CNS/ATM resource kit – Chapter 9: Instrument flight rules operations

In February 2016, GNSS became mandatory for all aircraft flying IFR. This chapter explores the transition to performance-based navigation and how GNSS is used in IFR operations through a number of area navigation (RNAV) applications, providing guidance on operating safely and addressing common issues. GNSS may also be used in VFR operations—see Chapter 10.

Pilots flying IFR must hold the applicable instrument rating aircraft endorsements.

CASA approves the use of GNSS for a variety of IFR applications. These include:

  • DR substitute
  • en route RNAV
  • required navigation performance (RNP) approach (also known as RNAV[GNSS] non-precision approach)
  • oceanic RNAV
  • GNSS landing system (GLS).

Transition to performance-based navigation

The number associated with an RNP or RNAV specification includes the navigational accuracy required (in nautical miles).

While both RNAV and RNP specify accuracy, RNP also specifies integrity.

Australian operational navigation specifications include:

  • RNP 1—for standard instrument departures (SIDs) and standard terminal arrival routes (STARs)
  • RNP 2—en route
  • RNP–APCH—LNAV approach.

Under GNSS-RNAV, these types of operations were previously known as terminal, en route and non-precision approach.

Navigation authorisations for typical IFR operations

The table below shows how operations have transitioned under advisory circular (AC) 91.U-01 Navigation authorisations.

This table contains how operations have transitioned under advisory circular (AC) 91.U-01 Navigation authorisations
Operation Applicable navigation specifications

Australian continental operations not entering oceanic airspace that include en route, terminal and:

  • RNAV (GNSS) approach
  • RNAV (GNSS) approach with Baro-VNAV
  • RNP 2
  • RNP 1
  • RNP APCH-LNAV
  • baro-VNAV
Operations entering oceanic airspace as well as continental operations that include en route, terminal and RNAV (GNSS) approach.
  • RNAV 10 (RNP 10)
  • RNP 2
  • RNP 1
  • RNP APCH-LNAV
Operations entering oceanic airspace as well as continental operations that include en route, terminal and RNAV (GNSS) approach that will also be operating in airspace with use of a satellite-based augmentation system (SBAS).
  • RNAV 10 (RNP 10)
  • RNP 2
  • RNP 1
  • RNP APCH-LNAV
  • RNP APCH-LP and LPV

Operations entering oceanic airspace with reduced separation (30 nm lateral and longitudinal separation) as well as continental operations that include en route, terminal and RNAV (GNSS) approach.

Note: There are likely to be additional requirements for aircraft to be equipped with CPDLC and ADS-C to support reduced separation operations in oceanic airspace.

  • RNAV 10 (RNP 10)
  • RNP 4
  • RNP 2
  • RNP 1
  • RNP APCH-LNAV
Aircraft that operate in B-RNAV airspace in Europe.
  • RNAV 5
Aircraft that operate in North Atlantic high level airspace (NATHLA).
  • NAT MNPS
  • NAT HLA MNPS
Aircraft that operate in European P-RNAV airspace or US RNAV Type A or Type B airspace.
  • RNAV 1
  • RNAV 2

IFR RNAV

Pilots flying IFR in Australian domestic airspace may use GPS for position fixing and long-range navigation in accordance with Airservices Australia’s Aeronautical Information Package (AIP). This applies to operations on designated RNAV routes, application of RNAV-based LSALT, deriving distance information for en route navigation, traffic information and air traffic control (ATC) separation. ATC may apply RNAV-based separation standards to aircraft meeting the requirement for IFR RNAV.

Position fix (PF) is determined with reference to navigation aid and systems using ground-based and/or satellite-based navigational systems.

GPS may also be used as a navigation aid to determine distance information for standard instrument departures (SIDs), standard terminal arrival routes (STARs) and instrument approach procedures where the use of GPS is specified on the instrument approach and landing (IAL) chart. GPS may be used to meet the IFR requirements for radio navigation systems specified in Airservices Australia’s AIP Part 1-General (GEN) 1.5.

