Performance-based navigation allows implementingapproach procedures in areas, where conventionalprocedures are not a feasible solution. This holdsespecially true for difficult topographic terrain. In case ofthe Meiringen Air Force base in Switzerland, which islocated in a valley surrounded by mountains up to10'000 ft, a cloud break procedure for rotary wingoperations has been developed. This procedure will allowthe Swiss Air Force and the local HEMS (HelicopterEmergency Medical Services) operator to fly underadverse meteorological conditions. Due to topographicconstraints a required navigation performance (RNP)value of 0.3 NM is envisaged for the whole approach,starting at the initial approach fix until the end of themissed approach.One of the most critical points is the loss of GNSSnavigation capability combined with a one-engine-outsituation. In the first part of this analysis, the approachavailability for a 30-satellites GPS constellation has beendetermined. A GNSS simulator has been used to assessdifferent avionics GPS receivers envisaged for thisapproach.Based on the mean time between outages (MTBO) ofGPS satellites, the probability of a satellite outage, andthe effect thereof, has been determined. The discontinuityprobability of the received GNSS signals have beenconsidered together with other main technical errorsources such as malfunction of navigation equipment andengine problems. Furthermore, statistics on meteorologicalconditions in the area of Meiringen have been takeninto account.A failure model has been used to determine theprobability of the hazardous situations for each helicoptertype individually. The resulted probability has then beencompared with the required target level of safety. For allhelicopter types, the achieved probability values are betterthan the required TLS, meaning that the minimum level ofsafety can be maintained.
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