ATM Modernization, Business & GA, Commercial, Embedded Avionics

Preparing For PBN

By By Frances Fiorino | February 1, 2011
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Airspace modernization is an enormously complex task. But aviation industry stakeholders are seeking safe passage through “NowGen” the period when performance-based navigation technologies are emerging to the reality of NextGen operations by 2025.

Performance-Based Navigation (PBN) is the umbrella term for various types of navigation procedures to be used in NextGen. PBN equipment allows an aircraft to fly precisely on a desired flight path within coverage of ground- or space-based navaids or with onboard aircraft systems, all of which leads to increased traffic capacity as well as reduced delays, fuel bills, emissions and noise.

There are two categories of PBN procedures, Area Navigation (RNAV) and Required Navigation Performance (RNP), each requiring increasing levels of navigation performance. RNAV is the broadest category and requires the lowest performance levels. RNAV-capable aircraft fly point-to-point using ground- or space-based navaids.

US Airways Director of Technologies Ron Thomas said his airline’s initial RNP goal is to build a foundation that enables continual improvements over time.

“Like many other operators, US Airways says, ‘Don’t focus on NextGen yet. Give us NowGen,’” Thomas said. “Give us the Required Navigation Procedures Authorization. Give us the optimum lateral and vertical profile arrivals and departures. … Then let’s start talking about NextGen, when we are utilizing the airplane’s capabilities available today.”

Under RNP, the simplest procedure is RNP APCH (approach), formerly referred to as GPS/GNSS RNAV approaches, designed for use to runways that do not have adequate ground-based navigation facilities, said Steve Fulton, chief technical officer with GE Aviation PBN Services (formerly Naverus). In 2007, ICAO passed a resolution calling for member states to deploy this procedure to all instrument runway ends by the end of 2016.

At the top of the RNP pyramid is RNP AR (Authorization Required), which requires the highest level of navigation performance, what Airbus Technical Marketing Manager Rafael Sheffield calls “the full package.” (Until early 2011, the United States will refer to it as RNP SAAAR, Special Aircraft and Aircrew Authorization Required).

A value of “RNP 0.3” means the aircraft is capable of remaining within 0.3 nautical miles to the right or left of center line 95 percent of the time within a defined containment area; RNP 0.1 means an aircraft must remain within 0.1 nm. The RNP AR-capable aircraft must have the ability to monitor navigation accuracy and alert the pilot to any deviation from required performance. This enhanced mode of PBN ensures an aircraft can fly predetermined, precise flight paths, shorter tracks, in congested airspace and in difficult terrain.

RNP AR procedures rely on a combination of GPS navigation and the aircraft’s flight management computer. And their defining characteristic is continuous descending turning trajectory, radius to fix (RF) turns in the final segment.

Four-dimensional trajectory based operations (4-DTBO) are at the core of NextGen and Europe’s Single European Sky ATM Research (SESAR) program, said Fulton. “Defined trajectories enable us to predict time specified on a known path. And when we can resolve for time, we can then begin to sequence and merge traffic,” he said.

As of December, according to FAA, among air carriers approved to conduct RNP AR operations, about 1,000 aircraft had been qualified for RNP approaches, and that number was expected to grow to 2,000 by spring 2011. In addition, 238 RNP SAAAR/AR approaches were in place in the United States as well as 4,925 non-SAAR/AR RNP approaches with LNAV minima.

Regulatory Approval

An operator wishing to qualify its fleet and flight crews for PBN operations will find guidance in the ICAO PBN Manual, as well as FAA Advisory Circular 90-101, Approval Guidance for RNP Procedures with SAAAR (both of which are to be revised by fall 2011) and the European Aviation Safety Agency (EASA) equivalent, AMC 20-26.

The operator must take a “three-legged stool” approach to gain regulatory approval for PBN operations: equipping the aircraft, developing the procedures and meeting operational/training requirements, said Mark Steinbicker, manager of FAA’s Performance Based Flight Systems Branch, Flight Standards division.

