BE30 CAPTAIN HAS ALT DEV WHILE DIAGNOSING LNDG GEAR PROBLEM.

Narrative

WHILE ON AN IFR FLT; I WAS THE CAPT OF A KING AIR 300 THAT EXPERIENCED A LNDG GEAR FAILURE DURING A VISUAL APCH TO RWY. 7 MI FROM TOUCHDOWN; THE GEAR WAS SELECTED TO THE DOWN POS AND NOTHING HAPPENED. WE IMMEDIATELY ADVISED TWR OF THE SITUATION AND WE WERE CLRED TO 3000 FT ON A HDG OF 240 DEGS. THE PLANE WAS BEING HAND FLOWN AT THE TIME. UPON HEARING THE NEW CLRNC; I THOUGHT I HAD REACHED DOWN TO ENGAGE THE AUTOPLT WHILE MY COPLT AND I ATTEMPTED TO RESOLVE OUR GEAR PROB. AFTER SEVERAL TRIES; IT WAS CLR THE GEAR WAS NOT GOING TO LOWER NORMALLY. DURING OUR EVALUATION OF THE PROB; APCH ASKED IF WE WERE HAVING TROUBLE HOLDING OUR ALT AND THAT WE WERE CLBING. I IMMEDIATELY DISCOVERED THE AUTOPLT NEVER ENGAGED AND THE ACFT WAS SLOWLY DRIFTING IN AN ASCENT. I REMEMBER SEEING 3400 FT WHEN I FORCIBLY PUSHED THE CTLS OVER AND REGAINED THE ASSIGNED 3000 FT. WE WERE THEN GIVEN A CLRNC TO 4000 FT ON A HDG OF 270 DEGS WHERE I REQUESTED A DIRECT RTE TO DIVERT TO ZZZ1 TO PREPARE FOR THE MANUAL EXTENSION PROCESS; LNDG; AND SUBSEQUENT REPAIRS. THE ALT EXCURSION LASTED LESS THAN 45 SECONDS FROM THE TIME I THOUGHT I HAD ENGAGED THE AUTOPLT AND RECEIVING A CALL FROM ATC ON THE ALTDEV; HOWEVER; IT WAS STILL AN EXCURSION. IN MY JUDGEMENT; DECLARING AN EMER WAS NOT NECESSARY. WE TRAIN ANNUALLY. ALTHOUGH TRAINING DOES HELP; IT NEVER COMPLETELY READIES YOU FOR THE ACTUAL EVENT IN YOUR OWN ACFT. THERE IS THAT SENSE OF CONCERN -- WILL MY SYS WORK AS ADVERTISED HAVING NEVER HAD TO LOWER THE GEAR MANUALLY IN 21 YRS OF FLYING THE SAME ACFT?