With the first operations utilizing performance-based navigation taking place in Europe, we take a look at how the concept may offer significant potential to improve reliability and safety in the helicopter industry.
By Mario Pierobon
As part of the PBN concept there exist two main macro-categories of navigation specifications: area navigation (RNAV) and required navigation performance (RNP). The main difference between RNAV and RNP is the requirement for on-board performance monitoring and alerting. A navigation specification that includes a requirement for on-board navigation performance monitoring and alerting is referred to as an RNP specification. One not having such a requirement is referred to as an RNAV specification.. The capability to undertake performance-based navigation (PBN) for both rotorcraft and fixed-wing aircraft is one of the “future” navigation concepts being embraced by industry experts around the globe in the present. Representing a shift from sensor-based navigation, the PBN concept sees performance requirements identified in navigation specifications, which also identify a choice of navigation sensors and equipment to be used to meet the performance requirements.
According to the International Civil Aviation Organization’s (ICAO’s) PBN manual (Document 9613), the PBN concept specifies that aircraft RNAV and RNP system performance requirements must be defined in terms of their accuracy, integrity, continuity and functionality.
Under PBN, generic navigation requirements are defined based on operational requirements. Operators then evaluate options in respect of available technology and navigation services, which could allow the requirements to be met. An operator thereby has the opportunity to select a more cost-effective option, rather than a solution being imposed as part of the operational requirements. Technology can evolve over time without requiring the operation itself to be reviewed, as long as the expected performance is provided by the RNAV or RNP system, according to ICAO.“The PBN concept suggests that RNAV specifications are effectively legacy specifications and that no new RNAV specifications will be developed,” states a PBN briefing paper from the European Organisation for the Safety of Air Navigation (Eurocontrol). “Indeed, PBN’s sights are firmly set on RNP which relies primarily on the use of satellite technologies. This explains why all the new navigation specifications in the 2013 update to the PBN manual are RNP specifications.”
FORGING A PATH
In Europe, some of the first applications of PBN have already taken place using the Localizer Performance with Vertical Guidance (LPV) approach and RNP 0.3, which are both part of the PBN family of navigation specifications.
In June 2015, a CHC Helikopter Service Sikorsky S-92 equipped with dual Universal Avionics SBAS-Flight Management Systems (FMS) car- ried out the first LPV approach to Florø, one of the first Norwegian air- ports to be equipped with LPV capability.
In Switzerland, Rega, one of the country’s leading helicopter emergency medical service (HEMS) providers, has been working closely with the Swiss Air Force on a RNP 0.3 low flight network (LFN) that links hospitals as well as military airfields and landing sites. The Norwegian Air Ambulance is involved in a similar project to achieve an RNP 0.3 instrument flight rules (IFR) route net in Norway.
RNP 0.3 is a navigation specification for all phases of helicopter operations with a requirement for on-board navigation performance monitoring and alerting. Under this navigation specification, the required accuracy is 0.3 nautical miles (nm) for all phases of flight, which means that during operations in airspace or on air traffic service (ATS) routes designated as RNP 0.3, the lateral total system error must be within ±0.3 nm for at least 95 percent of the total flight time. The along-track error must also be within ±0.3 nm for at least 95 percent of the total flight time. To meet this performance requirement, a flight technical error of 0.25 nm (95 percent) may be assumed.
With regard to the navigation needs of helicopter operations, ICAO’s PBN manual highlights that: “The helicopter community identified a need for a specification that has a single accuracy of 0.3 nm for all phases of flight, recognizing that such a specification would enable a significant part of the IFR helicopter fleet to obtain benefit from PBN.”
According to ICAO, the operations envisaged by the helicopter community included reduced protected areas, potentially enabling separation from fixed-wing traffic to allow simultaneous non-interfering operations in dense terminal airspace, as well as low level routes in obstacle- rich environments to reduce exposure to icing environments. Seamless transition from en route to terminal route was also envisaged, as well as a need for more efficient terminal routing in obstacle-rich or noise-sensitive terminal environments — something particularly relevant to HEMS IFR operations between hospitals, and around airports supporting the offshore industry. Other operational needs leading to the development of the RNP 0.3 navigation specification included transitions to point in space (PinS) approaches and for departures.
“Most modern IFR twins now come suitably equipped [for PBN] as standard,” Steve O’Collard, a technical pilot at CHC Helicopter, told Vertical. “In addition, there are after-market systems that are fully compatible.” According to Robert Clare, Universal Avionics’ director of sales, there are approximately 120 S-92s in the field with his company’s SBAS- FMS system installed.
Indeed, significant benefits are enabled by LPV approach procedures in that they allow approaches using on board global positioning system (GPS) rather than ground-based systems such as the instrument landing system (ILS). “LPV permits approaches to lower minima in cases where the local ground-based systems may not [such as VOR or NDB, which do not permit the same degree of accuracy], or where there are no ground-based systems, with potentially significant operational benefits,” said O’Collard. “The flight path is not affected by terrain interference; indeed ground-based signals can be ‘bent’ by terrain or by interference from, for example, other aircraft.”
