Aviation Stack Exchange is a question and answer site for aircraft pilots, mechanics, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
Generally speaking, which assumes a greater (steeper) minimum climb or descent angle – approach procedures or departure procedures? Which would typically require a larger extent of airspace protected down to any given elevation (e.g. surface, 700' AGL, 1200' AGL, etc.)?
I'm specifically interested in how the FAA designs the protected airspace in the US, though I suspect the answer would be similar everywhere.
The first departure OCS slope that is evaluated is 40:1 (run:rise), which is 1.43°. 200 ft/NM, which is ~2°, ensures clearing that surface if it's applicable. (And since those numbers are confusing, 200 ft/NM is about 650 ft/min when flying at 180 knots.)
The minimum climb gradient that will provide adequate ROC in the climb segment is 200 ft/nautical mile (NM), unless a higher gradient is specified.
... and anything above 500 ft/NM requires "approval of AFS-400 or the appropriate military authority."
On the other end, for designing an ILS, no angle less than 3° is authorized.
12-10-8. Glide Slope. [...] Angles less than three degrees are not authorized.
So, which assumes a steeper gradient? Generally speaking as requested, approaches, but since both ends are generally used, the departure OCS will be more demanding in terms of building structures around an existing airport.
Source: FAA TERPS 8260.3D (latest E revision here).
The descent is usually steeper, except in mountainous areas.
The standard descent angle is 3°, or about 300 feet per mile. Very few approaches are steeper than this; designers prefer to alter the lateral path instead, or just use high minimums.
The standard climb gradient is 200 feet per mile, or about 2°. However, that can go up to 500 feet per mile, or about 5°, if terrain or obstacles dictate.
The discrepancy seems odd since runways are normally used from both ends, but in mountainous areas, it’s fairly common for there to be approaches to only one end.
Your question seems to assume airspace is protected based on the procedures, but it’s actually the opposite; they survey what terrain and obstacles exist, then design procedures to fit within the airspace that is protected.
