What this calculator does
A departure procedure specifies a gradient in feet per nautical mile, and an aircraft climbs at a rate in feet per minute. Those are different units and the bridge between them is groundspeed, which is why a required gradient is harder to meet on a fast day than a slow one.
The consequence catches people out. A tailwind does nothing for the aircraft's climb performance but raises the groundspeed, so the same 700 feet a minute that comfortably clears a 200 foot per mile requirement at 90 knots only just clears it at 210.
The formula
The conversion is gradient = rate of climb × 60 ÷ groundspeed, because 60 minutes of climbing at that rate covers one hour of groundspeed in miles. Dividing the gradient by 6,076.12 and multiplying by 100 turns feet per mile into a percentage. Running it the other way, the rate of climb a published gradient demands is gradient × groundspeed ÷ 60.
| Term | Meaning |
|---|---|
| Gradient | Feet gained per nautical mile travelled. The unit procedures are written in. |
| Rate of climb | Feet per minute. What the vertical speed indicator shows. |
| Percentage gradient | Gradient divided by the feet in a nautical mile. A 3.3 per cent climb is about 200 ft/nm. |
| Required gradient | The minimum a departure procedure specifies, usually to clear terrain or obstacles. |
The inputs explained
| Field | What to enter |
|---|---|
| Rate of climb (fpm) | Rate of climb the aircraft is achieving, from the flight manual for the conditions rather than from a sea-level standard day. |
| Groundspeed (kt) | Groundspeed in the climb, which is true airspeed adjusted for wind rather than indicated airspeed. |
| Required gradient (ft/nm) | The gradient the procedure requires. Many departures specify 200 ft/nm, but obstacle-rich ones specify more. |
| Height to gain (ft) | Height you need to gain, used for the distance and time figures. |
When to use it
Checking a departure procedure
Enter the aircraft rate of climb and your expected groundspeed, then compare the gradient against the published requirement. The margin line is the one to read, and a negative figure means the procedure cannot be flown as planned.
Understanding the tailwind trap
Raise the groundspeed without changing the rate of climb. The gradient falls, because you are covering ground faster without climbing faster. A tailwind on departure reduces your obstacle clearance.
Working out the rate you need
The required-rate output works backwards from the procedure. At 200 ft/nm and 120 knots groundspeed you need 400 feet a minute, which is a different conversation from 300 at 90 knots.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Why does groundspeed matter so much?
The aircraft climbs at exactly the same rate in every row. Only the groundspeed changes.
| Groundspeed | Climb gradient | As a percentage | Margin over the requirement |
|---|---|---|---|
| 70 kt | 600 ft/nm | 9.87% | 400 ft/nm |
| 90 kt | 467 ft/nm | 7.68% | 267 ft/nm |
| 120 kt | 350 ft/nm | 5.76% | 150 ft/nm |
| 160 kt | 263 ft/nm | 4.32% | 63 ft/nm |
| 210 kt | 200 ft/nm | 3.29% | 0 ft/nm |
What rate of climb does the requirement need?
The requirement is fixed at 200 ft/nm and the achieved rate of climb changes.
| Rate of climb | Climb gradient | Margin over the requirement | Distance to gain that height |
|---|---|---|---|
| 300 fpm | 200 ft/nm | 0 ft/nm | 15.00 nm |
| 500 fpm | 333 ft/nm | 133 ft/nm | 9.00 nm |
| 700 fpm | 467 ft/nm | 267 ft/nm | 6.43 nm |
| 1000 fpm | 667 ft/nm | 467 ft/nm | 4.50 nm |
| 1500 fpm | 1,000 ft/nm | 800 ft/nm | 3.00 nm |
Questions
Why are procedures written in feet per nautical mile?
Because obstacles sit at a fixed distance from the runway, not a fixed time. A gradient relates height to distance, which is what clearance depends on, while a rate of climb depends on how fast you happen to be going.
Does a tailwind really reduce climb performance?
Not the aircraft's performance, but yes to the gradient over the ground, which is what matters for obstacles. The aircraft climbs at the same rate and covers more ground doing it, so it is lower at any given distance.
What gradient is 200 ft/nm as a percentage?
About 3.29 per cent. Dividing by 6,076.12 feet in a nautical mile gives the fraction, and procedures sometimes quote one and sometimes the other.
Should I use indicated or true airspeed?
Neither directly. Use groundspeed, which is true airspeed adjusted for the wind component. On a hot day at altitude true airspeed exceeds indicated, which already works against the gradient before any wind.
Is this enough to decide whether I can fly a departure?
No. It converts units; it does not know your aircraft, the temperature, the weight or whether the published gradient assumes all engines operating. Use the flight manual performance section for the decision.
For the descent side, see top of descent. Groundspeed in the climb comes from wind correction angle, and the performance behind the rate of climb is affected by density altitude.