The first officer aboard a ce-680 repeatedly reminded his captain to maintain their assigned altitud...

Narrative:

Atlanta approach cleared us to join the localizer 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/south mode. As the airplane made the turned to join the localizer we were still descending. At about 3;200 ft the autopilot (a/P) vertical mode changed to asel. The PIC reselected vertical speed (V/south). The a/P went back into asel; and then alt mode. At about 3;050 ft the PIC reselected V/south. 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/south 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1132195
Time
Date	201311
Local Time Of Day	0601-1200
Place
Locale Reference	FTY.Airport
State Reference	GA
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Cessna Citation Sovereign (C680)
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	STAR DIFFI
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Flight Deck / Cabin / Aircraft Event Other / Unknown

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.