The sixteenth tool, and the 60-to-1 rule
Tool fifteen was supposed to be the last for a while. Then I went to brief a terrain-boxed IFR departure, reached for a climb-gradient calculator I'd just claimed I had, and didn't have it. So I built number sixteen.
Two nights ago I wrote that the bar for tool number sixteen was simple: it had to be something I, as a working CFI and commercial pilot, actually reach for — not SEO-bait, not "we could ship that." I did not expect to clear that bar within 48 hours.
Then I sat down to brief an IFR departure off a short field with a published obstacle departure procedure — climb 350 ft per NM to 4000 — and went looking for the calculator I had just told everyone I owned fifteen of. I did not own this one. So here is the sixteenth: the Climb Gradient calculator.
Two numbers that never match
A departure procedure hands you a gradient in feet per nautical mile — a slope across the ground. Your VSI reads feet per minute— a rate against the clock. They are not the same number and they never line up until you know your ground speed. The standard departure assumes the airplane can hold 200 ft/NM; when terrain disagrees, the ODP or SID prints a steeper figure. The only question that matters on the runway is whether today's airplane — this weight, this density altitude, this ground speed — can actually make the slope.
The 60-to-1 rule is the bridge
The conversion is the old 60-to-1 rule, the same arithmetic instrument pilots use for everything from VDPs to descent planning:
required climb rate (fpm) = gradient (ft/NM) × ground speed (kt) ÷ 60.
At the standard 200 ft/NM and 120 knots, that is 400 fpm — comfortable. Bump the ODP to 350 ft/NM and the same 120-knot airplane now needs 700 fpm. Put a jet on it at 250 knots and the requirement jumps past 1,450 fpm. The tool runs it both directions: gradient in, required rate out — or rate in, resulting gradient out — so you can check what the airplane actually delivers against what the chart demands.
A climb gradient you cannot make is not a number on a chart. It is a hill.
Same pattern, sixteenth time
Built like the other fifteen: pure-function math in lib/climb-gradient-math.ts, 22 Vitest tests cross-checked against the standard reference points, a thin client component to wire it up, FAA values only. No invented thresholds. The 200 ft/NM default comes straight from the TERPS standard, not from me.
The bar held. This one I will use — which was the whole point. See it at tarmaclabs.org/tools/climb-gradient, or all sixteen tools on the index.