ALTDEV ALT OVERSHOT IN DSCNT.

Narrative

I SHOWED UP FOR WORK 1 HR BEFORE DEP TIME EXPECTING TO BE A PAX ON A TRIP TO PHL TO PICK UP MY FLT GOING TO JAX. I HAD BEEN FLYING OUR FALCON 900'S WITH LOTS OF FMS'S AND OTHER AUTOMATED GOODIES. IN FACT I WAS A DEAD HDG CREWMEMBER TO PHL TO CONTINUE AS CAPT ON A FALCON 900 TO JAX. HOWEVER; I WAS INFORMED I WOULD BE THE CAPT ON THE FALCON 50 GOING TO PHL FOR PURPOSES OF OUR COMPANY STANDARDIZATION OFFICER SEEING HOW I WORK WITH OTHER CREWMEMBERS. THIS CHANGE IN STATUS WAS KNOWN THE NIGHT BEFORE BY THESE FOLKS BUT THEY DIDN'T SEE THE NEED TO LET ME KNOW. SO I REVIEWED THE WX AND FLT PLANNING AND 'TKOF AND LAND' CARD FROM THE PREVIOUSLY ASSIGNED CAPT; ACCEPTED THE FLT; AND WE PROCEEDED ON OUR WAY. THE OPERATIONAL 'FLY IN THE OINTMENT' WAS A BARE VOR/DME INSTALLATION IN THE FLACON 50. DUE TO A DELAY IN BLACK BOX CERTIFICATION THIS FLACON 50 HAD NO LONG RANGE NAV; NO RNAV; JUST THE VOR/DME ON AIRWAY CAPABILITY. I DID MANAGE TO BRIEF MY EXCELLENT COPLT TO NOT ACCEPT ANY 'DIRECT TO'S' TO INTXNS; AND TO ASK FOR HDGS WHEN GIVEN DIRECT TO DISTANT VORS. YET; I DON'T BELIEVE I WAS MENTALLY PREPARED FOR THE INCREASED WORKLOAD THIS GAVE ME AND MORE PRECISELY; THE CHANGE IN TYPE OF WORKLOAD. AS WE STARTED OUR DSCNT TO MAKE XING RESTRS IN ICING LAYERS I FAILED TO SELECT THE ALT PRESELECT BUTTON ON THE AUTOPLT. THE FALCON 900'S I HAD BEEN FLYING OF COURSE DON'T NEED TO HAVE A BUTTON PUSHED TO ARM THE PRESELECT. WITH 800 WT TO GO I SAW THE ERROR AND HIT THE BUTTON; BUT THE POOR AUTOPLT DIDN'T REALLY HAVE A PRAYER BECAUSE OF MY DSCNT RATE TO MAKE THE RESTR WITH PWR UP FOR THE ICE AND BOARDS OUT TO KEEP US SLOWED DOWN. THIS IS A VERBAL DESCRIPTION OF BEING BEHIND THE AIRPLANE. I SAW THE FAILURE TO CAPTURE; DISCONNECTED THE AUTOPLT; AND LEVELED IT BACK UP TO FL200. I HAD GONE BELOW THE ALT BY 400 FT. THE GOOD NEWS IS I MADE THE RESTR. IN THE FUTURE I WILL RECOGNIZE THE NEED FOR A MENTAL ADJUSTMENT WHEN SWITCHING INTO A NON AUTOMATED COCKPIT. IN THE FUTURE I WILL SHRUG OFF A SLIGHT ATTITUDE PROB I PROBABLY CARRIED INTO THE COCKPIT ABOUT A CHANGE IN PLANS WHICH I WAS NOT INFORMED OF. IN THE FUTURE I SHOULDN'T HAVE TO WRITE THIS STORY AGAIN.