ADVANCED RNP TO ILS AUTOLAND APPROACHES FOR OPTIMAL BENEFITS FROM PBN: FLIGHT TESTING PROCEDURES WITH AN A320

摘要

We report on the the performance of our Airbus 320 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 three main areas of interest of the flight trials were the performance of the autoland capability, vertical path following during the RNP part of the procedure and lateral path following during the transition from RNP to localizer guidance. Today, precise area navigation systems have become more common in aircraft ranging from a small single engine piston airplane through helicopters and large jet transports. If these systems also provide continuous monitoring and display of the navigation accuracy they can qualify for operations under the required navigation performance (RNP) concept. Most recently, the aviation community recognized the potential for exploiting additional benefits from these systems and introduced the Performance Based Navigation (PBN) foundation through ICAO Doc 9613 [14] 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 into departure, en-route, arrival and approach procedures. These turns are called fixed radius transitions and are coded as radius-to-fix (RF) path terminators in the ARINC 424 standard 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 obstacle rich environments.Moreover, socio-economic factors can be included - such as circumnavigating noise sensitive areas with guaranteed track keeping performance precluding stray aircraft. 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 the Instrument Landing System (ILS) so that the guidance loops for flare and land modes of the auto flight guidance system can activate. This is often called RNP to ILS (or RNP2ILS). Naturally, the same considerations would apply to the GPS Landing System as well. Since traditional operations involving autoland are straight-in approaches, the behavior of the auto flight control system during maneuvers that involve a curved intermediate approach segment terminating at the final approach fix (FAF) is not known. In this study we investigated (a) the performance of the autoland capability (b) the vertical path during the entire approach and (c) the lateral path following during localizer capture.For the trials, we designed five instrument approaches to Braunschweig-Wolfsburg airport during which a RF curve terminates at the ILS intercept point at different heights.Each approach starts at an designated initial approach fix with a straight segment. The straight segment is followed by a radius-to-fix curve ending at the final approach fix, where the aircraft is fully established and centered on the localizer and glidepath of the ILS. We constructed the procedure such that the altitude constraints at initial, intermediate and final approach fix describe a continuous vertical path with minus two degree inclination.The chosen heights for the final approach fix were 550ft, 750ft, 1000ft, 1500ft and 2000ft and the approach names are ILS x, where x in {S,T,U,V,W}, respectively. The ILS at Braunschweig-Wolfsburg airport has a standard glide path angle of 3 degrees so that the aircraft intercepts the ILS glide path from below. Each approach had two different initial approach fixes which corresponded to a track angle change of 90 degrees and 180 degrees during the constant radius turn-to-final. Additionally, when beginning the approach from the 90 degree track offset, a 2 degree vertical path angle was included in the ARINC 424 code of the initial and intermediate approach segments.The procedure coding was conformal to the latest issue of ICAO PANS OPS [15], [16], but is not yet supported by the newest ARINC 424 [6] database standard and the packing software used by the database suppliers. Therefore it was necessary to designate the FAF as final approach course fix (FACF) and to insert a artificial and unused final approach fix on the extended runway centerline further downstream in the approach.For the trials, we used DLR's own Advanced Technology Research Aircraft (ATRA), an Airbus A320 MSN659 with flight test instrumentation and a FMS 2. The approaches were entirely flown using the auto flight guidance in managed mode and with auto-thrust activated. The approach mode was armed either at the FAF or before the initial approach fix.During no approach did the aircraft respect the intended initial vertical path of a continuous descent at 2 degrees downwards, but performed a descent with thrust at idle until reaching the next altitude constraint ("dive and drive"). This behavior repeated itself during all RNP segments before the ILS final. When the approach mode button was depressed at the FAF, the aircraft respected the entire lateral path guidance of the RNP part with a lateral total system error of less than 20m. When the approach mode was armed earlier, the autopilot established the aircraft on an intercept heading as soon as the course deviation indicator became alive. This resulted in a dogleg and a violation of the RNP corridor. When the approach mode button was depressed at the FAF, the intercept mode caused a slight and short oscillation with a maximum amplitude of 2 degrees about the vertical axis before full capture was indicated in the primary flight display. Finally, automatic landing was possible from all heights,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, airlines and other operators currently apply stabilization criteria following which the aircraft must be established on a straight final with the correct sink rate at 1000ft above aerodrome level in order to continue the approach. 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.