If your GNSS performance degrades to the point at which an alert is raised, or you have any other cause to doubt GNSS information integrity, you should stop using GNSS and carry out appropriate navaid failure procedures.

Common Australian operational navigation specifications under PBN
Common Australian operational navigation specifications under PBN

GNSS arrivals

Although classed as instrument approach procedures, GNSS arrivals and DME or GNSS arrivals (DGA) are included in the RNP 1 application and approval. Additional competency and recent qualifications apply to flying DGA.

For these procedures, the destination navaid (VOR or NDB) nominated on the approach chart must be used to provide primary track guidance during the arrival procedure and the distance information must be based on the ‘reference waypoint’ navaid nominated on the chart.

Lowest safe altitude (LSALT)

AIP GEN 3.3 permits IFR LSALT to be determined by GNSS capability. LSALT printed on terminal area and en route charts assumes RNP 2 capability and takes into accont an area of 5 nm surrounding and including the departure point, destination and nominal track. (see diagram).

Lowest safe altitude (LSALT)
Lowest safe altitude (LSALT)

For other routes and route segments, the obstacle clearance to be considered must be within an area of 5 nm surrounding, and including, an area defined by lines drawn from the departure point not less than 15 degrees each side of the nominal track to a maximum of 7 nm, thence paralleling track to abeam the destination and converging by a semicircle of 7 nm centred on the destination.

RNAV (GNSS) non-precision approach

Pilots operating under IFR may use GPS as an approach navigation aid to determine distance and track information for RNAV(GNSS), also known as RNP APCH-LNAV, non-precision approach procedures. As with other IFR applications, TSO-C129 is the minimum standard of approved receiver.

Overlay and T-pattern GNSS approaches are used in some other countries. Pilots should familiarise themselves with the design, procedures and naming conventions used before flying these approaches under IFR.

There are no published overlay approaches in Australian domestic airspace, as the TSO-C129 receiver is unable to accurately fly the base turn reversal procedure of the teardrop design commonly used in Australia.

Australian approach design

The Australian-developed Y-pattern runway aligned design has been adopted by ICAO and is published in PANS-OPS. The approaches are essentially ‘straight-in’ to the runway and can usually be joined at any of three initial approach waypoints without the need for a reversal or base turn manoeuvre.

Naming conventions

The approach name is based on airport identification and the runway used for alignment, or in some cases the direction of the approach in relation to the airport.

In general, waypoint names use the first three letters to identify the aerodrome, the next letter to identify the compass quadrant from which the approach is flown, and the final letter for the approach waypoint.

For example, the Runway 24 GNSS approach for Paraburdoo, WA (YPBO) uses ‘PBO’ as the first three letters, and ‘E’ as the fourth letter, of all waypoints.

Runway 24 GNSS approach for Paraburdoo

Runway 24 GNSS approach for Paraburdoo
Runway 24 GNSS approach for Paraburdoo

GPS non-precision approaches

GPS non-precision approaches
GPS non-precision approaches

Vertical navigation

GNSS does not provide accurate altitude guidance and all altitudes must be obtained from the aircraft altimeter. At runways where visual approach slope indicators are not provided, pilots should take extra care to maintain the correct approach angle at runways where visual slope indicators are not provided.

A distance altitude scale is usually provided on the approach plate to give a 3° approach profile, and a corresponding altitude may be included on the profile view at selected points.

Flying the approaches

GNSS receivers are essentially navigation management computers and require more pilot attention than ILS, VOR and ADF receivers, particularly during approach. Pilots should take advantage of receiver simulation modes and ground training prior to undertaking airborne training.

Prior to flight, the receiver should be set to the required aviation parameters—nautical miles, knots, altitude in feet, pressure in hectopascals and WGS84 reference system. The nominal en route CDI scaling is 5 nm full-scale deflection.

The database must be current and contain the relevant approach. The approach should first be retrieved from the database and then selected along with the desired initial approach waypoint. The intermediate, final and missed approach segments must be flown only in that sequence. Select the desired approach and the initial approach waypoint and add this to the flight plan. Check waypoint sequence, tracks and distances against the approach chart.

