A fractional crew on the ril 26 ILS was 200 ft low at the jigom crossing restriction after the first...

Attributes
ACN	1137085
Time
Date	201312
Local Time Of Day	1201-1800
Place
Locale Reference	RIL.Airport
State Reference	CO
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	Light Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 91
Flight Phase	Descent Initial Approach
Flight Plan	IFR
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Crossing Restriction Not Met Deviation - Altitude Overshoot Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.

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.