En-route Operations

There are 6 possible navigation specifications that support en-route domestic continental operations.  However, A-RNP and RNP 0.3 are applications which are also used in terminal operations as well.  Similarly, A-RNP and RNP 2 can qualify for oceanic/remote continental operations provided that the aircraft carries additional avionics for this type of operation; this will include dual long range navigation systems meeting the high continuity requirement as well as CPDLC and ADS-C as a minimum.

The European certification standard, JAA Temporary Guidance Leaflet No. 2 (TGL2) was first published in July 1996.  This certification standard aimed to capture as many aircraft as possible so the requirements were not particularly demanding and could be met by 1980 built aircraft with first generation digital avionics.  When EASA superseded the JAA, TGL 2 Revision 1 was re-issued as AMC 20-4.  The FAA’s equivalent certification document was AC 90-96.  Both documents have now been superseded again, in Europe by CS-ACNS and in the US by AC 90-105A.

This was the European’s first area navigation application and in April 1998, Basic RNAV (B-RNAV) was mandated for European en-route airspace.  The RNAV 5 navigation specification is based on AMC 20-4, therefore B-RNAV is RNAV 5.  The ICAO Regional Supplementary Procedures (ICAO Doc 7030) clearly identifies that the term B-RNAV is replaced by RNAV 5.

RNAV 5 is for use within the en-route phase of flight and calls for a +/- 5NM lateral accuracy along ATS routes without a requirement for on-board performance monitoring and alerting.  RNAV 5 can be employed in surveilled and unsurveilled airspace; however, where there is no surveillance then route spacing will need to be increased commensurate with the assurance of meeting the SSR.  The specification does not require the carriage of dual RNAV systems so the potential for loss of RNAV capability requires the aircraft to be fitted with an alternative navigation source.

The ANSP is to have direct controller pilot (voice) communications.  The ANSP may provide an ATS Surveillance service to assist contingency procedures, to mitigate the effect of blunder errors and to enable a reduced route spacing.

When reliance is placed on the use of ATS surveillance services to these ends, performance of the ATS Surveillance system should be fit for purpose, ensuring that the  routes lie within the ATS surveillance and communications service volumes and the ATS resources are sufficient for these tasks.

Where GNSS is used as the sole basis for both ATS surveillance relies upon the same system that also supports the navigation function (e.g. ADS-B) and navigation, consideration should be given to the risks associated with loss of GNSS which may result in the loss of navigation and impact on the ATS surveillance. This should typically be addressed through the regional or local State safety case prepared in support of the application.

RNAV 1 and 2 have exactly the same requirements and therefore the RNAV 1 and 2 navigation specification covers both performance requirements.  The RNAV 1 and 2 specification is applicable to all ATS routes, including routes in the en-route domain, SIDs and STARS and can support operations up to the FAF.

The RNAV 1 and 2 specification is primarily developed for RNAV operations in a radar environment (for SIDs, radar coverage is expected prior to the first RNAV course change). The RNP 1 specification is intended for similar operations outside radar coverage.  However, RNAV 1 and RNAV 2 may be used in a non-radar environment or below minimum vectoring altitude if the implementing State ensures appropriate system safety and accounts for lack of on-board performance monitoring and alerting.

RNAV 1 and RNAV 2 operations are is required to be conducted with Direct pilot to ATC (voice) communications (a DCPC environment).

The following navigation infrastructures can support RNAV 1 and 2: GNSS and/or DME/DME.

Where DME is the only navigation service used for position updates, gaps in DME coverage can prevent position update. Integration of IRUs can permit extended gaps in coverage but patently the aircraft must be fitted DME/DME/IRU; it should be noted that the growth in position error after reverting to IRU can be expected to be less than 2 NM per 15 minutes.  If the aircraft is not fitted with an IRU then the aircraft can revert to dead reckoning. However, additional protection, in accordance with PANS-OPS, will be needed to cater for the increased error. GNSS should be authorized whenever possible and limitations on the use of specific system elements should be avoided.

Most modern RNAV systems prioritize input from GNSS and then DME/DME positioning.   However, if VOR is also fitted to the multi sensor computer then its influence on the navigation solution must be managed.  The navigation specification identifies that some aircraft systems may revert to VOR/DME-based navigation before reverting to inertial coasting.  Therefore, the impact of VOR radial accuracy, when the VOR is greater than 40 NM from the aircraft, must not affect aircraft position accuracy and should be excluded from the position solution.

RNP 2 is intended for a diverse set of en-route applications, particularly in geographic areas with little or no ground NAVAID infrastructure. This Nav Spec was introduced into the fourth edition of the PBN Manual in 2013.  RNP 2 is designed to cover two completely different environments and two completely different user groups.  Originally when developing the Nav Spec, the PBN Study Group had considered introducing a Basic and an Advanced RNP 2.  However, it was decided to develop a hybrid specification that covered both environments and user groups with continuity playing an important role.  Therefore, the application of RNP 2 in continental applications requires a lower continuity requirement than application in oceanic/remote airspaces. In the latter application, the target traffic is primarily transport category aircraft operating at high altitude, whereas, continental applications may include a significant percentage of GA aircraft.

The application of RNP 2 in oceanic or continental airspace considered by a State to be remote may require different considerations for aircraft eligibility based on whether the remote areas support suitable landing airports for the target aircraft population, or support reversion to an alternate means of navigation. Thus for remote airspace applications, a State may choose to designate either continental (low continuity) or oceanic/remote (high continuity) aircraft eligibility.

The RNP 0.3 specification is intended for the exclusive use of helicopters and the navigation specification addresses continental, remote continental and offshore operations.  The specification takes credit for the fact that the large majority of IFR helicopters are already equipped with SBAS receivers and moving map displays, and require autopilot including stability augmentation for IFR certification.

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 significant benefits from PBN.  Specific operations that RNP 0.3 can enable included:

  • Reduced protected areas, potentially enabling separation from fixed wing traffic to allow simultaneous non-interfering operations in dense terminal airspace;
  • Low-level routes in obstacle-rich environments reducing exposure to icing environments;
  • Seamless transition from en route to terminal route;
  • More efficient terminal routing in an obstacle-rich or noise-sensitive terminal environment, specifically in consideration of helicopter emergency service IFR operations between hospitals;
  • Transitions to helicopter point-in-space approaches and for helicopter departures; and
  • Helicopter en-route operations are limited by range and speed and can often equate to the dimensions of terminal fixed wing operations.

RNP 0.3 supports operations en route and in the terminal airspace supporting operations to and from airports, heliports and for servicing offshore rigs. RNP 0.3 accuracy may also be needed en route to support operations at low level in mountainous remote areas and, for airspace capacity reasons, in high density airspace.

The specification is applicable to departure, en route, arrival (including the initial and intermediate segments of the approach), and to the final phase of the missed approach; for the final approach segment (FAS) a RNP approach certification and approval is required.

RNP 0.3 can be applied with and without ATS surveillance.  The specific ATM requirements will be specified in such documents as operating rules, AIPs and the Regional Supplementary Procedures (Doc 7030).

The RNP 0.3 specification is based upon GNSS; however, its implementation is not solely dependent on the availability of SBAS. DME/DME based RNAV systems will not be capable of consistently providing RNP 0.3 performance, and States should not plan on implementing RNP 0.3 operations through application of DME/DME-based navigation. States must also not use RNP 0.3 in areas of known navigation signal (GNSS) interference.

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