Above 400 AGL, the diverse departure lets you continue the climb in whatever direction takes you on course, provided you continue to climb at least 200 feet per nautical mile. That’s feet per mile, not feet per minute. If you climbed at 60 knots, which is 1 nautical mile per minute, it would be a climb of 200 feet per minute. But if you climb at 120 knots, you need 400 feet per minute. What matters is groundspeed, not airspeed, so even a Cessna 172 with a bit of tailwind could need at least 400 feet per minute. Therefore, the safe zone for a diverse departure isn’t a cylinder of airspace around each airport so much as an inverted cone, albeit a pretty shallow one.
Most discussions of diverse departures end there, but that omits one last requirement. The diverse departure doesn’t extend infinitely in every direction around the airport. This cone of safety only extends 25 nm in non-mountainous areas and 46 nm in mountainous ones. If you fly a diverse departure, you must reach a known safe instrument altitude before getting more than 25 nm (or 46 nm in the mountains) away from the airport.
A safe instrument altitude means an MEA for an airway you’re joining, which is shown on the chart along the airway, or the off-route obstruction clearance altitude (OROCA) for the sector you’re in. OROCAs are the IFR counterpart for the brown Minimum Elevation Figures (MEFs) you see for each quadrant of a Sectional Chart. MEFs provide as little as 100 feet of obstacle clearance. OROCAs provide at least 1000 feet, so the published altitudes are higher.
Even climbing only 200 feet per mile, after 25 miles you’d be 5000 feet AGL. By the time you traveled 46 nm from the airport, you’d be over 10,000 feet AGL. You’re also only concerned about maintaining your own obstacle clearance until getting a vector from ATC, at which point they take over that responsibility. However, just hearing “radar contact” doesn’t constitute a vector, so it behooves you to understand these details and ensure you have a plan to get from pavement up to altitude entirely within the protected airspace.
Sometimes the airport would qualify for the diverse departure if the airplane could climb just a little bit steeper for the first thousand feet or so. Or 200 feet per nm would work if the airplane could see and avoid that one annoying cell tower before entering the clouds. Adding such a restriction and then saying it’s good for a diverse departure from that point would be so much easier than creating a whole ODP.
So there’s one more thing to check: the Takeoff Minimums. The most common format is that you need a certain ceiling and visibility to see and avoid obstacles while you climb at least 200 feet per nm, or you need a climb rate greater than the standard 200 feet per nm until a certain altitude, after which you can continue to climb at the 200 feet per nm.
Technically, complying with takeoff minimums or following published ODP isn’t required for Part 91 flight. However, I like to treat these restrictions as if they are required, because I’m a member of the Pilots Who Like Continued Birthdays club. Besides, my tax dollars paid for that obstacle survey, and I’m going to use it, dammit.