With regard to PBN aeroplane operations, terminal and en-route navigation requires less navigation accuracy (RNP 1 and RNP 2, respectively). “ICAO PANS-OPS now allows RNP 0.3 operations for helicopters ‘in all phases of flight,’ ” said Lukas Kistler, lead pilot of Rega’s EC145 fleet. “This enables more flexible helicopter-dedicated en route IFR segments in difficult terrain with many restricting obstacles. By minimizing the size of the obstacle-free corridor, the routes can be designed at lower altitudes where generally warmer temperatures prevail. This in turn allows non de-iced helicopters to operate IFR on days where higher airways are already in an icing zone.”
In Europe, there are not many large-scale implementations of PBN for helicopter operations beyond the use of LPV approaches to aerodromes that have published RNP approach procedures to LPV minima. The acceptable means of compliance to the PBN section of European Aviation Safety Agency (EASA) air operations regulations dealing with operations that require specific approval aren’t even published for the RNP 0.3 specification. This means that operators wanting to implement RNP 0.3 — to derive the benefits of PBN for phases of flight other than the approach phase — must refer to ICAO’s PBN manual.
Kistler said that bad weather currently prevents around 600 people from receiving emergency assistance from the air each year in Switzerland. “We see the possibility to increase the number of patients we are able to transport in marginal weather conditions [through the LFN],” he said. “We also believe that by routinely flying on an established, well known IFR network, we can further improve the level of safety as opposed to NVIS-aided VFR [visual flight rules] night operations.” According to Kistler, another setting where RNP 0.3 helicopter operations could provide benefit to both helicopter operators and wider air navigation systems are IFR transit routes through busy airspace at inter- national airports.
Over in the U.S., Hughes Aerospace Corporation, based in the Woodlands, Texas, is one of just two companies both endorsed by ICAO and certified by the Federal Aviation Administration (FAA) to develop, validate, and maintain FAA public (part 97) IFR procedures, as well as special IFR procedures for helicopter and fixed-wing operators worldwide. Hughes implemented the first public RNP approach procedures at Chicago O’Hare International Airport, which the company still maintains on behalf of the FAA. “We also develop and maintain helicopter PBN procedures,” Chris Baur, president and chief executive officer of Hughes Aerospace, told Vertical. “In the U.S., there are no IFR heliports, hence [helicopter] IFR procedures are categorized as special instrument procedures, developed as “PinS” or ‘Point in Space’ procedures. . . . There are [also] hundreds of special LNAV instrument approaches and departures, along with more contemporary LPV procedures.”
The company has worked with the FAA to prototype the use of the Wide Area Augmentation System (WAAS) and the use of “RF” (radius-to-fix) segment coding for
helicopters, leveraging the accuracy level and superior containment of WAAS and advanced ARINC 424 binary code. Baur noted that the FAA does not characterize helicopter RNP instrument approaches as RNP 0.3 naviga- tion, but it recently published criteria for RNP
0.3 IFR low level routes, supported by WAAS. Raw GPS has an accuracy of 10 meters, but if augmented by either space-based or ground- based augmentation systems, it can achieve accuracy levels of one meter and 10 centime- ters, respectively.
“In order to execute an RNP approach, you need to have special equipment and aircrew training,” said Baur. “The equipage requirements include dual flight management computers [FMC] and inertial reference systems [IRS]. In the event of a catastrophic reception failure of GPS updating to the FMC, the IRSs are used to perform an extraction maneuverer [missed approach], providing sufficient contain- ment until the aircraft is positioned at a safe altitude. Helicopters do not normally have a dual inertial reference system — the project we did with the FAA was to see illustrate how operators could use the containment accuracy of WAAS (SBAS) demonstrated at 0.3 RNP to alleviate the need to have [one].”
TECHNICAL AND OPERATIONAL REQUIREMENTS
For a helicopter company to perform PBN operations, O’Collard said it must first install equipment that satisfies the “aircraft requirements” of navigation specifications. Next, it must have operational approval from the applicable civil aviation authority to use the equipment. “This approval will cover not only normal and abnormal operational procedures, but also training,” he said.
ICAO’s PBN manual highlights that airworthiness certification and recognition of RNP 0.3 aircraft qualification alone does not authorize RNP 0.3 operations. “Operational approval is also required to confirm the adequacy of the operator’s normal and contingency procedures for the particular equipment installation applied to RNP 0.3 operations,” the manual states.
According to ICAO, the operating procedures to be developed, docu- mented and implemented include pre-flight planning, RNP 0.3 availability prediction, general operating procedures, RNP 0.3 standard instrument departure (SID) and standard instrument arrival route (STAR) specific requirements, as well as contingency procedures in case of loss of the RNP 0.3 capability.
