Originally Posted by safetypee
I refer to a very useful pilots’ guide on instrument flight procedures – “From Take Off to Landing”. This booklet was written by Olle Akerlind of SAS and published some time ago. A revised edition was published last year, sponsored by Honeywell Safety Systems (EGPWS) as a contribution to CFIT and ALAR. I regret that I cannot identify an easily accessible source for additional copies, but I recommend that you beg, borrow, or steal one that you may see.
The revised book refers to PANS-OPS and TERPS, but heed the usual warnings on master document revision status.
For PANS-OPS the primary area for obstacle clearance (984ft/300m), is 2.5 nm either side of the required track. There is a secondary area of a further 2.5 nm where the obstacle clearance reduces linearly from 984ft/300m to zero. Thus there is up to a 5nm wide corridor for a DME arc procedure.
For TERPS the primary area is 4 nm either side of the required track for an obstacle clearance of 1000ft. The secondary area is a further 2nm where the clearance starts at 500ft and reduces to zero. The minimum DME arc radius is 7nm (15nm for high level procedures).
I cannot find any reference for the required accuracy of flight (normally a value for flight technical error), but as with any flight procedure we should always aim for the highest accuracy.
The actual signal tolerance of the arc as PANS-OPS applies is ±0.25 NM + 1.25 of the distance from the DME antenna. The 2.5NMprimary/2.5NM secondary semi width is a) only for obstacle assessment (95%/99.7% containment areas) and b) only valid if the arc is on initial approach.