SF340 first officer experiences position uncertain message on FMS during arrival. A clearance direct...

Narrative:

Enroute to destination message flag occurred on FMS. Message stated position uncertain. ATC started giving heading vectors before problem needed to be solved. After several heading vectors from ATC; they gave us direct to an enroute waypoint. It was input into the FMS by the PNF and verified by PF. No message flags were shown on the FMS at that time. When initiated the auto pilot turned the aircraft to the right; opposite the direction of the waypoint. According to the FMS that was in fact the direction to the waypoint. As the crew was confirming this with other navigational sources ATC gave us a heading back towards the airport. Remainder of flight was continued without incident. The FMS location was not correct. Our situational awareness was slightly off due to the repeated vector turns from ATC as we were in IMC. We were about to question ATC about a vector when they offered one on their own. The FMS was written up upon landing for both the position uncertain and the incorrect positioning during the flight.

Original NASA ASRS Text

Title: SF340 First Officer experiences POS UNCERTAIN message on FMS during arrival. A clearance direct to a waypoint results in aircraft turning the wrong way. Vectors are provided to a normal landing.

Narrative: Enroute to destination message flag occurred on FMS. Message stated POS UNCERTAIN. ATC started giving heading vectors before problem needed to be solved. After several heading vectors from ATC; they gave us direct to an enroute waypoint. It was input into the FMS by the PNF and verified by PF. No message flags were shown on the FMS at that time. When initiated the auto pilot turned the aircraft to the right; opposite the direction of the waypoint. According to the FMS that was in fact the direction to the waypoint. As the crew was confirming this with other navigational sources ATC gave us a heading back towards the airport. Remainder of flight was continued without incident. The FMS location was not correct. Our situational awareness was slightly off due to the repeated vector turns from ATC as we were in IMC. We were about to question ATC about a vector when they offered one on their own. The FMS was written up upon landing for both the POS uncertain and the incorrect positioning during the flight.

Data retrieved from NASA's ASRS site as of April 2012 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	853882
Time
Date	200909
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	SF 340B
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	First Officer
Qualification	Flight Crew Commercial
Experience	Flight Crew Last 90 Days 5 Flight Crew Total 2500 Flight Crew Type 1500
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Clearance Deviation - Track / Heading All Types

SF340 first officer experiences position uncertain message on FMS during arrival. A clearance direct to a waypoint results in aircraft turning the wrong way. Vectors are provided to a normal landing.