EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Distraction
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Aircraft; Human Factors
I was Pilot Monitoring as we were being vectored on arrival. The First Officer had turned off the autopilot. I went heads down to program the FMS and pull up a message on ACARS.  Shortly thereafter I heard the altitude alert sound and the First Officer was correcting altitude to return to our assigned altitude.  He said he had looked at something else and didn't notice he was losing altitude. We immediately returned to altitude and no mention or query was made by ATC. This aircraft had a configuration where the FMS was full aft on the center pedestal which makes it impossible to program the FMS and also keep an eye on the PFD/MFD. I didn't notice the altitude loss until the horn went off. I debriefed the First Officer regarding obligations while hand flying.
EMB-145 Captain reports an altitude deviation by the First Officer who is hand flying on vectors for approach. The Captains attention was diverted by FMC work which is physically located on aft end of the center pedestal.