Design and Testing of RNP AR to SBAS LPV approaches into Salzburg Airport

Kurzfassung

We designed and tested a Required Navigation Performance (RNP) Authorization Required (AR) to Localizer Performance with Vertical guidance (LPV) supported by the space-based augmentation system of GPS approach for runway 33 of Salzburg Airport, Austria. Approach to landing on runway 33 is severely impacted by mountainous terrain to the south of the airport that lies within the runway extended centerline, endering all straight-in approaches but those based on RNP AR impossible. That is due to the high navigation accuracy available under RNP AR. It makes it possible to keep the obstacle protection areas, which are constructed around the approach path andmust be clear of obstacles to ensure the required obstacle clearance, very small so that they do not extend into the terrain. The combination of RNP AR and LPV, in turn, makes it possible to use the more precise angular guidance for the final approach available under LPV while still being able to exploit the significantly smaller RNP AR protection areas in all approach segments. In Salzburg, this enables us to reduce the minima from 368 ft to 218 ft above runway threshold level while at the same time establishing a longer final approach segment compared to existing RNP AR approaches because critical terrain and obstacles now fall outside of the protection areas. For the intermediate approach, we use curved legs based on a constant radius between two fixes (Radius to Fix legs) that lead onto the final approach course, followed by a horizontal segment to account for possible altitude differences when changing from barometric to geometric LPV altitude. The length of this segment is minimized based on current procedure design rules for standard LPV approaches, which are also used as a basis for the merging of the different protection areas. The approach is coded as advanced RNP with 0.1 nautical mile values for the 95-percent lateral accuracy (RNP 0.1) on each sequence as RNP AR can otherwise not be combined with LPV under the database coding standard ARINC 424. Level D full flight simulator tests with an Airbus A350 showed that this coding is sufficient to achieve RNP 0.1 performance under all permitted conditions with the approach being presented to the pilots as RNP AR. However, further research must go into the flyability of approach at very high temperatures, where the LPV glide path was missed due to configuration errors made by the pilots and the approach could not be repeated due to time constraints.