent Calculator

Frequently Asked Questions

How is top-of-descent distance calculated?

TOD (NM) ≈ altitude to lose (ft) ÷ descent gradient (ft/NM). For a standard 3° glidepath, gradient ≈ 318 ft/NM (often rounded to 300), so 10,000 ft loss ≈ 30–33 NM from the descent fix.

What vertical speed do I need?

V/S (fpm) ≈ ground speed (kt) × gradient ÷ 60. At 120 kt GS on a 3° path: 120 × 318 ÷ 60 ≈ 636 fpm. A quick mental rule: V/S ≈ GS × 5 for a 3° gradient.

How does wind affect TOD?

Tailwind increases GS and shortens TOD distance for a given V/S; headwind does the opposite. Always compute V/S from current GS, not TAS, or you'll be high in a tailwind and low in a headwind.

What about Mach descents at altitude?

Jets descend at constant Mach (e.g. 0.80) until crossing to constant CAS (e.g. 300 KIAS), then standard 3°. FMS computes TOD across both segments; for hand-flying GA, the 3°/3-NM-per-1,000-ft rule is the rule of thumb. Educational only - verify with the AFM/FMS.

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Aviation & Marine Disclaimer: Educational only. Not for flight or navigation operations.

This calculator is for educational and informational purposes only and is not a substitute for official flight or navigation planning. Always use current performance charts, an approved POH/AFM, certified navigation tools, and follow all applicable FAA, ICAO, USCG, and other regulatory guidance. Verify all results independently before operating any aircraft or vessel.

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How This Calculator Works

The top-of-descent (TOD) is the point along your route where you must start coming down to arrive at a target altitude over a target fix at the published or desired descent angle. Plan it too late and you arrive high (cabin uncomfortable, unstabilised approach, possible go-around); plan it too early and you burn extra fuel at low altitude. The standard ILS glideslope is 3° (sometimes 2.5° or steeper for terrain), and most arrival charts and STARs are sized around the same value. This calculator takes cruise altitude, target altitude, descent angle, and ground speed, then computes the lateral distance to begin descent, the vertical speed required to fly that angle at the ground speed, and the total descent time. Use for planning study; ATC clearances, approach charts, and current winds govern the actual descent.

Formula / Method

Altitude to lose = cruise − target (ft). Ground distance to start of descent = altitude ÷ tan(angle); convert ft to nautical miles by dividing by 6076.12. The handy 3° rule simplifies to distance (nm) = altitude (ft) ÷ 300. Vertical speed needed = ground speed × tan(angle), converted to feet per minute: vs (fpm) = GS × tan(angle) × 6076.12 ÷ 60. For 3° this reduces to vs ≈ GS × 5 (so 140 kt → 700 fpm). Wind matters because vertical speed is fixed by groundspeed, not airspeed - a tailwind raises required fpm.

Worked Example

Cruise 8,500 ft, target 2,500 ft (pattern altitude at the destination), 3° descent, ground speed 140 kt. Altitude to lose = 6,000 ft. Distance to TOD = 6,000 ÷ 300 = 20.0 nm (quick rule) or 6,000 ÷ tan(3°) ÷ 6076.12 = 6,000 ÷ 0.0524 ÷ 6076.12 ≈ 18.8 nm via exact math - the "divide by 300" rule overstates slightly because it linearises the tangent. Vertical speed needed = 140 × 5 = 700 fpm. Descent time ≈ 18.8 nm ÷ 140 kt × 60 ≈ 8 min. Start your descent about 20 nm out, pull the power to set 700 fpm, and you arrive comfortably at pattern altitude.

Common Mistakes

Using true airspeed or indicated airspeed instead of ground speed for the vs calculation - with a 30-kt tailwind your required fpm goes up by ≈20%. Forgetting to set the descent early when ATC issues a crossing restriction (“cross XYZ at and maintain 6,000”); you may already be past the natural TOD. Treating the 3° rule as a fixed law - some arrivals (mountainous terrain, noise-abatement, jet operations) use 2° or 4° descents. Forgetting to factor in a level-off for a step-down or speed reduction; subtract a mile or two for each level segment you expect. Not adjusting for headwind/tailwind changes during descent - ground speed often drops 20–30 kt as you enter denser, slower air.

Tips & FAQ

The 3-1 rule: 3 nm per 1,000 ft of altitude to lose - the simplest mental form, identical to “altitude ÷ 300.” Why GS × 5 for vertical speed? Because tan(3°) ≈ 0.0524, and 0.0524 × 6076.12 ÷ 60 ≈ 5.3 - close enough for cockpit math. When does the angle change? Approach plates state the final-approach descent angle (often 3.0–3.5°); enroute descents on a STAR may use 2–3°. What about engine cooling? Avoid abrupt power reductions in piston engines - plan a smooth, gradual descent at cruise power minus an inch or two of manifold pressure. Disclaimer: planning study only; comply with ATC instructions, approach chart, and POH/AFM.

Frequently Asked Questions

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