Originally Posted by
TeeS
I'm not sure whether the attached image will be of help or interest for anyone. I've overlaid a construction of the protection areas for the NDB approach to Runway 27 at Gloucester EGBJ.
At the GST NDB on the airfield, a semi-width of 1.25 NM (it would be 1.0 NM for a VOR) extends out each side of the approach centreline; the inner half of each semi-width (shown in orange) defines the start of the primary protection area, the outer half of each semi-width (Shown in yellow) defines the start of the secondary protection area.
Each secondary area outer edge then diverges from the centreline at an angle of 10.3° (it would be 7.8° for a VOR) . At the same time the primary areas diverge so that the width of the primary and secondary areas, when measured perpendicular to the centerline, remain equal.
Within the primary area, bounded by the orange lines, a minimum obstacle clearance (MOC) is maintained at 300 m (984 ft) until the IF at 8 DME where the MOC reduces to 150 m (492 ft) until the FAF where the MOC reduces to 76 m (246 ft). In the secondary area, the MOC starts at the same value as the adjacent primary area and reduces linearly to zero at the outer boundary of the secondary area.
Cheers
TeeS
The answer you’re looking for is contained within PANS OPS. (CAP 5862 I think, can’t remember!)