Date: 201210
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: Beechjet 400
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Contributing factors: Procedure; Aircraft
Primary problem: Ambiguous

Narrative: [We were] cleared to descend via the STAR by Approach. Pilot not flying dialed in 7;000 FT. I noticed altitudes were missing from the FMS and began to pull up the STAR from the iPad to review the altitudes. As I was doing so; the aircraft descended below 13;000 FT prior to the fix. We reached approximately 12;800 FT and ATC advised we were not supposed to descend below 13 before the fix then further cleared us to descend to 11;000 FT and continue the arrival. Nothing further occurred. We were confused by the missing altitudes on the FMS and were distracted and failed to catch the gradual descent. Altitudes/fixes were all confirmed prior to departure. Better vigilance to the current FMS may have reduced the likelihood of this event.


Second narrative: I was the Captain on aircraft. During this particular flight I was the pilot not flying. We were talking with Approach within 100 miles of our destination at the time of this incident. Shortly after receiving the ATIS I updated the FMS with the appropriate approach which was going to be the visual. So I selected the ILS to [Runway] 19 on the FMS; and verified it with the First Officer. Soon after this the Controller told us to descend via the STAR which we had in the FMS and previously verified. I also then armed the VNAV and selected 7;000 FT in the altitude pre-selector. I told the First Officer to grab the iPad because I wanted to verify that the final altitude was 7;000 FT on that particular arrival. As he handed me the iPad we noticed the plane began to descend out of 13;000 FT. At about 12;800 FT. the First Officer quickly disconnected the autopilot to begin pitching up; as the Controller came on the radio and advised that we were not to descend below 13;000 FT until after the first fix. The Controller then said we were to just maintain 11;000 until crossing the next fix. I read back 11;000 FT and selected 11;000 FT in the preselected and the plane leveled off at 11;000 and we continued to descend via the STAR. I am not sure why the airplane would have begun a descent from 13;000 FT if we had 7;000 FT selected in the altitude pre-selector and VNAV armed and the next crossing altitude was 11;000 FT at the second fix which was several miles ahead and we had not reached the top of descent anyway. The plane should have stayed at 13;000 FT until it was time to descend to cross the second fix at 11;000 FT.


Synopsis: A BE-400XP FMS with VNAV engaged dropped the 13;000 FT constraint and descended 200 FT before ATC and crew saw the descent to 7;000 FT begin which was set in the altitude alerter.