A DSNDING LEAR 55 FLC FAILS TO MAKE THEIR ASSIGNED LEVELOFF ALT IN TIME TO AVOID A LOSS OF TFC SEPARATION WITH OPPOSITE DIRECTION TFC ABOVE. CREW IS CITED BY ZID; IN.

Narrative

WHILE ENRTE TO BNA; ATC CLRED US TO DSND FROM FL390 TO FL350. THE AUTOPLT WAS ALREADY ENGAGED AND BEGAN A NORMAL DSCNT TO FL350. THE AUTOPLT CONTINUED TO DSND AT AN APPROX RATE OF 1500 FPM. WITHOUT OUR AWARENESS; THE ACFT STOPPED ITS DSCNT AND STARTED A SLIGHT CLB AT 35700 FT. WHEN WE REALIZED THIS THE PF USED AUTOPLT NOSE DOWN TRIM TO PUT THE ACFT BACK INTO A DSCNT. THEN ATC QUESTIONED OUR DSCNT RATE AND WE INFORMED HIM OF A POSSIBLE AUTOPLT MALFUNCTION AND THAT WE ARE CONTINUING OUR DSCNT TO FL350. AT THIS TIME; ATC POINTED OUT CONVERGING TFC AT FL370 AND TOLD US NO DELAY ON YOUR DSCNT AND TO RPT LEVEL AT FL350. WE COMPLIED AND CONTINUED TO ADD MORE NOSE DOWN AUTOPLT TRIM. THE ACFT DID START TO DSND; BUT AGAIN BEGAN TO LEVEL OFF PREMATURELY AROUND FL352. AT THIS TIME; THE PF DISENGAGED THE AUTOPLT AND MANUALLY LEVELED THE ACFT AT FL350. DURING THIS EVENT; ATC REPEATEDLY ASKED IF WE WERE LEVEL AND WE RESPONDED EACH TIME WITH A NEGATIVE. ONCE WE WERE LEVEL AT FL350 WE NOTIFIED ATC. WE DID HAVE VISUAL CONTACT ON THE TFC AND ALSO HAD THE TARGET ON TCASI. WHEN THE TFC PASSED OVER US; THE TCASI INDICATED THAT THE TARGET WAS 1800 FT ABOVE OUR POS. OUR TCASI ALARM NEVER ENGAGED DURING THIS CHAIN OF EVENTS. SHORTLY AFTER THIS OCCURRED; ATC INFORMED US OF A POSSIBLE PLTDEV AND INSTRUCTED US TO CALL A CERTAIN NUMBER ONCE ON THE GND. SUPPLEMENTAL INFO FROM ACN 475518: CONTRIBUTING FACTORS: DID NOT DETECT PROB FAST ENOUGH. NOT AGGRESSIVE ENOUGH ON TRIM INPUT TO COMPLY WITH ATC COMMAND. SHOULD HAVE BEEN LESS CONCERNED WITH PAX COMFORT AND MORE AGGRESSIVE.