SMT PLT HAD ALTDEV IN ZMP CLASS E.

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WHILE IN A CRUISE CLB TO FL250; ZMP GAVE AN ALT RESTR OF 16000 FT. I ACKNOWLEDGED AND SET 16000 FT IN THE ALT PRESELECT AND ENGAGED THE ALT SELECT ON THE AUTOPLT. CLOUDS BASES AND TOPS WERE ABOUT 12000 FT AND 15000 FT RESPECTIVELY. WHILE IN IMC; CTR ADVISED OF TFC OPPOSITE DIRECTION AT 17000 FT. WHEN I BROKE OUT OF THE TOPS AT ABOUT 15000 FT; I BEGAN LOOKING FOR THE TFC; WHICH I NEVER DID SEE. I RECALL LOOKING AT THE ALT ALERT ANNUNCIATOR AT 15700 FT WHERE IT INDICATED THE AUTOPLT WAS GOING TO CAPTURE THE 16000 FT ON THE PRESELECT. SHORTLY AFTER THAT (SECONDS) I GOT A 'FAULT WARNING' LIGHT INDICATING A TRIM FAILURE. I BECAME DISTR BY THIS WARNING AND FAILED TO NOTICE THAT THE AUTOPLT HAD NOT CAPTURED ALT AT 16000 FT. MY RATE OF CLB WAS AT ABOUT 1700 FPM. BY THE TIME I REALIZED WHAT WAS HAPPENING AND DISCONNECTED THE AUTOPLT TO REVERSE THE CLBING TREND TO GO BACK DOWN TO 16000 FT; I SAW 16600 FT ON MY ALTIMETER. I PUSHED THE NOSE DOWN TO GO BACK TO 16000 FT. CTR CALLED TO SAY THERE HAD BEEN LOSS OF SEPARATION. THEY SAID THEY HAD 1 HIT SHOWING MY ACFT AT 16800 FT AND THE OTHER ACFT HAD ALREADY PASSED BY AT 17000 FT AND 2 OR 3 MI AT THE POINT OF LOSS OF SEPARATION. I WAS ADVISED TO CALL ZMP AND DID SO AS SOON AS I COULD. TEMPS ON THE GND WERE -5 DEGS F AND ABOUT -30 DEGS F AT 16000 FT. I BELIEVE THAT DUE TO THE SEVERE COLD AND THE HIGH RATE OF CLB; THE AUTOPLT WAS UNABLE TO TRIM TO HOLD OR CAPTURE THE SELECTED ALT. AFTER DISCONNECTING THE AUTOPLT I WAS ABLE TO RE-TRIM THE ACFT OR ELEVATOR; BUT IT WAS VERY STIFF AND DIFFICULT TO MOVE. IN RETROSPECT; I SHOULD HAVE DISENGAGED THE AUTOPLT AS SOON AS THE TRIM FAILURE WARNING APPEARED AND MANUALLY HELD THE ASSIGNED ALT. WE ARE CONSTANTLY TOLD DURING TRAINING EXERCISES TO 'FLY THE AIRPLANE' WHEN SOMETHING FAILS. I HAVE LEARNED THIS TO BE TRUE.