As part of SESAR, the participating institutions and companies have shown that RNP-based procedures are suited to shortening the approach routes for ordinary approaches by instrument landing system (ILS) and to being more flexible in their planning by using curved sections. In RNP2ILS the Institute of Flight Guidance is concerned with testing "RNP to ILS" approaches.
RNP-based procedures are different in that aircraft are not obliged to follow fixed beacons on the ground but plot their route using waypoints which they choose for themselves. The aircraft positioning necessary to achieve this requires a particular level of navigation performance, established as the Required Navigation Performance (RNP).
The research project RNP2ILS investigates the possibility of carrying out an automated landing following an approach of this type in order to gain further advantages with regard to the availability and reliability of such procedures. To this end, suitable test flights were carried out with the DLR testbed A320-ATRA in July 2015 in which the ATRA flew out of a curve at low altitude onto the instrument landing system's approach path. The objective of these experiments was to reduce the straight final approach required to a minimum. This procedure is intended not only to allow aircraft to fly around areas dense with obstacles or areas sensitive to noise, but also to perform an automatic landing.
The focus was on two areas in particular during the test flights. On the one hand it was about investigating the autoland capacity for maintaining the vertical path during the RNP part of the procedure. On the other, lateral deviations from the intended path during the transition from RNP to instrument landing system were investigated.
Because traditional ILS approaches always guide the aircraft straight onto the ILS approach path, relatively little is known about the behaviour of the on-board computers and autopilots when they fly onto these landing systems out of a curve. For the study, DLR developed five instrument approaches to Braunschweig-Wolfsburg airport in which the curve ended on the ILS approach path at a different altitude in each case. The results show that an automatic landing was possible from all flight altitudes.
The results of the test flights have exceeded expectations from previous simulator test. Automatic landing was possible with no limitations even when the transition to the ILS took place at only 550 feet (170 m) above ground. Thus RNP to ILS approaches represent a promising possibility for the creation of more efficient and more flexible landing procedures.