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Small aircraft pilot on IFR flight cruises 200' low because mode C on transponder misreads 200' high, airborne conflict with VFR aircraft results.
| Attributes | |
| ACN | 152973 |
| Time | |
| Date | 199007 |
| Day | Sun |
| Local Time Of Day | 1201 To 1800 |
| Place | |
| Locale Reference | atc facility : sac |
| State Reference | CA |
| Altitude | msl bound lower : 6800 msl bound upper : 7000 |
| Environment | |
| Flight Conditions | VMC |
| Light | Daylight |
| Aircraft 1 | |
| Controlling Facilities | tracon : eug |
| Operator | general aviation : instructional |
| Make Model Name | Small Aircraft, High Wing, 1 Eng, Retractable Gear |
| Flight Phase | cruise other |
| Route In Use | enroute airway : zoa |
| Flight Plan | IFR |
| Aircraft 2 | |
| Make Model Name | Any Unknown or Unlisted Aircraft Manufacturer |
| Flight Phase | cruise other |
| Route In Use | enroute : direct |
| Person 1 | |
| Affiliation | Other |
| Function | flight crew : single pilot |
| Qualification | pilot : instrument pilot : commercial |
| Experience | flight time last 90 days : 54 flight time total : 6823 flight time type : 280 |
| ASRS Report | 152973 |
| Person 2 | |
| Affiliation | Other |
| Function | flight crew : single pilot |
| Qualification | pilot : private |
| Events | |
| Anomaly | aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : far non adherence : clearance |
| Independent Detector | other flight crewa |
| Resolutory Action | flight crew : took evasive action |
| Consequence | Other |
| Miss Distance | horizontal : 1500 vertical : 0 |
| Supplementary | |
| Primary Problem | Flight Crew Human Performance |
| Air Traffic Incident | Pilot Deviation |
Narrative:
Cleared from rbl VORTAC direct to eca VORTAC at 7000'. We had been advised earlier that our mode C readout was 200-400' high. Consequently, I was maintaining 100-200' below assigned altitude to keep controllers readout within their 'tolerance'. Controller called, 'traffic Y at 12 O'clock, opp direction indicating 6800'.' traffic Y was subsequently sighted at same altitude. I turned left approximately 30 degree and easily cleared it. Aircraft Y was apparently 300' above the appropriate quadrantal altitude for his direction of VFR flight, had we been in IMC conditions, I would have been flying with mode C disabled. Upon arriving at home base, the transponder was squawked and the recommendation made to have both the transponder system and the static pressure system rechked.
Original NASA ASRS Text
Title: SMA PLT ON IFR FLT CRUISES 200' LOW BECAUSE MODE C ON TRANSPONDER MISREADS 200' HIGH, AIRBORNE CONFLICT WITH VFR ACFT RESULTS.
Narrative: CLRED FROM RBL VORTAC DIRECT TO ECA VORTAC AT 7000'. WE HAD BEEN ADVISED EARLIER THAT OUR MODE C READOUT WAS 200-400' HIGH. CONSEQUENTLY, I WAS MAINTAINING 100-200' BELOW ASSIGNED ALT TO KEEP CTLRS READOUT WITHIN THEIR 'TOLERANCE'. CTLR CALLED, 'TFC Y AT 12 O'CLOCK, OPP DIRECTION INDICATING 6800'.' TFC Y WAS SUBSEQUENTLY SIGHTED AT SAME ALT. I TURNED L APPROX 30 DEG AND EASILY CLRED IT. ACFT Y WAS APPARENTLY 300' ABOVE THE APPROPRIATE QUADRANTAL ALT FOR HIS DIRECTION OF VFR FLT, HAD WE BEEN IN IMC CONDITIONS, I WOULD HAVE BEEN FLYING WITH MODE C DISABLED. UPON ARRIVING AT HOME BASE, THE XPONDER WAS SQUAWKED AND THE RECOMMENDATION MADE TO HAVE BOTH THE XPONDER SYS AND THE STATIC PRESSURE SYS RECHKED.
Data retrieved from NASA's ASRS site as of July 2007 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.