A CFI AND HIS STUDENT TAKE A PA28-161 ON AN IFR TRAINING FLT WITH A XPONDER KNOWN TO HAVE HAD INTERMITTENT FAILURES IN THE MODE C. FLT CREATES EXTRA WORKLOAD FOR APCH CTLR AT BPT; TX.

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RPTR IS A CFII CONDUCTING INST FLT TRAINING WITH A PP- SEL; WHOSE VISION WAS RESTR BY FOGGLES AND WHO MANIPULATED CTLS FOR ENTIRE FLT. THE FLT WAS ON AN IFR FLT PLAN FROM T41 TO BPT AND RETURN; WITH 2 APCHS CONDUCTED AT BPT. WX WAS VFR (3500 FT SCATTERED; PLUS 10 MI VISIBILITY). THE ACFT WAS A PA28-161 RENTED FROM A LCL FBO; AT WHICH RPTR IS A PART-TIME EMPLOYEE. THE XPONDER ON THIS ACFT WAS OCCASIONALLY INTERMITTENT IN BOTH MODE A AND MODE C. IT HAD FREQUENTLY AND RECENTLY BEEN TAKEN TO AN AVIONICS SHOP FOR REPAIR. DURING THE WK PRECEDING THE INCIDENT IN QUESTION; BOTH THE RPTR AND THE STUDENT HAD EXPERIENCED INTERMITTENT XPONDER FAILURE IN THIS ACFT. BOTH INSTANCES INVOLVED MODE C ONLY; WERE TRANSIENT; AND OCCURRED IN THE SAME REGION AROUND BPT. THE RPTR AND STUDENT FILED THE IFR FLT PLAN TO BPT AND RETURN AFTER WEIGHING THE FOLLOWING POINTS CONCERNING POTENTIAL XPONDER PROBS: 1) MOST RECENT FAILURES SEEMED TO BE INTERMITTENT AND TRANSIENT; LASTING ONLY ABOUT 5 MINS. 2) FAILURES SEEMED TO INVOLVE ONLY MODE C. 3) FLT WAS CONDUCTED UNDER 'SEVERE CLR' CONDITIONS; PERMITTING REVERSION TO VFR OPS AT ANY TIME. 4) XPONDER WAS REPORTEDLY REPAIRED SINCE LAST INCIDENT; BUT THIS RPT WAS NOT VERIFIED. 5) 'REPLY' LIGHT WAS KNOWN TO BE OPERATING ON FLT PRECEDING THIS ONE. THE FLT PROCEEDED UNEVENTFULLY FROM T41 TO BPT. WHEN BEING VECTORED FOR THE FIRST APCH AT BPT; THE MODE C PORTION OF THE XPONDER FAILED. SEVERAL ATTEMPTS TO RESET THE UNIT BY CYCLING THE PWR SWITCH DID NOT RESTORE MODE C. REMAINDER OF TRAINING FLT WAS CONDUCTED WITH MODE A ('PRIMARY RETURN ONLY;' AS DESCRIBED BY BPT APCH) ON IFR FLT PLAN. ALT WAS WELL MAINTAINED AND RPTED APPROPRIATELY. RPTR WAS TOLD TO TELEPHONE HOUSTON APCH CTL ON THE GND AFTER RETURN TO T41. THE RPTR DID SO. THE MGR AT APCH HAD BEEN ALERTED BY THREE FAILURES IN THE SAME ACFT; AT THE SAME PLACE; IN 1 WK. HE STRONGLY SUGGESTED THAT ANOTHER TRIP TO THE AVIONICS SHOP WAS IN ORDER. THE RPTR AGREED. CONCLUSIONS: THE RPTR THINKS THAT THE DECISION TO CONDUCT THE FLT WAS OK; BUT NOT CONSERVATIVE. THERE WAS REASON TO BELIEVE THAT XPONDER DIFFICULTIES WOULD NOT RECUR AND IF THEY DID; NO RISK TO THE FLT OR TO OTHER FLTS WOULD RESULT EVEN UNDER THE WORST CIRCUMSTANCES (ALTHOUGH SLIGHTLY GREATER WORKLOAD WAS REQUIRED FROM BOTH OUR FLT AND ATC). THE RPTR CERTAINLY WISHES THAT THE INCIDENT HAD NOT OCCURRED. HE ALSO FEELS THAT ATC HANDLED THE SIT WELL. BOTH AT BPT AND HOU; THE PERSONNEL WERE HELPFUL AND COURTEOUS; PERMITTING THE FLT TO CONTINUE EVEN WITH AN ENHANCED WORKLOAD. THE MGR AT HOUSTON APCH; BY HIS MANNER ON THE TELEPHONE TO THE RPTR; ASSURED THAT THE SIT WOULD BE RESOLVED.