Advanced RNP to xLS Approaches with Optimized Speed Profiles and Automatic Landings: Flight Testing Procedures with an A320

Abstract

We report on the performance of our A320 during novel advanced required navigation performance (RNP) procedures which contain a fixed radius turn that delivers the aircraft onto a short ILS precision final. The approaches were flown automatically with guidance and autothrust as computed by the flight management system. Main areas of interest of these flight trials were the impact of the use of the described ARINC424 coding options onto (a) the performance of the speed profile for arrival time optimization (b) the vertical path during the RNP part of the procedure and (c) the performance of the autoland capability after a curved transition onto an ILS final. Most recently, the aviation community recognized the potential for exploiting additional benefits from precise area navigations systems and introduced Performance Based Navigation (PBN) through ICAO Doc 9613 for new kinds of en-route, departure and approach procedures. RNP is part of the PBN concept. Within the PBN concept exists the possibility to incorporate turns with a precise ground track. These turns are called fixed radius transitions and are coded as radius-to-fix (RF) path terminators in the ARINC 424 database. They offer the advantage of repeatable ground tracks during the turn and thus more freedom for the procedure designer when route planning in dense traffic, high terrain or environments witch obstacles. Moreover, socio-economic factors can be included - such as circumnavigating noise sensitive areas with guaranteed track keeping performance precluding stray aircraft. Additionally, ARINC 424 allows specifying altitude constraints at waypoints and vertical path angles for each RNP segment terminating at such a waypoint. Whilst offering these benefits, such advanced RNP approach operations are still non-precision procedures and automatic landings cannot be performed after their successful completion. Hence, to enable automatic landings and to extract maximum benefits from RNP operations, they must transition into a precision final approach segment provided by any precision landing system (ILS, GLS, MLS). This is often called RNP to xLS (or RNP2xLS). Moreover, the vertical path angle feature is currently largely unused and unexplored, except for the final approach segment of an RNP approach. The approaches were entirely flown using the auto flight guidance in managed mode and with auto-thrust activated. We show supporting evidence that RNP2ILS approaches can be safely flown all the way to an automatic landing using the flight management guidance computer and the auto flight control system. In order to fly the desired path with vertical path angle during the RNP initial and intermediate approach, a separate mode (such as LNAV/VNAV) different from the singular approach mode would need to be implemented in the aircraft. Additionally, operators currently apply stabilization criteria following which the aircraft must be established on a straight final with the correct sink rate, fully configured at 1000ft above aerodrome level. For landings in low visibility conditions, more stringent criteria are often applied. An operational implementation of RNP2ILS approaches with a curved final intercept would require a rephrasing of the criteria to include a concept, such as RNP established.