FO OF A DEHAVILLAND DASH 8-102 (DH8) OVERSHOT ASSIGNED DSCNT ALT AND CORRECTED BACK AT ONCE. THE OVERSIGHT WAS POSSIBLE DUE TO THE FAILURE OF THE AUTOPLT TO MAINTAIN THE CAPTURE MODE AFTER ANOTHER MODE IS SELECTED.

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WE HAD BEEN CLRED DOWN TO 5000 FT (OUT OF 6000 FT; I BELIEVE) AND WERE IN THE DSCNT. I HAD CALLED OUT '1000 FT TO GO' OUT OF 6000 FT PER OUR POH. I WAS AT THE TIME OF THE DEV 'INSIDE' THE FLT DECK; WHEN I SUDDENLY FELT OUR LOAD FACTOR INCREASE SIGNIFICANTLY. I LOOKED UP AND OUTSIDE; SAW NOTHING UNUSUAL; THEN LOOKED AT THE ALTIMETER; FOLLOWED BY THE ALT ALERTER. WE WERE JUST ABOVE 4500 FT; AND THE FO WAS CORRECTING BACK TO OUR ASSIGNED AND SET ALT OF 5000 FT. I'M NOT COMPLETELY CERTAIN; BUT I BELIEVE THAT THE AUTOPLT WAS ENGAGED AND OPERATING THE CTLS. THE FO QUICKLY RETURNED TO OUR ASSIGNED ALT; AND WE HEARD NOTHING FROM ATC. THE FO THEN SMIRKED AND SAID 'I'VE NEVER DONE THAT BEFORE!' TO WHICH I RETORTED 'AND YOU'LL NEVER TO IT AGAIN.' MY FO HAS ONLY BEEN WITH US FOR A FEW MONTHS; BUT FROM THE 2 DAYS THAT I FLEW WITH HIM SEEMED VERY COMPETENT AND PROFESSIONAL; AND NOT GIVEN TO LAXITY OR SLOPPINESS; AND HE HAS A CONSIDERABLE BACKGND FLYING LARGE; COMPLEX ACFT. STILL; HE IS RELATIVELY NEW TO THE DASH 8; WHOSE AUTOPLT HAS THE NASTY HABIT OF ELIMINATING THE ALT SELECT FUNCTION IF CERTAIN CHANGES ARE MADE WHEN THE AUTOPLT IS 'CAPTURING' THE ALT. I'M NOT CERTAIN THAT THIS IS WHAT HAPPENED; BUT IT'S A COMMON SIT; WHICH ONLY REQUIRES A MOMENTARY DISTR TO MISS NOTICING ITS OCCURRENCE. IT'S ALSO POSSIBLE THAT THERE WAS A SYS MALFUNCTION; BUT THE AUTOPLT HAD AND CONTINUED TO OPERATE NORMALLY. THE SOLUTION; OF COURSE; IS TO CAREFULLY MONITOR THE AUTOPLT AS IT CAPTURES THE ALT; WHICH IS THE NORMAL PROC; BUT AS I SAID ABOVE; THIS IS ONE OF THOSE SITS WHERE EVEN A VERY BRIEF DISTR CAN BITE YOU.