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A300 flight crew reports that the FMC will routinely place a down track fix off the direct course line; causing a track deviation. The error is normally small but in this case the deviation is noticed by ZID.
| Attributes | |
| ACN | 832423 |
| Time | |
| Date | 200904 |
| Local Time Of Day | 1801-2400 |
| Place | |
| Locale Reference | ZID.ARTCC |
| State Reference | IN |
| Environment | |
| Flight Conditions | VMC |
| Light | Night |
| Aircraft 1 | |
| Make Model Name | A300 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | FMS/FMC |
| Person 1 | |
| Function | Pilot Not Flying Captain |
| Experience | Flight Crew Last 90 Days 119 Flight Crew Total 17125 Flight Crew Type 119 |
| Person 2 | |
| Function | First Officer Pilot Flying |
| Experience | Flight Crew Last 90 Days 90 Flight Crew Total 7500 Flight Crew Type 90 |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Track / Heading All Types |
Narrative:
At FL410 we were issued a descent to FL300 95 NM east of darby intersection. At the time we were about 145 NM east of darby. We entered a 'place bearing distance' (pbd) into the FMC as 'darby/-95.' at this time the course line to the pbd shifted about 8 degrees to the left. The center controller (ZID) noticed it immediately and told us that he thought we would fly south of darby if we remained on that course. The CDU showed us direct to that pbd then darby. The FMC course line always shifts when a pbd is entered on a direct course. This time it was excessive. It is the first time ATC ever questioned me about it. Later on we were cleared to cross 35 NM east of darby at FL240; this time the course shift was only 2 degrees. I believe the course line should not shift at all. If this shift is normal; could you please tell us how many degrees of shift we should expect and at what distances. This 8 degree course change was unexpected and was caught by a sharp controller almost at once. The airbus FMC always changes the course line. What's the best way to compensate for this anomaly?
Original NASA ASRS Text
Title: A300 flight crew reports that the FMC will routinely place a down track fix off the direct course line; causing a track deviation. The error is normally small but in this case the deviation is noticed by ZID.
Narrative: At FL410 we were issued a descent to FL300 95 NM east of DARBY Intersection. At the time we were about 145 NM east of DARBY. We entered a 'Place Bearing Distance' (PBD) into the FMC as 'DARBY/-95.' At this time the course line to the PBD shifted about 8 degrees to the left. The Center Controller (ZID) noticed it immediately and told us that he thought we would fly south of DARBY if we remained on that course. The CDU showed us direct to that PBD then DARBY. The FMC course line always shifts when a PBD is entered on a direct course. This time it was excessive. It is the first time ATC ever questioned me about it. Later on we were cleared to cross 35 NM east of DARBY at FL240; this time the course shift was only 2 degrees. I believe the course line should not shift at all. If this shift is normal; could you please tell us how many degrees of shift we should expect and at what distances. This 8 degree course change was unexpected and was caught by a sharp controller almost at once. The Airbus FMC always changes the course line. What's the best way to compensate for this anomaly?
Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.