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A320 captain experiences an FMC/GPS position disagree at FL290 at the same time ATC informs the crew they appear to be 500 ft low. After three minutes the ECAM disappeared; and center informed the crew they were back on altitude; and the TCAS showed all traffic in the vicinity back at cruise altitudes.
| Attributes | |
| ACN | 978938 |
| Time | |
| Date | 201111 |
| Local Time Of Day | 1201-1800 |
| Place | |
| Locale Reference | ZZZ.ARTCC |
| State Reference | US |
| Aircraft 1 | |
| Make Model Name | A320 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Cruise |
| Flight Plan | IFR |
| Component | |
| Aircraft Component | GPS & Other Satellite Navigation |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Experience | Flight Crew Last 90 Days 200 Flight Crew Total 12000 Flight Crew Type 5000 |
| Events | |
| Anomaly | Aircraft Equipment Problem Critical Deviation - Altitude Overshoot |
Narrative:
At cruise; had a FMC/GPS position disagree come up on the ECAM; lasted for about 3 minutes. Difference in GPS latitude position of about 1.5 minutes; longitude was the same; didn't note what the GPS altitude read. Center calls us for an altitude check; says he was showing us 500 ft low; and TCAS also showed 500 ft low (compared to other cruise traffic in the area). All three altimeters never left FL290. Is this a known airbus glitch? Does the mode C normally get altitude input from the GPS? I see where this could have easily led to a TCAS RA with our aircraft reporting the wrong altitude; leading to an erroneous RA; and possibly having two aircraft both responding to RA's coming closer together;(worse yet; midair) instead of moving apart. After the ECAM disappeared; center said he showed us back on altitude; and the TCAS showed all traffic in the vicinity back at cruise altitudes.
Original NASA ASRS Text
Title: A320 Captain experiences an FMC/GPS position disagree at FL290 at the same time ATC informs the crew they appear to be 500 FT low. After three minutes the ECAM disappeared; and Center informed the crew they were back on altitude; and the TCAS showed all traffic in the vicinity back at cruise altitudes.
Narrative: At cruise; had a FMC/GPS position disagree come up on the ECAM; lasted for about 3 minutes. Difference in GPS latitude position of about 1.5 minutes; longitude was the same; didn't note what the GPS altitude read. Center calls us for an altitude check; says he was showing us 500 FT low; and TCAS also showed 500 FT low (compared to other cruise traffic in the area). All three altimeters never left FL290. Is this a known Airbus glitch? Does the Mode C normally get altitude input from the GPS? I see where this could have easily led to a TCAS RA with our aircraft reporting the wrong altitude; leading to an erroneous RA; and possibly having two aircraft both responding to RA's coming closer together;(worse yet; midair) instead of moving apart. After the ECAM disappeared; Center said he showed us back on altitude; and the TCAS showed all traffic in the vicinity back at cruise altitudes.
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.