The origin of the info: ICAO Doc 8168 PANS-OPS Sect 4 Ch5
5.5.5.2 The protection area assumes that the pilot does not normally deviate from the centre line more than halfscale deflection after being established on track. Thereafter the aircraft should adhere to the on-course, on-glide path/elevation angle position since a more than half course sector deflection or a more than half course fly-up deflection combined with other allowable system tolerances could place the aircraft in the vicinity of the edge or bottom of the protected airspace where loss of protection from obstacles can occur.
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