A NEWLY TRAINED AIRLINE B737-200 FO; FLYING AS A B-1B RESERVE MIL PLT PIC; FAILS TO USE THE AUTOPLT FOR ALT HOLD FUNCTION AND ALLOWS THE ACFT TO SETTLE 400 FT LOW FROM ITS ASSIGNED ALT BEFORE CORRECTING BACK NEAR PUB; CO.

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THIS IS A HUMAN FACTORS EVENT CAUSED FROM FLYING MULTIPLE ACFT TYPES WITH DIFFERENT SYS. I RECENTLY COMPLETED TRAINING WITH AIRLINE ON THE B737-200 (1 WK AGO). THE B737 HAS AUTOTHROTTLES WHICH ARE ENGAGED ON TKOF AND STAY ON UNDER NORMAL CONDITIONS UNTIL LNDG. AT ALT; YOU ENGAGE THE AUTOTHROTTLES TO MACH HOLD AND LEAVE THEM THERE. UPON COMPLETION OF TRAINING AT AIRLINE; I TOOK SOME MIL LEAVE TO GO THROUGH TRAINING ON A NEW VERSION OF THE B-1B. THE B-1 RECENTLY RECEIVED SOME NEW UPGRADES; ONE OF WHICH IS AN FMS. I WAS FLYING MY FIRST RIDE IN THIS VERSION AFTER NOT HAVING FLOWN IN APPROX 6 WKS. THERE WAS A LOT OF 'HEADS DOWN TIME' LEARNING THE NEW FMS. WE HAD JUST COMPLETED SOME SIMULATED BOMB RUNS AT ALT BTWN HGO AND LAA 253039 AND WERE HDG DOWN TO TCC; NM. HAVING JUST FLOWN THE B737 ON IOE AND NOT FLOWN THE B-1; I WAS A BIT RUSTY. THE B-1 ALSO HAS AUTOTHROTTLES BUT THEY ARE USUALLY ONLY USED FOR MAINTAINING A CRUISE SPD. ANYWAY; I HOOKED UP THE AUTOPLT IN THE 'ALT HOLD MODE' WHICH IS 1 OF 3 PITCH MODES (IAS HOLD AND MACH HOLD ARE THE OTHERS; ONLY 1 PITCH MODE CAN BE ENGAGED AT ONCE). TO HOOK UP THE AUTOTHROTTLES IN THE B-1; YOU SHOULD PRESS 'AT ENGAGE.' ON MY FLT; THIS WAS NOT LIT. I ACQUIRED THE SPD I WANTED; DIDN'T SEE THE CORRECT BUTTON AND HIT 'MACH HOLD' JUST LIKE ON THE B737. THE ACFT DID EXACTLY WHAT I TOLD IT TO -- MACH HOLD VERSUS ALT HOLD. LONG STORY SHORT -- WE WERE HEADS DOWN WITH THE NEW FMS AND DSNDED ABOUT 400 FT LOW. ATC DIDN'T SAY ANYTHING AND I'M NOT SURE THEY EVEN NOTICED. BOTTOM LINE: IF YOU ARE FLYING MULTIPLE ACFT TYPES; BE CAREFUL -- ESPECIALLY IF YOU ARE MORE CURRENT IN ONE THAN THE OTHER AND WATCH THE HEADS DOWN TIME WITH FMS; ETC.