High-end RNP: flying through a soda straw

By Charlotte Adams

High-end required navigation performance (RNP) is coming into its own. Many of today's commercial and business jets are equipped to fly these ultra-narrow flight paths and many operators are jumping on board. High-end RNP -- known as RNP SAAAR (special aircraft and aircrew authorization required) or RNP AR (authorization required) -- is popular with the airlines.

In addition to pioneers like Alaska Airlines and Canada's WestJet, Delta Air Lines is a major user, and Southwest began RNP operations this year. Other SAAAR operators include Qantas, American, Continental, Air New Zealand, and JetBlue. RNP SAAAR approach procedures are particularly popular. The Federal Aviation Administration (FAA) points to 238 RNP SAAAR approaches in the U.S., as of Dec. 16, 2010.

The approaches feature path widths as low as 0.2 nautical miles -- 2x the lowest RNP 0.1 value -- with vertical guidance. To fly RNP 0.1 procedures, the aircraft must be equipped, the crew trained, and the carrier approved to keep to the desired track within 0.1 nautical miles of the centerline 95 percent of the time. RNP procedures require not only the avionics to stay on the desired flight path, but onboard monitoring and alerting if actual navigation performance (ANP) should exceed RNP.

Compared with traditional non-precision approaches (NPAs), RNP SAAAR offers increased safety, lower minimums -- and the increased probability of making destinations in adverse weather conditions -- more direct routing, shortened approach paths, reduced pilot workload, and even greater passenger comfort.RNP SAAAR also allows curved segments inside the final approach fix and guided missed approaches. Within the "vertical error budget," the aircraft's barometric altimeter, together with the flight management system (FMS), creates the glide path angle to the runway threshold crossing height, assuming the pilot was able to see the runway at the decision altitude.In worst-case scenarios, RNP procedures and avionics can increase safety on missed approaches. If an aircraft coming in to land in a valley, for example, gets down to minimums and can't see the runway, and then loses GPS, causing it to make a missed approach, the aircraft still has inertial guidance to make a safe exit, explains Chad Cundiff, vice president of crew interface products at Honeywell Aerospace in Phoenix. Honeywell has developed an FAA-approved RNP consultancy that helps business aviation clients obtain approval for SAAAR procedures.Operators also find RNP useful in avoiding noise-sensitive areas and achieving "precision-like" approaches at non-instrument landing system (ILS) airports. WestJet, for example, uses an RNP approach procedure into Palm Springs International Airport, Calif., which lacks ILS. Unlike ILS, RNP SAAAR is not a precision approach because it relies on internal avionics capabilities, such as temperature-corrected barometric altimeter data, to generate the vertical path to the runway. ILS, by contrast, uses a completely independent, ground-based system which is unaffected by temperature.According to Advisory Circular AC 90-101 on SAAAR approval guidance, aircraft typically must have at least dual global navigation satellite system sensors, dual FMS, dual air data systems, dual autopilots, and a single inertial reference unit. Down in Appendix 2, the AC also notes that aircraft must display barometric altitude from two independent altimetry sources, one in each pilot's primary optimum field of view. SAAAR approaches assume barometric-vertical navigation (baro-VNAV) capability.Airlines

Alaska Airlines started RNP work with its approach to Juneau (JNU) via the Gastineau Channel -- RNP 0.15 at its lowest value. The driver was increasing access to an airport surrounded by terrain and plagued by poor weather conditions. Before the

approach was developed nearly 10 percent of the carrier's flights into JNU were canceled or diverted, recalls Sarah Dalton, Alaska's director of airspace and technology.WestJet in Canada initially adopted RNP in order to enhance safety on approaches into difficult terrain by reducing exposure to traditional "dive-and-drive" NPAs. It is the first Canadian carrier to achieve approval for RNP 0.1 approaches. WestJet has been approved to fly down to RNP 0.1 in Canada and the U.S.WestJet dividends

"We were trying to reduce the number of traditional non-precision approaches we were exposed to, and RNP fit that bill perfectly," says Darcy Granley, director of flight technical operations for WestJet. NPAs in mountainous terrain can require maneuvers such as circling (within a valley) to land. High-end RNP design criteria involve narrow, linear, lateral protection areas, and FMS-based flight path guidance. This, plus the ability to add curved segments -- known as radius-to-fix, or RF legs -- enables an aircraft to snake around obstacles rather than overfly them. Minimums can be much lower than those for traditional NPAs and flight paths can be safely shortened, reducing fuel burn. WestJet can save about 80 pounds of fuel per arrival for straight-in RNP vs. conventional "dive-and-drive" approaches, where the aircraft constantly powers up and powers down as it comes down to the runway. On some approaches the airline can save almost 50 track miles.RNP allows the Canadian carrier to lift more weight into terrain-rich environments like Kelowna, British Columbia, Granley says. This RNP 0.1 approach involves a number of RF turns through the valley to avoid terrain. RNP also allows WestJet to build

departure profiles that allow a safe exit from a terrain-challenged airport even on a single engine. "You can work your way through the valleys...so you're not restricted by obstacles," he explains.WestJet's B737NGs currently fly 82 RNP procedures -- 78 arrivals and four departures. Carriers tend to have more RNP arrivals than departures. Arrivals are typically more complicated than departures, giving airlines more opportunities to reduce operational risk and increase savings.WestJet has worked with Naverus in Kent, Wash., on designing its RNP procedures under approval from Transport Canada. The carrier continues to collaborate with Naverus to optimize its RNP procedures. Naverus has added curved segments to some approach procedures so that the aircraft can set up in line with the runway closer in than before. This shortened transition cuts by five to 10 miles, on average, off the approach.Down in the U.S. carriers would like to get more bang for their RNP buck in busier terminal airspace. According to one expert, if RNP SAAAR is good enough to get to terrain-challenged airports like Vail, Colo., airlines ought to be able to use RNP precision to increase capacity at other U.S. airports. Airlines want to exploit RNP SAAAR's tight lateral and vertical containment to be separated from adjacent traffic and fly much shorter final approaches.Carriers, in particular, want to be able to fly RNP approaches alongside straight-in, ILS approaches onto parallel runways. The RNP airplane would be able to make the turn onto its final approach precisely enough to keep it at a safe distance from the ILS airplane heading onto the parallel runway. Known as RPAT (RNP parallel approach transition), this concept requires the RNP aircraft to visually acquire the ILS aircraft before crossing the final approach fix. Alaska advantage
Alaska Airlines calculates RNP benefits in terms of approach and departure "saves," Dalton explains. A save is a flight that, without RNP and its lower minimums, would not have gotten into or out of an airport. In 2010 Alaska enjoyed more than 1,500 RNP saves, the vast majority of which were due to procedures the airline developed itself. Dalton says Alaska is the first airline to be 100 percent RNP-equipped. Alaska flew its first RNP procedure in revenue service in 1996, before there were procedure development criteria or third parties to help out. Today the carrier has the authority to conduct obstacle assessments and flight validation for its own procedures. The FAA participates in the flights, however, and Alaska submits its procedures to the agency for approval. Alaska flies proprietary procedures into 11 airports, Dalton says. The airline has 22 special approaches and 15 special departures. The remaining procedures are FAA public ones. Last year the airline flew nearly 12,000 RNP approaches and departures, and only 48 of the approaches used FAA-developed procedures. This month the carrier announced that it has developed new RNP approach procedures into Adak and Ketchikan, Alaska.