How do I calculate a safe descent rate for an approach?

In real-world aviation and flight simulation, estimate a normal 3° approach descent rate by multiplying groundspeed in knots by 5: 120 kt × 5 gives about 600 ft/min. Use groundspeed, not indicated airspeed, and adjust this starting value to stay on the published glidepath or visual slope; aircraft limits and stabilised-approach criteria take priority.

Approach descent-rate formula

The standard mental rule for a 3° approach is descent rate (ft/min) ≈ groundspeed (kt) × 5. Another way to perform the same calculation is to halve the groundspeed and add a zero: half of 140 is 70, giving approximately 700 ft/min.

The geometrically exact formula is vertical speed = groundspeed × 101.3 × tan(glide angle). A 3° path therefore uses a factor of about 5.3 rather than 5, but the simpler rule is normally adequate as an initial setting because the pilot must still correct for wind and glidepath deviation.

For a non-standard approach angle, use the exact formula or the charted vertical-speed table. Approximate factors are 4.4 for 2.5°, 5.3 for 3° and 6.2 for 3.5°.

How do I calculate the rate step by step?

A safe calculation starts with the published approach path and ends with a continuous path check rather than a fixed vertical-speed command.

1. Find the required angle. Use the published glide angle or visual guidance. Do not assume every runway has a 3° path; displaced thresholds, terrain and approved steep approaches can change it.
2. Read the groundspeed. Take it from the GPS, FMS or other navigation display. Indicated airspeed does not include the wind effect over the ground.
3. Calculate an initial rate. For 120 kt groundspeed on a 3° path, start near 600–640 ft/min.
4. Cross-check the height. A 3° path loses about 318 ft per nautical mile, often rounded to 300 ft/NM. At 5 NM, expect to be approximately 1,590 ft above the runway threshold elevation, subject to the published procedure.
5. Adjust while descending. Use the glideslope, glidepath, PAPI or VASI rather than chasing the calculated number. Allow for indication lag; our explanation of VSI indications and descent-rate rules covers how the instrument should be interpreted.

If no glide angle is available, divide the altitude to lose by the time remaining. For example, 6 NM at 120 kt takes 3 minutes; losing 1,800 ft in that time requires 600 ft/min. This method must not be used to bypass published altitude restrictions, minimum descent altitude or decision altitude.

What makes an approach descent rate safe?

A descent rate is safe only when it keeps the aircraft on the intended path within its operating limitations and stabilised-approach criteria. The calculation alone provides no obstacle clearance and does not prove that the aircraft can land safely.

• The aircraft should be on the correct lateral and vertical path.
• Landing configuration, target speed and power should be established by the applicable stabilisation gate.
• The rate should remain within the aircraft flight manual, operating procedure or instructor's limits.
• Only small corrections should be required; a late dive towards the runway is not a valid way to recover the profile.
• If the approach cannot be stabilised by the specified gate, go around rather than forcing the landing.

What if the result is over 1,000 ft/min?

A calculated rate above 1,000 ft/min is not automatically unsafe, but it warrants a careful check. On a normal 3° path it corresponds to roughly 190 kt groundspeed, which is unusually fast for many aircraft on final; an excessive tailwind, incorrect groundspeed, steep path or poor configuration may be the cause.

Many airline stabilised-approach policies require rates above 1,000 ft/min to be specifically briefed or treat them as unstable unless expected. This is not a universal regulatory limit: an approved steep approach may legitimately require more, while a light training aircraft may become uncomfortable or unstable at a much lower rate. Follow the aircraft manual, approach chart and applicable operating procedure.

How does wind change the required descent rate?

Wind changes descent rate through groundspeed: a headwind requires fewer feet per minute, while a tailwind requires more to maintain the same angle. At 120 kt indicated airspeed, an approximate 20 kt headwind might produce 100 kt groundspeed and a 500 ft/min estimate; a 20 kt tailwind might produce 140 kt groundspeed and about 700 ft/min.

Use the displayed groundspeed rather than assuming the wind correction, especially with crosswind or changing wind near the surface. If groundspeed decreases while vertical speed remains fixed, the path becomes steeper; if groundspeed increases, it becomes shallower.

Should I hold the calculated rate on an ILS?

No: on an ILS or another approach with vertical guidance, the calculated rate is only an initial value and the glidepath indication is controlling. Holding a fixed vertical speed despite changing wind will eventually move the aircraft above or below the path.

Capture the path from the correct position, establish the expected rate, then make measured pitch-and-power corrections. Our FSX ILS capture and tracking procedure demonstrates the same principle in a simulator. On a visual approach, use the PAPI or VASI, altitude checks and runway aiming point; see our technique for controlling a visual final.

Common descent-rate calculation mistakes

Most descent-rate errors come from using the wrong speed or treating a planning estimate as a command.

• Using indicated airspeed: calculate from groundspeed because the aircraft must lose height over ground distance.
• Forgetting runway elevation: a 1,500 ft height above the threshold is not necessarily 1,500 ft on the altimeter; add threshold elevation when working in mean sea level altitudes.
• Confusing distance and rate: the 3-to-1 rule estimates where descent should begin, while groundspeed × 5 estimates feet per minute. Our guide to planning a top-of-descent point with the 3-to-1 rule explains the distance calculation.
• Locking onto one VSI number: update the rate as groundspeed changes and correct according to the actual glidepath.
• Trying to salvage a late descent: high sink rate, excess speed and rushed configuration are signs to discontinue the approach rather than descend more aggressively.