These steps must be accomplished simultaneously. “An operator cannot say, ‘I’ll certify the aircraft, then design the procedure, then go to the regulatory authority,’” Sheffield advised.

Aircraft must be equipped with appropriate navigation equipment to handle three basic RNP AR capabilities, according to GE Aviation PBN Services:

➤ Ascertain the aircraft’s position in space through GPS and other navigation receivers, Inertial Reference System and barometric altitude sensing equipment.

➤ Create a navigation path/trajectory through space, provided by the FMS, which is capable of storing thousands of trajectories.

➤ Fly a prescribed RNP path. Equipment such as autopilot/auto throttle and display systems must be able to depict for the flight crew the path and the aircraft’s progression along that path, and alert the flight crew of any fault in the system.

RNP capability is basic to all new aircraft coming off the production line, according to Boeing and Airbus. Manufacturers offer additional features and options for older aircraft, depending on an airline’s RNP requirement, said Gary Limesand, program manager for Avionics Systems, Boeing Modification Services. For example, flight control computer software updates could provide additional functionality not inherent in older airplanes, such as Take Off/Go Around (TOGA) to LNAV, which allows an aircraft to remain at the required navigation mode during the go-around.

Boeing Business Development/Modification Services Senior Manager Rudy Bracho said Boeing strives to make certain older aircraft are able to upgrade to the capability that is delivered in production aircraft. “We continue to invest heavily in developing RNP-related solutions for out-of-production aircraft so they can operate like modern production aircraft,” Bracho said, calling this a “no-aircraft-left-behind” approach.

Down time for installation of equipment and software updates varies greatly depending on aircraft age and the operator’s desired configuration. A software load on a Boeing 777 can be done in an overnight turn, said Limesand, but a 737 Classic that might require hardware and wiring updates would require an extended time period.

Upgrade costs also vary greatly, depending on requirements. Airbus’s Sheffield estimates upgrade costs for a 15-year-old aircraft, for example, can be “a fairly expensive exercise, perhaps as high as $1 million if the aircraft would require a multimode receiver, GPS receiver or has an older FMS1 box.” In 2009, Southwest Airlines committed to spending at least $175 million to make its 500-aircraft fleet of Boeing 737s RNP-capable.

An airline seeking to operate a specific aircraft model to a specific airport would seek the design of what is called a “special” or “tailor-made” RNP AR procedure from a regulatory authority. FAA designs mainly “public use” or “non-special” PBN procedures, which have lower navigation performance requirements and are intended for use by all aircraft equipped to meet those requirements.

Operators may also approach a RNP consultant firm. The latter include Airbus subsidiary Quovadis or FAA-approved RNP consultants GE PBN Services, Jeppesen and Honeywell Go Direct Services.

Another option is for an airline to design the procedures. US Airways is developing RNP procedures in-house, using TARGETS (Terminal Area Route Generation Evaluation and Traffic Simulation) software, the same system FAA uses to design terminal airspace procedures, Thomas said. The airline is expecting RNAV RNP AR authorization from FAA on its 15 Embraer 190s and on its 216 Airbus A310s in early 2011.

The regulating agency must carry out a flight operational safety assessment (FOSA). Here, philosophical differences between EASA and FAA emerge.

Under FAA guidelines, the aircraft is required to obtain basic equipment certification and validation, Sheffield said. The operator and national authority then carry out the operational aspects according to the specifics of each approach, demonstrating the flyability of the procedures, the FOSA. The pilot can be part of the equation and is then required to demonstrate the ability to fly the aircraft to a given level of performance.

According to Sheffield, EASA’s philosophy emphasizes the manufacturer’s role in demonstrating the aircraft’s capability to execute RNP operations. The national aviation authority can rely on the EASA certification in performing the FOSA. The emphasis is on ensuring pilot workload remains consistent and comfortable, enabling pilots to concentrate on monitoring the approach.

