Cessna Citation Sovereign (C680)
Flight Crew Air Transport Pilot (ATP)
Confusion; Human-Machine Interface; Training / Qualification
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Aircraft; Human Factors; Manuals
Atlanta Approach cleared us to join the LOC Runway 8 course at TIINI from the DIFFI2 STAR. While on the STAR; we were cleared to 3;000 FT; and the PIC selected V/S mode. As the airplane made the turned to join the LOC we were still descending. At about 3;200 FT the Autopilot (A/P) vertical mode changed to ASEL. The PIC reselected vertical speed (V/S). The A/P went back into ASEL; and then alt mode. At about 3;050 FT the PIC reselected V/S. I stated 'altitude' to indicate the potential deviation. The PIC allowed the airplane to descend to 2;850 FT; again I told the PIC he needed to climb. The PIC started to correct the deviation with the A/P; [and] then disconnected the A/P. The PIC corrected to 2;950 FT and reconnected the A/P. The airplane started a descent; (I stated that the plane was descending and he needed to climb) and the PIC disconnected the A/P. The airplane was allowed to descend to 2;750 FT before returning to the assigned altitude of 3;000 FT. Approach did not say anything; and switched us to the Tower shortly thereafter.After landing; I asked the PIC why he selected the V/S mode after the mode changed to ASEL; and he responded that he wanted to make sure that the autopilot captured the altitude. I think that this is a fundamental lack of understanding of the autoflight system on the part of the PIC. Though this was taught in initial there have been changes to the autoflight system (phase 5.2 i.e. autotransitioning; etc) that a review of the system to make those less comfortable with relying on and trusting the system the newer edition; and the new limits (i.e. the autotuning occurs between 25-30 nm from destination) may be helpful. I could have taken the controls. However; this endeavor did not seem needed as the PIC was responding to my verbal cautions as he attempted to recover to the correct altitude.
The First Officer aboard a CE-680 repeatedly reminded his Captain to maintain their assigned altitude as the Captain appeared uncertain of the autopilot vertical navigation functions.  Several modest deviations were initiated and arrested without comment from ATC.