Right Approach

Airlines and aviation authorities in the region are beginning to investigate the advantages offered by Required Navigation Performance (RNP) approaches.

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By  Neil Denslow Published  July 4, 2005

|~|qr_takeoff_m.jpg|~||~|Required Navigation Performance (RNP) approaches potentially offer a host of advantages to both airlines and governments in the region. The system promises both higher levels of safety and lower fuel usage, as well as allowing for the more efficient usage of crowded airspace. Some carriers in North America are already making use of the technology, and there is now emerging interest in the Middle East and North Africa region as well. Aircraft making conventional approaches rely upon ground-based radio sensors to give them a positional fix, both horizontally and vertically. However, such systems have only limited accuracy, which approach designs need to take into consideration even for Area Navigation (RNAV) ‘precision approaches.’ The necessary margin of error is allowed for through the design of Terminal Instrument Procedures (TERPS) planes — essentially 2 km wide approach tubes that the aircraft can fly down. The design also includes a missed approach surface that planes can fly out on from the decision altitude in the event of an aborted attempt at landing. If the runway is in a wide open area, then TERPS can provide a direct approach straight onto the end of the runway. However, if there is an obstacle in the way, such as a mountain or a restricted noise area, then the TERPS plane needs to be shifted so that the aircraft has a safe channel to fly in. This though, means that the plane needs to approach the runway at an angle, which adds complications to the landing procedure. Furthermore, in a mountainous area, for instance, it may be impossible to draw up a TERPS corridor. For these runways, pilots have to rely on non-precision approaches, which only provide guidance down to a minimum descent altitude. This presents a number of problems, including the fact that if the landing is aborted below the minimum descent altitude, then TERPS provides no extraction surface for the plane to leave on. Also, if an instrument approach is not available then pilots have to undertake a complex circling procedure instead. This all increases the risks on landing, particularly in low visibility, and, as such, non-precision approaches have a worst safety record than precision approaches. For instance, according to the Flight Safety Foundation, 60% of all controlled flight into terrain (CFIT) incidents occur during non-precision landings. Furthermore, the risk of a crash during such a landing is five times greater than the risk during a precision approach. TERPS is still used for approaches at almost all airports around the world, including in this region, as it provides guidance for all aircraft types. However, most newer planes, including all Boeing and Airbus aircraft made since around 1995, are technologically capable of using a safer and more efficient approach method called RNP for landings instead. RNP-capable aircraft are kitted out with GPS receivers and have enhanced software in the dual flight management systems (FMS) that monitors sensor inputs and the actual navigation in realtime. This combination then allows the aircraft to much more accurately position itself in the sky than when using RNAV. For instance, RNP .11, which is widely used by Alaska Airlines in the US, ensures that the aircraft stays within 0.11 nautical miles (177 m) left or right of the centreline 95% of the time, and within 0.22 nautical miles (354 m) 99.99999% of the time. All new airliners meet at least RNP .3, which allows planes to track within 0.6 nautical miles (966 m) left or right of the centreline, 99.99999% of the time. ||**|||~||~||~|For pilots, the advantages of RNP approaches are numerous, starting with the fact that they are simpler and more consistent to land. This is both because of the more accurate knowledge that the pilot has about the position of the plane, and because the same approach can be used in all conditions. This removes complexity for the pilots, which shows up in a number of ways. For instance, the amount of radio traffic in RNP-enabled airports has been shown to fall by as much as 75%. RNP approaches are also safer than non-precision ones because they provide vertical and lateral guidance down to a much lower level. The decision altitude for aborting a landing can therefore be cut to as low as 250 feet, which gives the pilot more time to safely adjust their approach. This improves safety levels and also means there are fewer aborted landings, as the pilot does not have to be as conservative. A further benefit of RNP is the ability to safely fly round obstacles on approach to landing. If there is a residential area or hill in the ideal approach path, for instance, RNP allows the aircraft to make a fixed radius turn around the obstacle rather than using a TERPS approach, which would force it to come in to land from a completely different direction. This greater precision can have a dramatic effect on journey times. WestJet in Canada, for instance, which has implemented RNP at many of its airports, managed to shave 16 miles off its 45 minute flight between Calgary and Kelowna. With each mile in the air costing around US $33 to fly, RNP will clearly generate huge cost savings for the airline over the course of time. Further cost savings can also be generated through reduced pilot training. Traditionally, pilots have had to be trained on an alphabet spaghetti of different landing types, such as ILS, VOR, NDB-DME and VOR-DME to name just a small sample. The implementation of RNAV cuts this list down to only a few types, but if an airline rolls out RNP approaches across its network, then pilots only need to be trained on this type. This then enables the carrier either to reduce the amount of training the pilots receive, as WestJet has done, for instance, or to train its pilots more thoroughly in the same amount of time, as less needs to be covered. This was the approach adopted by US Airways and Alaska Airlines. For governments, one of the major benefits of RNP, along with the enhanced safety levels, is that it allows airspace to be used much more efficiently. Because ATC has a much more accurate awareness of where each plane is, more aircraft can be safely handled in the same amount of space. This will be particularly advantageous in the Gulf, where an increasingly large number of flights are using numerous airports, and thereby causing congestion in the skies. Also, because RNP is reliant on satellites rather than ground-based radios, it is much less likely to be interfered with. This was particularly apparent after last year’s tsunami, when a number of runways were shut down because the radio sensors were destroyed or swept away. “If we had had RNAV and GPS at these airports, the opportunity to continue operations [after the tsunami] would have been much better,” recalls Capt. Mike O’Grady, vice president, flight operations performance, Emirates. Given these advantages, RNP is attracting strong interest around the world. A recent seminar in Dubai hosted by Emirates on the subject, for instance, had a high turnout from airlines and aviation authorities from across the region. However, while the benefits of RNP are clear, the challenges of implementing it are equally apparent, as a host of factors have to be prepared before it can be rolled out. “There are a lot of pieces that need to be put in place in order to have an operation,” says Dr. Kathy Abbott, chief scientific & technical advisor, flight deck human factors, FAA. “We have had the aircraft capability for quite a while… but what we have not had is the procedure design criteria, we have not had the airspace set-up to accommodate it, we have not had the crew training [and] we have not had the public access to all of the different pieces that have to come together,” she explains. Up to now, RNP has only been implemented where a particular airline, such as WestJet or Alaska, has driven the process and done the work itself or subcontracted to a third party, such as Naverus. However, in the USA, the FAA is starting to approve public approaches that can be used by all aircraft, beginning with an approach to Reagan Washington National Airport. Reagan is clearly not in a mountainous area, but the numerous restricted flying areas in the US capital, especially since 9/11, make it a complex approach. Within North America, the adoption of RNP is still far from universal, and within the Middle East it is even more limited. The vast majority of Emirates’ fleet, for instance, is RNP-capable, but it currently does not fly a single RNP approach because of the lack of procedures in place. However, this presents the region with an opportunity, as airlines and aviation authorities here are able to work together on designing public RNP approaches from day one, rather than duplicating work on separate private designs. This would then cut the development costs of such projects and also speed the implementation of the technology. “In order for RNP to work in our network we need our regional regulators to work with us,” says O’Grady. The high attendance at the Emirates’ recent RNP seminar in Dubai clearly showed that these regulators are beginning this process, but there is a long way to go before it is complete. The complexity involved in designing RNP approaches will certainly ensure that it does not happen overnight, but once it does there will be immediate benefits to all users of the airspace in terms of more efficiency, greater safety levels and simplified operations for pilots. However, this simplicity depends on a lot of complexity behind the scenes.||**||

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