Chicago’s O’Hare airport recently implemented its first RNP (required navigation performance) instrument approach. The satellite-based precision approach is part of the new generation of GPS approaches popping up at airports around the nation and the world. Chris Baur, president and CEO of Hughes Aerospace, the company that developed the approach, told Examiner on Wednesday that the new approach is an environmentally friendly alternative to current land-based approaches.
Instrument approaches are the procedures used by pilots to locate the landing runway when the weather precludes a visual approach. At large airports like O’Hare (KORD), the typical method of tracking to the runway involves an ILS (instrument landing system) approach. ILS approaches use land-based transmitters to send signals to the pilots that bring the airplanes to the touchdown point on the runway both laterally (with a localizer) and vertically (with a glideslope). This requires two separate transmitter facilities for each runway that utilizes an ILS. This can be cost-prohibitive for small airports.
RNP approaches utilize satellite navigation technology instead of ground-based navigational facilities. The aircraft’s flight management computer is programmed to fly a predetermined course along a series of waypoints, locations on a map that do not have to coincide with any geographic feature, toward the runway. Because the airplane is following GPS waypoints, approaches can easily be planned to avoid terrain features or noise sensitive areas. They can also be developed over water where land-based facilities are impractical. At some airports, RNP approaches even follow a curved path to the runway using RF (radius to fix) segments, such as this approach at Atlanta’s Peachtree-DeKalb airport (KPDK).
The RNP, required navigation performance of the approach, determines the accuracy of the approach and has an effect on the minimums, how low the pilot can go without seeing the runway. The required navigation performance for approaches can be as low as 0.1. This means that an airplane’s navigational system must be accurate to with 0.1 nautical mile radius 95 percent of the time. This is referred to as performance-based navigation (PBN).
Baur says the new approach has several advantages over traditional ILS approaches. The RNP approach uses GPS satellites so there are no “costly ground based Infrastructure, architectural weakness and repetitive flight inspections” that are required of traditional ILS facilities. This can reduce costs for maintaining the approach.
Ground-based facilities are also subject to interference from local weather conditions. Baur notes that snow from the infamous Chicago winters can accumulate on antennas and cause a degradation of the navigation signals just when accuracy is needed most. “In the event of the loss or degradation of the ILS, traffic flows and arrival rates can be maintained” with RNP approaches, Baur says. This can translate into fewer delayed or canceled flights for airline passengers. Baur also points out that, because RNP approaches are designed with the stabilized approach concept in mind, they will likely result in fewer missed approaches or “go-arounds.”
“Collectively this reduces the overall environmental impact to the airport and surrounding community,” Baur says.
There are some disadvantages to RNP approaches as well. RNP approaches require special pilot and aircraft certification. Baur notes that many, but not all, modern airliners are equipped with avionics that are capable of RNP approaches. Many airlines have already incorporated RNP approaches into their training programs. Many, if not most, private airplanes are not equipped to fly RNP approaches. This means that smaller aircraft flying into smaller, rural airports cannot benefit from RNP approaches where their value could be greatest.
Further, RNP approaches are nearly, but not quite, as accurate as an ILS. An ILS typically has minimums of 200 feet and ½ mile visibility, but can go even lower in some cases. This means that the airplane can descend as low as 200 feet above the ground without making visual contact with the runway. The RNP approach at O’Hare has minimums of 330 feet and ¾ mile visibility. In most cases, the higher minimum will not make a difference, but in a snowstorm or fog, the extra 130 feet might make the difference between landing and going around.
In spite of the drawbacks, Baur feels that RNP and PBN are the future of aviation. “Today most of the major US Airlines and many of the foreign airlines have received authorization for RNP AR [arrival] procedures, as well as many corporate operators,” he says. “I feel RNAV (RNP) as well as RNP AR will continue to grow and provide benefits. The number of participating operators will also increase with improved access through avionics.”
Hughes Aerospace has already developed RNP approaches in numerous countries around the world. Baur notes that the approaches would be valuable to airports that are concerned with terrain, noise abatement, airspace restrictions or maintaining arrival rates in the event of a disruption of ground-based facilities. They are also an alternative to the high cost of traditional ground-based approach facilities. As technology improves as entry costs for RNP-capable avionics improve, RNP approaches will become more and more common around the world.
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