Date: 201210
Local time of day: 1201-1800
Location: PHX.Airport, AZ
Flight conditions: VMC
Light: Daylight
Aircraft: Citation Excel (C560XL)
Operator: Fractional
Operating under: Part 91
Flight phase: Descent
Airspace: Class E PHX
ATC / advisory: TRACON P50
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Confusion; Distraction; Troubleshooting
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC Overrode Automation
Contributing factors: Procedure; Human Factors; Aircraft
Primary problem: Ambiguous

Narrative: On the MAIER FIVE ARRIVAL to PHX we were descending via the arrival. We were originally assigned Runway 26; but were later given our preferred runway; 25L around HUBUB on the arrival. The autopilot was engaged in long range NAV and the VNAV was selected to make the crossing restrictions on the arrival. The altitude selector was set to 7;000 FT; the lowest altitude on the arrival. I was busy setting up the new runway assignment to 25L when ATC called and asked if we were going to make 9;000 FT at EDDNA. When I looked up at the instruments I observed that the Flight Director was still set on Long Range NAV and the VNAV was still engaged as well as the autopilot; however the aircraft was not descending on the computed glide path. The aircraft was still descending; but not at the rate needed to make the crossing restriction at EDDNA. I indicated that we would likely not make the restriction at EDDNA. ATC immediately gave us a turn to the left and a new altitude assignment with which we complied. The rest of the flight was uneventful. Something similar had happened earlier in our tour descending in to Vancouver BC. I was also the non-flying pilot. When I took control of the aircraft for the approach briefing I saw that the aircraft was flying above the computed glide path. I immediately disengaged the VNAV and reprogrammed the VNAV to make the crossing restriction ahead of us (DIRECT to the altitude) and reengaged the VNAV. The automation worked as it was designed to and the descent rate increased to make the crossing restriction. I was unable to determine what had gone wrong with the VNAV in the first place as it had been working perfectly earlier and everything was still set correctly when it started to deviate from the programmed glide path. After that incident the VNAV worked perfectly again so I was inclined to write it off as some kind of undetermined operator error. I used the VNAV later that day to try to duplicate the error; but was unable to observe any fault with it. As I was also unable to observe any fault with the way it was being operated by the First Officer I finally decided that it must be an equipment malfunction and wrote up the discrepancy. I wish I had been flying when the malfunctions occurred as I may have been able to better determine the root cause with more certainty. The only way to prevent events like this is to make sure you are flying the aircraft even when the autopilot is engaged. The First Officer said earlier that he had not slept as well as he would have liked the night before and perhaps he should have called in fatigued. I know from personal experience that it is not always easy to evaluate when I am too tired to perform as necessary.




Synopsis: When the C560-XL's FMS appeared unlikely to comply with the imminent charted 9;000 MSL crossing at EDDNA on the MAIER RNAV STAR to PHX; ATC intervened and provided vectors and altitudes to complete the arrival. The aircraft's VNAV path control had shown an occasional tendency to err on previous flights but had alternately appeared to function as intended.