Display and Cross-check of Ground-based Navaids Against FMS Map Display
Human Factors in Aircraft Automation - issue #4
SECOND REPORT from the Human Factors Committee, Automation Subcommittee, Air Transport Association
prevExcept for those aircraft designed to meet Required Navigation Performance (RNP) for the Approach Phase (B-737 or B-777 with Advanced FMS, for example). Flight Management Systems are certified for enroute and terminal navigation, but not for approaches. Except for special FMS, GPS, and RNAV procedures, approaches are flown relative to or from ground-based navaids. So presumably, a pilot may (from an FAR perspective) take off, accomplish a SID and its transitions, navigate enroute, and accomplish a STAR and its transitions to the initial approach fix, all in LNAV without direct reference to ground-based navaids. But by the initial approach fix, the pilot must tune, identify, and monitor the navaids that define the approach; to ensure the path flown by the aircraft complies with the required track.
Are those FAR requirements sufficient for ATA member carriers? A review of procedures across carriers suggests it is not. The subcommittee encountered the following range of policies:
- At least one airline has not published a ground-based navaid policy.
- One requires one pilot to tune, identify, and display ground-based navaids on any approach where raw data can be displayed on the EHSI. For an NDB approach, cross-check is required. (This is very close to the FAR requirement.)
- Another requires one pilot to display ground-based navaids below 25,000 ft. in Latin America, and lists other requirements specific to the type of approach in aircraft Training and Operating Manuals.
- Another stipulates that the PNF will have ground-based navaids selected on precision and non-precision approaches prior to final course interception and the final approach fix, whenever below grid MORA in Latin America, and during departures or approaches to airports listed in Company pages (such as European, Mexican, and special US airports).
- Another requires the PNF to tune, identify, and monitor ground-based navaids throughout the flight. The PF may display and use LNAV guidance. Accuracy of FMS updating is traded for constant pilot cross-check.
Clearly, no industry standard exists though individual carriers have established when ground-based navaids must be displayed or cross-checked. However, other potential requirements have been left undefined by one or more carriers. When is a cross-check acceptable (i.e., against the RDMI) and when is a deviation display required (i.e., EHSI)? Who must display ground-based navaids (CA/FO, PF/PNF)? Do region-specific requirements generalize to any other International or Domestic operations?
These issues change with the introduction of Advanced FMS aircraft certified to comply with Required Navigation Performance (RNP) in the Approach Phase. Example aircraft include B-737 and B-777 with Advanced FMS. These aircraft are certified to accomplish published RNAV approaches solely by reference to the FMS. However, for all other approaches, cross-check against ground-based navaids is required. Pilots transitioning to and from these aircraft must understand these distinctions. Inappropriately generalizing RNAV procedures to non-RNAV approaches on these aircraft, or transferring these procedures to non-RNP aircraft must be prevented.
Supporting Evidence
Each member carrier participating in the subcommittee reported events that highlight misunderstanding of or absence of guidance concerning ground-based navaid requirements:
- Map shifts have been encountered occasionally at a variety of locations (for example within the U.S. at BNA, DFW, LAX, and SAT). These must be detected and corrected by the crew by reference to ground-based navaids or by ATC advisory. Detection becomes critical in the approach environment, but is potentially an issue any time the aircraft is below grid MORA.
- Approach deviations have been reported where a crew followed FMS course guidance that diverged from the localizer or radial defining the approach segment. The ground track specified by the procedure must be flown, so ground-based navaids defining the procedure must be at least cross-checked. Such events are evidence that the cross-check was not accomplished or properly weighted by the crew.
- Similarly, crews have continued approaches in instrument conditions after the failure of a ground-based navaid when the pilots persuaded themselves the map display sufficed. It clearly does not.
- Terminal area deviations have involved incorrectly flown SIDs and STARs where the wrong transition has been entered or wrong course built by one pilot. There is no FAR requirement to cross-check these procedures against ground-based navaids, though procedures at certain airports have resulted in company requirements. Only one carrier specifically requires pilots to cross-check against ground-based navaids defining all SIDs and STARs, though several have emphasized in procedure having the chart out and available during the departure and arrival and have encouraged their pilots to cross-check those courses as a technique.
Most policy requirements beyond those required by FAR appear to be geared towards protecting pilots against map shifts. If they were written to support identification and correction of erroneous FMS data or pilot inputs, would they be any different? The subcommittee is concerned that pilots may not understand the reasons underlying a company’s ground-based navaid policy and may not apply it correctly in the critical situations it was designed to resolve.
Policy Guidance for Pilot Action
The Subcommittee recommends that member carriers evaluate and define their requirements for reference to ground-based navaids. Member carriers are observing a variety of techniques being carried out by pilots where procedure might clarify their understanding and prevent or correct error. The key question is, "Who must tune, identify, and display or cross-check what ground-based navaids for each phase of flight:"
- Takeoff?
- Departure Climb including SID and transition?
- Enroute navigation including unique requirements of overwater, European, and Latin American operations?
- Arrival including STAR and transition?
- Approach including unique requirements of precision and non-precision approaches?
- Published RNAV approaches on RNP aircraft?
A key question surrounds when a CDI display is required -- and this may be one of the best ways to approach decisions about a ground-based navaid policy.
The subcommittee offers the following prototypical statement for publication in operating manuals:
Except for those aircraft designed to meet Required Navigation Performance (RNP) for the Approach Phase (B-737 or B-777 with Advanced FMS, for example), Flight Management Systems are certified for enroute and terminal navigation, but not for approaches. Except where prohibited by bulletin or company-specific pages in the Airway Manual, pilots may accomplish a SID and its transitions, navigate enroute, and accomplish a STAR and its transitions to the initial approach fix solely by FMS navigation, but not approaches.
Except for published FMS, GPS, and RNAV instrument approach procedures, approaches are flown relative to ground-based NAVAIDs. For all other approaches, prior to the initial approach fix, one pilot must tune, identify, and monitor (on a CDI display, where available) the navaids that define the approach. These actions are necessary to ensure the path flown by the aircraft complies with the ground track required by the approach procedure. The function of the FMS and Nav display during an approach is to assist your situation awareness -- not to fly the approach. Any discrepancy between the Nav Display or Flight Director based on FMS/GFMS guidance and raw data from navaids defining the approach must be challenged and resolved immediately. Should the ground based signal be lost, the crew must abandon that approach if in instrument conditions. On all instrument approaches inside the final approach fix in IMC weather conditions, a go-around is required whenever unreliability or full scale deflection of the ground-based approach navaids is encountered. [Note: this paragraph describes what is necessary for the pilot to comply with FMS certification.]
Specific autoflight and display modes required for precision and nonprecision approaches are specified in each aircraft flight manual. Requirements to accomplish published FMS, GPS, and RNAV instrument approaches are published in the operating manual of fleets so equipped. In addition, ground-based navaids defining a course must be tuned, identified, and monitored where specified by bulletin or company-specific pages in the Airway Manual, and when operating in Latin America below FL250. [Note: this paragraph describes additional, company-specific requirements.]
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