POTENTIAL CONFLICT ENSUES AFTER PARTIAL LOSS OF ACFT CTL DURING FLC TROUBLESHOOTING OF AN ACFT EQUIP MALFUNCTION.

Narrative

I SERVED AS CAPT AND PIC OF AN AIRLINE FLT ON MAY/WED/93 FROM SDF TO BHM. THE FLT PROCEEDED NORMALLY UNTIL JUST S OF THE BWG VOR. WE RECEIVED AN INSTRUCTION TO CLB AND MAINTAIN 24000 FT. I ACCOMPLISHED THE CLB USING THE VNAV FEATURE OF THE AUTOPLT. THE AUTOPLT INDICATED A CAPTURE; BUT LEVELED THE ACFT AT 23900 FT. THE ACFT; STILL UNDER AUTOPLT CTL; THEN BEGAN AN UNCOMMANDED DSCNT AT A RATE OF APPROX 300 FPM. I ATTEMPTED TO CORRECT THE DSCNT USING THE VERT SPD FUNCTION OF THE AUTOPLT SELECTING 300 FPM UP. THE ACFT DID NOT RESPOND. I THEN GENTLY PULLED UPON THE YOKE. THE ACFT THEN CAPTURED A SECOND TIME AND LEVELED OFF AT 24000 FT. SHORTLY THEREAFTER; WE RECEIVED A CLRNC TO CLB AND MAINTAIN 29000 FT. I ACCOMPLISHED THE CLB USING THE VNAV FUNCTION OF THE #1 AUTOPLT. THE AUTOPLT INDICATED A CAPTURE BUT LEVELED AT 28900 FT. THE ACFT THEN BEGAN AN UNCOMMANDED DSCNT AT APPROX 300 FPM. I AGAIN ATTEMPTED TO CORRECT THIS DSCNT USING THE VERT SPD FUNCTION OF THE AUTOPLT #1. I INITIALLY SELECTED 300 FPM UP. THE ACFT DID NOT RESPOND. I THEN INCREASED THE VERT SPD COMMAND TO 800 FPM UP WITH STILL NO RESPONSE. I IMMEDIATELY PULLED BACK ON THE YOKE TO CORRECT THE DSCNT. THE ACFT THEN BEGAN TO CLB TOWARD 29000 FT. THEN THE ACFT BEGAN TO PITCH UP ABRUPTLY AT ABOUT 2000 FPM. I THEN DISCONNECTED THE AUTOPLT; PUSHED FORWARD HARD ON THE YOKE AND ARRESTED THE CLB. WE GOT A TCASII TA PERTAINING TO ANOTHER ACR FLT. WE HAD THE ACFT IN SIGHT. HE WAS BEHIND US; ABOVE AND ON A PARALLEL COURSE. I DO NOT KNOW HOW THIS HAPPENED. I FEEL STRONGLY THAT WE HAD AN AUTOPLT MALFUNCTION. SUPPLEMENTAL INFO FROM ACN 241488: WE TURNED OFF THE AUTOPLT TO HAND FLY THE ACFT AND THE ALT WINDOW WAS CHANGED FROM 290 TO 300 (BY ACCIDENT HIS HAND MAY HAVE BRUSHED THE WHEEL MOVING IT 1 DIGIT). THE NEXT THING I KNOW WE'RE FLYING THROUGH 290 ON THE WAY TO 300 (WHICH DIDN'T MAKE SENSE; BUT I WAS JUST GETTING OFF THE COMPANY RADIO AND THOUGHT I MISSED SOMETHING) AND ON OUR WAY TO FL305 AND I KNEW THERE WAS TFC AT 310. I ASK ATC IF WE'RE SUPPOSED TO BE AT 290 OR 300. THE TCASII IS GOING OFF 'MONITOR VERT SPD' AND THE CAPT IS TRYING TO GET THE ACFT BACK DOWN TO 290 WITHOUT THROWING EVERYONE OUT OF THEIR SEATS. THIS ALL HAPPENED IN WHAT SEEMED WAS SECONDS. THE AUTOPLT MUST HAVE HAD SOME TYPE OF PROB THAT IT WOULDN'T CAPTURE ON ALT. ALSO A TRIM OUT LIGHT WAS ON EARLIER. ITS POSSIBLE IT WAS OVER TRIMMED NOSE UP WHICH CAUSED IT TO CLB AT SUCH AN EXCESSIVE RATE. IF THE CAPT HAD NOT TURNED OFF THE AUTOPLT TO GET IT BACK DOWN TO FL290 THERE IS NOT TELLING HOW HIGH THE ACFT WOULD HAVE CONTINUED TO CLB. CORRECTIVE ACTION: WE SHOULD HAVE SWITCHED AUTOPLTS THE FIRST TIME THIS HAPPENED; SHOULD HAVE TURNED OFF AUTOPLT ONCE IT DIDN'T RESPOND THE WAY WE WANTED. I SHOULDN'T HAVE CALLED THE COMPANY ON RADIO (BEING OUT OF LOOP) UNTIL AT FINAL CRUISE ALT.