The EASA philosophy requires the aircraft manufacturer, in this case, Airbus, to certify aircraft for all known conditions. “As a result, Airbus has had to take the most stringent procedures and demonstrate that the aircraft is able to fly the procedure in the most constraining cases,” said Sheffield.

Aircraft manufacturers work with regulatory authorities to build training programs. Airbus offers airline instructor pilots one full day of classroom training and four hours in the simulator, which includes flying a procedure specific to the carrier.

US Airways began offering RNP AR training in March 2009 in Qualification training and in May that year in Continuing Qualification training. Each pilot in training is required to fly at least two RNP AR approaches with RF legs (one to a landing and one to a missed approach) as “pilot flying” and repeat same as the “pilot monitoring.”

Other aviation industry stakeholders require PBN training. FAA is developing courses and offers ongoing training to air-traffic controllers as NextGen technologies roll out, said Steinbicker. With RNP, for example, controllers must learn how to safely separate RNP traffic operating on precise trajectories from “normal” traffic flying traditional paths.

Inspectors responsible for approving FOSAs also require training. Steinbicker said FAA is working to build expertise by assigning expert staff at the regional level.

National authorities worldwide have varying levels of experience with emerging PBN operations, Sheffield said. The common reaction from regulators of smaller countries who are suddenly faced with requests for RNP AR approvals, he said, is: “You want to fly down mountainous terrain in bad weather? And you want me to sign off on this?”

The time from request to final approval for a specific-use RNP AR approach varies greatly. Some earlier requests took more than a year. But approval of Air China’s request for an RNP AR procedure designed by GE PBN Services for its Airbus A330 into Lhasa, Tibet, the most challenging environment that exists, took just four months. “The airport doesn’t have ground instrumentation, the airfield elevation is 12,000 feet and the surrounding terrain is at 18,000 feet,” said Sheffield. “Lhasa has bad weather conditions and low visibility 300 days of the year. And when it is not raining or snowing, there’s a sandstorm.”

Most RNP AR approvals are for single-aisle aircraft, but interest in gaining approvals for widebodies has stepped up since the A330 went into Lhasa, Sheffield said.

Stakeholders worldwide already are reaping the benefits of PBN. Air New Zealand became the first operator to have RNP AR capability across its Airbus A320 fleet and is the first to fly an actual RNP AR 0.1 procedure at Queensland.

RNAV procedures implemented at Dallas/Fort Worth International Airport in September 2005 have allowed for an extra 10 departures an hour, reduced air delays by 5 percent and saved airlines $300 million in the last three fiscal years, mostly in fuel consumption, according to Steinbicker. In addition, airlines performing PBN procedures at Atlanta have annually saved 700,000 gallons of fuel and reduced carbon emissions by 6,700 tons each year.

Ahead, Optimized Profile Descents (previously called continual descent approaches, or CDAs), already in use at certain airports, will become more commonplace. In this procedure, an aircraft makes a continuous descent path with engines near idle from the top of the descent to touch down. OPD are designed to reduce fuel consumption, emissions and noise. A FAA flight simulation model, for example, estimates an average 1,000-to-3,000-pound fuel savings per flight. US Airways’ Thomas said the airline is seeing “substantial savings” from OPDs it helped develop at Las Vegas and Phoenix airports.

The challenge of OPD, Fulton said, is being able to perform descents in more complex traffic environments, such as San Francisco, Los Angeles or Miami, where operators can gain significant benefits. Boeing points out that work has just started at FAA and in Europe using RNP guidance in parallel runway operations and to abbreviate track miles into same operations.

Could airspace modernization inadvertently create safety risks? “Certainly we could, if we are not careful with the transition,” said Fulton. “Any time there is change, we must pay close attention to the discontinuities. My biggest concern is that members of various work groups may advocate things be done quickly without fully understanding the full color and flavor of the change.”

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