The training program should provide sufficient training (in a simulator, training device, or aircraft) on the aircraft RNP system to the extent that the pilot is familiar with the content of the RNP 0.3 navigation specification in ICAO’s PBN manual. Flight crew training should also include required navigation equipment and minimum equipment list for operation on RNP 0.3 ATS routes and RNP system-specific information. Simulator and/or aircraft training should also be delivered to familiarize the flight crews with RNP equipment operating procedures and as contingency procedures.
To make it possible for the air navigation system to enable the high standards of PBN operations, a very peculiar system component comes into play: the variety of satellite based augmentation systems (SBASs) that operate in different regions of the world. While the GPS has been used successfully in aviation for many years, Universal Avionics said the basic technology does not produce adequate precision and accuracy to allow it to be used as a sole source of navigation. “The accuracy and integrity of GPS is greatly enhanced by the use of augmentation information from a variety of sources,” states a Universal Avionics’ document on SBAS. “[SBAS] augments the GPS signal to produce an increased accuracy, integrity, reliability and availability of information for aviation. With the decommissioning of legacy ground-based navigation systems, regional SBAS programs have grown substantially over the past five years. This technology is a critical component of the FAA’s Next Generation [NextGen] program and the Eurocontrol SESAR [Single European Sky ATM Research] initiative.”
An SBAS consists of a network of precisely surveyed ground reference stations strategically positioned to monitor, collect and process satellite signals. The ground reference stations send satellite signal data to ground master stations, which then take measurements of signal delay and other errors (such as ionospheric and/or solar activity) that may impact the signal. Using the signal error measurements, master sta- tions develop corrections to the information obtained from the ground reference stations and send a corrected, or augmented, message to Geostationary Earth Orbit (GEO) communication satellites. These GEOs then broadcast the message to the internal SBAS receiver in an SBAS- capable FMS. Paul Damschen, Universal Avionics’ manager of airworthiness and flight operations, said the LPV approach does not require RNP 0.3 — in fact, it requires a much lower RNP value. “LPV approaches require a horizontal and vertical alarm limit of 50 meters, so positional accuracy to support those operations is much lower than basic RNP 0.3,” he said. “Basic RNP 0.3 does not require SBAS augmentation, and TSO C129 as well as TSO C146 equipment can support RNP 0.3 for en route, terminal, and approach. LNAV-only approach can be conducted under RNP 0.3, again, without SBAS augmentation.”
Several regional SBAS programs have been implemented so far, each complying with a common global standard. Therefore, all are compatible and interoperable, and do not interfere with each other. An operator with an SBAS-capable receiver can benefit from the same level of service and performance no matter which coverage area they are in. Existing and in-work SBAS include the WAAS in North America; the European Geostationary Navigation Overlay Service (EGNOS) in Europe and North Africa; the Multi-functional Satellite Augmentation System (MSAS) in Japan and GPS-Aided Geo-Augmentation Navigation (GAGAN) in the Indian subcontinent.
To achieve the high level of safety that RNP operations provide, espe- cially in terminal areas where approaches without a vertical glide path will be history, the airspace providers and the operators have a signifi- cant responsibility ahead of them. “While GSA in Europe is finalizing the SBAS coverage to achieve lowest LPV minima at all airports, the national airspace providers at airports should be designing new RNP approaches to replace existing conventional approaches,” said Jørgen
Staffeldt, a type technical pilot of the S-92 at CHC Helicopter. “In parallel, the operators should be updating aircraft to SBAS capability and training the crew to achieve RNP authorization.
“On the other side, the helicopter OEMs should be working on the certification of aircraft to not just SBAS capability, but future NAV SPEC capability, including today’s RNP AR [Authorization Required] that cur- rently include Radius-to-Fix legs to be used in approach procedures, to achieve shortened and more flexible approaches,” Staffeldt continued. Baur noted that the several hundred public LPV procedures existing in the world are published for fixed-wing operations, while the majority of helicopter LPV procedures are special instrument approach procedures. “[Helicopter] instrument procedures in the United States are developed by third party service providers, such as Hughes Aerospace,” he said. “The FAA has issued Hughes with the authority to perform heliport evaluations, flight, simulator and obstacle evaluations as well as proce- dure maintenance.”
Baur notes that one of his company’s recent achievements has been to publish the first LP [Localizer Performance] helicopter approach procedure in the United States. “This type of approach provides sig- nificant benefit to the operator, in a situation where you are not able to provide the vertical guidance because the aircraft is WAAS [or SBAS] equipped but does not have the necessary equipage for LPV,” he said. “LP approaches provide lower minima than non-augmented LNAV approaches. The WAAS constellation supports a tighter accuracyand containment, with the potential to eliminate obstacles and reduce minima required with LNAV.”
PBN for rotary wing operations has significant potential to improve reliability and safety as well as air traffic management. However, it still has to develop a critical mass. While the availability of technology installations and upgrades put PBN within easy reach for operators, without the sup- port of air navigation service providers (who must develop more public PBN procedures) and industry regulators (who must oversee the opera- tors wanting to upgrade to PBN capability), the benefit of PBN operations for the helicopter community will remain largely unfulfilled.