ALTDEV ALT EXCURSION.

Narrative

THE CAPT INITIATED THE HOLDING PATTERN ENTRY. I ALERTED HIM THAT WE HAD NOT CROSSED THE FIX. AS HE WAS INTERRUPTING THE AUTOMATIC FLT CTL SYS TO OVERRIDE THE AUTOPLT'S TURN THE AUTOPLT DISCONNECTED. THE ACFT DSNDED APPROX 1000 FT BEFORE THE CAPT RECOVERED. CONFUSION OVER WHEN TO START THE HOLDING ENTRY RESULTED IN PART; BECAUSE THE CAPT'S DATA IS DISPLAYED FROM THE FLT MGMNT SYS WHILE THE COPLT DISPLAY IS VOR BASED ONLY. THE CAPT'S DISPLAY INDICATED STATION PASSAGE WHILE MY SIDE DID NOT. OTHER CONTRIBUTING FACTORS. 1) WHEN THE AUTOPLT DISCONNECTED THERE IS ONLY A MOMENTARY LIGHT ON THE ANNUNCIATOR PANEL WITH NO WARNING TONE OR MASTER CAUTION LIGHT. 2) WHEN THE AFCS IS INTERRUPTED THE COMMAND BARS ON THE ADI STAY ON THE MINIATURE ACFT EVEN IF ALT HOLD IS SELECTED THEREFORE THEY DON'T INDICATE VARIATIONS IN SELECTED ALT. 3) COPLT'S ALTIMETER IS SEPARATE FROM THE AIR DATA COMPUTER AND INDICATES HIGHER THAN THE CAPT'S ALTIMETER AT THE SAME ALTIMETER SETTING. ALTHOUGH ALT CORRECTION WAS INITIATED WHEN THE ALT ALERTER CHIMED AT 100 FT LOW THE ACFT LOST A CONSIDERABLE AMOUNT OF ALT IN THE CONFUSION. CREW COORD SHOULD HAVE RESULTED IN CONFIRMATION OF STATION PASSAGE PRIOR TO INITIATING THE TURN. THIS WOULD HAVE COMPENSATED FOR THE DIFFERENCE IN THE RNAV (FMS) INDICATIONS AND THE VOR.