sentences:  Where buildings may (under certain circumstances) be permitted to penetrate the OLS, they cannot be permitted to penetrate any PANS-OPS surface, because the purpose of these surfaces is to guarantee pilots operating under IMC a descent path free of obstacles for a given approach.In 1989 the very first commercially available off the shelf instrument procedure design software conforming to ICAO document 8168 Pans-Ops was programmed.This software was first demonstrated at Bailbrook College in Bath, England in 1992 to a procedure design course.