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AA5 pilot at 8500 feet reported being informed by ZAB that his mode C showed 9000 feet. The reporter believed that the problem was with ZAB.
| Attributes | |
| ACN | 1459661 |
| Time | |
| Date | 201706 |
| Local Time Of Day | 0601-1200 |
| Place | |
| Locale Reference | ZAB.ARTCC |
| State Reference | NM |
| Aircraft 1 | |
| Make Model Name | Cheetah Tiger Traveler AA5 Series |
| Operating Under FAR Part | Part 91 |
| Flight Phase | Cruise |
| Route In Use | Direct |
| Flight Plan | VFR |
| Component | |
| Aircraft Component | Altitude |
| Person 1 | |
| Function | Pilot Flying Single Pilot |
| Qualification | Flight Crew Instrument |
| Events | |
| Anomaly | ATC Issue All Types Aircraft Equipment Problem Less Severe |
Narrative:
ATC reported me as traffic to another aircraft; telling him I was at 9000 ft; when I was at 8500 ft. I had the correct altimeter setting; and cross checked against the mode C report display on my transponder; and it was reporting 8500 in corrected PA. (Altimeter setting was 29.91 so corrected and uncorrected should be close). Communicating to ATC; he was unconvinced there was something unusual happening; but I further cross checked against the pressure altimeter in my ipad mini4; and it agreed that I was at about 8500 ft. The GPS altitude was about 9000 ft; though; and I have ads-B out. After frequency change to la center; a very similar discrepancy seemed to occur between a different aircraft and ATC. Is it possible that ATC displays are showing altitudes that are sometimes derived from ads-B and sometimes from mode-C? That seems really dangerous under conditions where GPS altitude differs from corrected pressure altitude.
Original NASA ASRS Text
Title: AA5 pilot at 8500 feet reported being informed by ZAB that his Mode C showed 9000 feet. The reporter believed that the problem was with ZAB.
Narrative: ATC reported me as traffic to another aircraft; telling him I was at 9000 ft; when I was at 8500 ft. I had the correct altimeter setting; and cross checked against the Mode C report display on my transponder; and it was reporting 8500 in corrected PA. (Altimeter setting was 29.91 so corrected and uncorrected should be close). Communicating to ATC; he was unconvinced there was something unusual happening; but I further cross checked against the pressure altimeter in my iPad mini4; and it agreed that I was at about 8500 ft. The GPS altitude was about 9000 ft; though; and I have ADS-B out. After frequency change to LA center; a very similar discrepancy seemed to occur between a different aircraft and ATC. Is it possible that ATC displays are showing altitudes that are sometimes derived from ADS-B and sometimes from Mode-C? That seems really dangerous under conditions where GPS altitude differs from corrected pressure altitude.
Data retrieved from NASA's ASRS site 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.