How to brief an IFR departure you've never flown (the 60-to-1 rule in practice)
Every published ODP or SID hands you a gradient in feet per nautical mile. Your VSI reads feet per minute. They're not the same number until you know your ground speed. Here's how to bridge them — and when to say no.
You're sitting in a rental at a field you've never flown out of. The published ODP says climb 350 ft per NM to 4000 before proceeding on course. You know this airplane and you know today's conditions. Can you make the slope?
The conversion: gradient → climb rate
An ODP gradient is a spatial requirement — feet per nautical mile, a slope across the ground. Your VSI is a temporal instrument — feet per minute, a rate against the clock. The bridge between them is ground speed:
Required climb rate (fpm) = gradient (ft/NM) × ground speed (kt) ÷ 60
This is the 60-to-1 rule applied to departure planning. It works because 1 NM per minute = 60 knots, so dividing by 60 converts the spatial slope into a temporal rate at your specific ground speed.
Standard ODP: 200 ft/NM
The standard obstacle-clearance gradient from TERPS is 200 ft/NM. At different ground speeds:
• 90 knots GS → 300 fpm
• 120 knots GS → 400 fpm
• 150 knots GS → 500 fpm
• 200 knots GS → 667 fpm
Most piston singles at pattern altitude and full power can sustain 500–800 fpm. At 120 knots GS, the standard 200 ft/NM = 400 fpm — comfortable. But when the chart says 350 ft/NM, the same airplane at 120 knots now needs 700 fpm. That might be fine on a cold day. On a hot day at 5,000 feet DA, you might not have it.
When to say no
If the chart's gradient at today's expected ground speed produces a climb rate your airplane can't reliably sustain — taking into account density altitude, weight, and whether you'll have flaps up by then — the answer is: don't depart IFR. Depart VFR if you can see the terrain, or wait for conditions to improve.
The gradient on the chart is the minimumto clear obstacles with TERPS separation margins. If your airplane delivers exactly that number at full power, you have zero margin. TERPS assumed you could do better than the published minimum. If you can't: don't go.
A climb gradient you cannot make is not a number on a chart. It is a hill.
Run the numbers
We built a free Climb Gradient Calculator that does this conversion both directions — gradient in, required rate out; or rate in, resulting gradient out. Plus a four-band performance advisory and the exact 60-to-1 math with real TERPS reference points.
If you're teaching instrument students and want to track their progress through ODPs, SIDs, and the rest of the IFR curriculum, Trim keeps the whole §61.65(d) record building as you fly.