Obstacle Departure Gradient Calculator
Real terrain, real numbers: from runway end to an obstacle's height and distance, the gradient required — compared against the 200 ft/nm standard and your aircraft.
Ridges manufacture their own downdrafts on the windward pilot's lee side — 500 ft/min of sink subtracts 300+ ft/nm at light-single speeds. Demand margin, not equality, near terrain.
Formula
⚠️ For planning and education only. Weight & balance must be computed from YOUR aircraft's actual empty weight, arm and current equipment list, and verified against the POH/AFM envelope before flight.
Real terrain, real numbers: from runway end to an obstacle's height and distance, the gradient required — compared against the 200 ft/nm standard and your aircraft.
About Obstacle Departure Gradient Calculator
Instrument procedures pre-chew terrain into published gradients; VFR mountain strips hand you the raw ingredients. This calculator does the procedure designer's division for your actual obstacle: height plus your demanded clearance, over distance, equals required ft/nm — compared against both the 200 ft/nm standard (to calibrate how serious the terrain is) and your aircraft's honest gradient today, with the verdict insisting on margin near rocks.
How to use Obstacle Departure Gradient Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula required ft/nm = (obstacle height + clearance) / distance; compare with aircraft gradient substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Obstacle Departure Gradient Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula required ft/nm = (obstacle height + clearance) / distance; compare with aircraft gradient with sources cited on the page
- ✓Ridges manufacture their own downdrafts on the windward pilot's lee side — 500 ft/min of sink subtracts 300+ ft/nm at light-single speeds. Demand margin, not equality, near terrain.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How do I measure the obstacle's height and distance honestly?+
Sectional contours and spot elevations give ridge heights (add the obstacle's own height for towers); any mapping app's ruler gives distance from the departure end along your actual track. Measure to the terrain you'll cross, not the valley's scenic centerline — and use the runway end elevation, not the airport reference point, as your zero.
What clearance should I demand over terrain?+
IFR design uses 48 ft per nm of flight (the difference between the 152 ft/nm surface and the required 200) growing with distance; a VFR planning floor of 300–500 ft over the controlling obstacle is common, doubled when wind crosses the ridge (downdraft territory) or at night. The input is yours precisely because the conditions are.
My margin is positive but small — what eats gradient without warning?+
Three thieves: lee-side sink (a 500 ft/min downdraft costs ~330 ft/nm at 90 kt), the climb-rate sag between sea-level book figures and today's density altitude (our DA tools quantify it), and the turn — banking 30° toward the pass cuts climb rate 10–15%. A margin smaller than one thief's appetite isn't a margin.
Is circling to gain altitude before crossing a legitimate plan?+
It's the standard mountain technique when the straight-out gradient fails: climb in the valley's wide part (over landable terrain, mindful of the drainage's own slope) until the crossing altitude plus clearance is banked, then cross. The calculator's role flips — confirming the requirement is impossible straight-ahead is the evidence that circling isn't optional.
Related Field tools
Sunrise & Sunset Calculator
Exact rise, set, solar noon and day length for any place and date — the NOAA solar equations with the refraction fine print included.
● LiveGolden Hour & Blue Hour Calculator
Tonight's golden hour and blue hour, computed from sun elevation — the photographer's light windows with the angles that define them.
● LiveDay Length Calculator
Hours of daylight for any date and latitude, how fast it's changing, and the swing between your solstices.
● Live