Light Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Distraction; Human-Machine Interface
Aircraft Equipment Problem Critical; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC Overrode Automation; Flight Crew Took Evasive Action
Procedure; Environment - Non Weather Related; Aircraft; Weather
ILS Runway 26 into RIL. We utilized VNAV function to descend via the ARC from COMFR to the final approach course. As the aircraft began to descend the pilot not flying noticed first that the FMS had sequenced early as in it would be performing a 'smart descent'. I then disengaged the automation and manually leveled the aircraft off and initiated a climb. As we passed the fix JIGOM we were 200 feet low. The remainder of the ILS approach was in VFR conditions with no disruption to our customer's arrival into RIL. Upon clearing the runway and canceling IFR the ATC Controller made no mention of any issues. I suggest not using VNAV on a feeder route or an ARC on an instrument approach. I recommend sticking to the tried and true method of selecting VS and performing the step downs. Yes VNAV would create a smoother more comfortable environment for our customers. However as we see; it is not reliable for this purpose.
I believe the sequencing of the Honeywell avionics on the VNAV function got confused as our base for the descent prior to the ILS portion of the approach was too close. I suggest crews not to use VNAV for vertical navigation on this approach.
A Fractional crew on the RIL 26 ILS was 200 FT low at the JIGOM crossing restriction after the First Officer disconnected the autopilot to slow the descent because the FMS for unknown reasons disregarded the constraint.