Required Rate of Descent

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What vertical speed is required?

deg
Required rate of descent over one minute A side view showing the horizontal distance and vertical descent completed in one minute. 637 ft 2.0 NM in 1 min 3.0° START OF ONE MINUTE ONE MINUTE LATER
Path gradient 319 ft/NM
Distance per minute 2.0 NM/min
Required ROD 637 ft/min 3.0° mental: GS × 5 ≈ 600 ft/min

Method

From groundspeed to vertical speed

01

Find the path gradient

Tangent converts the angle into vertical feet lost during one horizontal nautical mile.

tan(angle) = vertical ÷ horizontal tan 3.0° × 6080 = 318.6 ft/NM

One nautical mile is normally 6076 ft. We use the rounded value 6080 ft.

02

Convert knots to NM/min

A knot is one nautical mile per hour. Divide by 60 to find the distance covered in one minute.

120 ÷ 60 = 2.0 NM/min
03

Find feet per minute

Multiply feet per nautical mile by nautical miles per minute. The NM units cancel.

318.6 ft/NM × 2.0 NM/min = 637 ft/min

Mental calculation

Why the ROD factor works

Rate-of-descent factor over one minute The distance flown in one minute is combined with the approximate height lost per nautical mile. 2.0 NM in 1 min 600 ft 3.0°
The factor converts groundspeed directly into feet per minute.

The 1-in-60 rule gives about 100 ft/NM per degree. Dividing that gradient by 60 turns it into a multiplier for groundspeed.

Approximate gradient 300 ft/NM 3.0 × 100
ROD factor ×5 300 ÷ 60 = 5.00
Mental ROD 600 ft/min 120 × 5

For 3.0°: use groundspeed × 5. The calculated factor is exactly 5.00.

Exact tangent result 637 ft/min 318.6 × 2.0
Mental factor result 600 ft/min 120 × 5

Difference: 37 ft/min (5.8%)

Why divide by 60? At 1 kt, an aircraft flies 1 NM in 60 minutes. Therefore, ft/NM ÷ 60 gives the feet per minute required for each knot of groundspeed.

Practice · Rate of descent

Find the required vertical speed

Your groundspeed is 110 kt. For a 3.0° descent, use factor ×5.

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