Re AIM para 5-2-8, Instrument Departure Procedures I read it a few times. I have a simple question for the case when there is a SID gradient required for obstacle clearance that is above 200 feet per nm (fpnm). Will there be a 48 fpnm obstacle clearance or is it perhaps greater? For example, the AIM tells me that for a diverse departure obstacle assessment, if the 40:1 surface is clear of obstacles, then the climb gradient of 200 fpnm will assure at least 48 fpnm obstacle clearance. So I’m assuming that in the case where an obstacle is just a hair under the 40:1 surface, the obstacle clearance will be almost exactly 48 fpnm. The aircraft may be laterally some distance from the obstacle though. Does that all seem correct? Now lets consider a location where a SID is specified in the clearance, and it is required for obstacle clearance, and is 500 fpnm. Do we still have the same 48 fpnm obstacle clearance? I realize the assessment starts at departure end of the runway, so an aircraft has that extra buffer. Can anyone help? Stan

The 76% formula is used. As you say, a 40:1 is 152 feet per mile. The 40:1 is an obstacle clearance surface (OCS). The 200 feet per mile is a performance surface. 152 is 76% of 200. Assuming an ODP or SID with a climb gradient of 500 feet per mile, the OIS would be 380 feet per mile (16:1). The obstacle clearance per mile is 120 feet per mile, far greater than the nominal 48 feet per mile. The theory of the 76% rule is that the aircraft is more apt to fall below a more demanding performance surface.

Excellent, thanks aTerpster. It would be nice to see that in the AIM, though I guess pilots don't really need to know this much detail.

It is covered in the Instrument Procedures Handbook. At least the one I have, which might not be current.

Yes, and available here: https://www.faa.gov/regulations_pol..._procedures_handbook/media/FAA-H-8083-16B.pdf Page 1-16 "The obstacle environment may require a climb gradient greater than 200 ft/NM. In these cases, the ROC provided above obstacles is equivalent to 24 percent of the published climb gradient. The required climb gradient, for obstacle purposes on ODPs and SIDs, is obtained by using the formulas... Excellent reading, didn't know it was available on line.

Just about all the FAA's current publications are available online. LOL. Most wouldn't want to. Enough to know what to do to avoid hitting the terrain without having to understand the math.

...and you have no idea how many times I had to read it (in the TERPS, actually) before it made sense.