Approach design

Approach design
Approach design

Pilot identification of each waypoint is essential for situational awareness during the approach, and to ensure compliance with limiting altitudes. The distance provided by the receiver is to the next approach waypoint (not to the airport) and the receiver will adjust the CDI scaling through the approach. The tracking tolerance is half of full-scale deflection regardless of the CDI scale.

An aircraft that is not required to hold or to lose height in a holding pattern may commence the approach without entering the holding pattern for procedures using GPS, provided the aircraft is tracking to an initial approach waypoint from within the capture region for that waypoint. Capture regions ensure that the radius of turn will permit interception of the approach segment prior to the next waypoint.

Should a missed approach be required, the missed approach mode must be manually selected. This expands the CDI scale to the terminal mode of 1.0 nm.

Oceanic RNAV

CASA may issue an approval for an operator to use GNSS as an en-route navigation aid in oceanic and remote areas outside the boundaries of Australian domestic airspace. The oceanic RNAV approval is based on FAA Notice 8110.60 and designed for operations over the high seas and in remote areas such as Antarctica.

Equipment requirements

The GPS equipment requirements include dual installations of FDE-capable receivers to ensure adequate redundancy and navigation performance. Installations in Australian registered aircraft must be approved and equipment capable of carrying out an appropriate en route RAIM prediction analysis for the route to be flown must use avionics manufacturer-specified software.

Operational requirements

You must operate GPS navigation equipment in accordance with the operating instructions and any additional requirements specified in the approved aircraft flight manual or flight manual supplement. These instructions must be carried on-board the aircraft.

In addition to GPS, aircraft must also be equipped with serviceable radio navigation systems as specified at GEN1.5 Section 2, or the operator’s minimum equipment list.

Before each flight, you must do an appropriate en route GPS prediction analysis, using the software provided by the GPS manufacturer. For this analysis, you must use the following parameters or equivalents:

  • the route or airspace RNP, where published
  • a centreline space of 20 nm for flight in classes A, C, D and E airspace or 50 nm for flight in OCA
  • for Australian operators, a record of the GPS prediction analysis must be retained as required by the instrument of approval.

Oceanic lateral offsets

ICAO has analysed the technical safety issues associated with the use of lateral offsets when flying in oceanic areas. Parallel offset tracking is only approved for oceanic operations. The current separation standards, safety height calculations and tracking requirements for IFR aircraft are based on the requirement that the pilot will attempt to maintain track as closely as possible. Offsets are not approved for non-oceanic IFR aircraft.

Authorisation from AIP ENR 2.2 states that aircraft operating in oceanic controlled airspace in the Australian flight information region (FIR) are authorised to use lateral offsets in accordance with certain requirements. This approval is no longer specific to operations with GNSS and full details of the requirements are listed in AIP. Contact your local CASA field office to apply for an oceanic approval.

Key points

  • GNSS must not be used as navigation reference for flight below the lowest safe altitude (LSALT) or minimum safe altitude (MSA), except as specified in IFR applications or as authorised by CASA.
  • Pilots operating under IFR may use GPS in lieu of dead reckoning (DR) navigation techniques for that part of the flight that is outside the rated coverage of terrestrial navigation aids.
  • Pilots operating in Australian domestic airspace under IFR may use GPS for RNAV—position fixing and long range navigation—in accordance with Airservices Australia’s AIP en route (ENR) documentation.
  • Pilots operating under IFR may use GPS as an approach navigation aid to determine distance and track information for RNAV(GNSS) non-precision approach procedures.
  • CASA may issue an approval for an operator to use GNSS as an en-route navigation aid in oceanic and remote areas outside the boundaries of Australian domestic airspace.

Resources

Further reading

References

Online version available at: https://www.casa.gov.au//search-centre/safety-kits/cnsatm-resource-kit/cnsatm-resource-kit-chapter-9-instrument-flight-rules-operations
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