Procedural implications of functional differences in FMS and ground-based navigation
Human Factors in Aircraft Automation - issue #3
SECOND REPORT from the Human Factors Committee, Automation Subcommittee, Air Transport Association
prevFMS/GFMS systems navigate in fundamentally different ways from pilot tracking or autopilot coupling to ground-based navigation signals. The FMS uses inertial, satellite, and/or ground references to determine its position and navigate to waypoints defined by latitude and longitude. Pilots and earlier-generation autopilots track courses, bearings, or radials provided by a ground-based navaid. An FMS makes use of databases of waypoints, navaid locations, and procedures to accomplish the ground track for published procedures.
However, charts and databases are not identical on a variety of measures. Databases do not always correspond with charted fix names, bearings, or radials, and do not include all charted procedures. In the majority of situations, these differences are of no consequence -- despite naming or depiction differences, the FMS navigates the same ground track for an approach, SID, or STAR as a pilot flying via radio navigation. However, from time to time, pilots have brought to the attention of their carriers situations where the FMS did not fly a procedure as defined by radio navigation. Reasons for differences in approaches were described in a 1997 report by Transport Canada and include:
- Not all approaches are in the database.
- Waypoint identification may differ from chart to FMS.
- Waypoints may be added to or deleted from a procedure.
- Courses and bearings may differ from chart to FMS.
- Duplicate identifiers require database conventions that may not be apparent to the pilot.
- Revision cycles for charts and databases are not identical.
Pilots must determine whether they are flying the charted procedure for which they are cleared. Given the capabilities and limitations of existing nav databases, pilots will encounter discrepancies, ranging from a simple name difference that leads to the same ground track to selection/activation or database errors that result in a substantially different course. Significant discrepancies more often result from selecting and activating a procedure incorrectly, but may also signal a database error that needs to be brought to the airline’s and database provider’s attention and corrected. Detection and correction requires a thorough briefing and cross-check that is not called for in most member carrier policies, procedures, or training.
Supporting Evidence
Each member carrier participating on the subcommittee was able to provide example events.
- Pilots have selected an approach other than the one for which they were cleared. This usually occurs when there are multiple approaches of the same type to a runway. For example at Reno, Dallas/Fort Worth, and Washington National, there are multiple ILSs to 16R, ILSs to 13R, and LDAs to 18, respectively. Except for very-recently introduced Advanced FMSs, navigation databases are limited to one approach of each type to each runway. The others simply are not in the database, and pilots have selected and flown one when cleared for another. Depending on the procedure, this can result in significant track differences on the approach or missed approach.
- Pilots have failed to select a correct transition for a SID, STAR, or approach. This usually occurs where charting/database convention incompatibilities, or a failure of the procedure-designer to comply with conventions, result in a leg that is part of the charted procedure being labeled a transition in the database. Depending on the procedure, this can result in track differences such as cutting a corner defined by the transition waypoints.
- Pilots have failed to cross-check step-down fixes on an approach that do not appear in the database, but are honored through vertical navigation constraints. This could result in a significant error on a non-precision approach, because VNAV is not used and the constraints are not depicted. However, the error has been reported to member carriers to date only for fixes outside glideslope intercept on precision approaches. In these cases, pilots have often engaged the approach mode and failed to monitor crossing altitude on non-depicted fixes outside the depicted ILS "feather."
- Pilots have continued on an FMS track or course that deviated from the track required on an arrival. For example, an arrival into one airport ended at a waypoint from which a turn to downwind was required. At this point, the aircraft would revert to heading hold and fail to make the turn. Controllers at the airport described this as a frequent error, and reports resulted in the heading off the waypoint being added to the database.
- Pilots have entered an incorrect holding pattern after having been instructed to hold as published at a fix. In these cases, two patterns were published at the same fix, usually one at high and one at low altitude. Only one appears on each chart and the database cannot store multiple patterns at the same fix. Depending upon the traffic pattern in the area, this can result in loss of separation with other traffic.
A variety of other examples could be provided. Each may be viewed in two different ways — as an operator error or as a database problem. The subcommittee is aware of and supports the activities of the industry working group pursuing standardization between charts and nav data. However, we believe that these limitations will always exist in some form and pilots must be prepared to detect and correct them.
Policy Guidance for Pilot Action
The Subcommittee recommends that member carriers review the extent to which their pilots need to know the underlying database issues, but more importantly, the extent to which their procedures emphasize the pilot actions required to ensure the aircraft flies the track required by a procedure. We offer the following prototypical statement for publication in policy manuals:
- Before departure, thoroughly review your assigned departure and cross-check the waypoints obtained with your desired course. If you select or build a transition, verify between pilots that it matches your clearance and produces the desired track. Ask ATC for clarification if any conflict exists.
- Before arriving in the terminal area, thoroughly brief the arrival and approach you expect to fly and cross-check fixes presented by the FMS against fixes depicted on the approach chart. Should the runway or approach change and you wish to use the FMS for the new approach, that same level of cross-check is essential. If time constraints or circumstances prevent your cross-check, decline the clearance or tune and identify radio aids to navigation and fly the approach in a lower level of automation.
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