When a BE-400 flight diverted to TWF due to weather below minimums at their destination; an unanticipated late clearance for the VOR Runway 7 approach caused substantial confusion because their FMS database included only one of the two available VOR approaches for which they had approach plates. The one in their database; a VOR DME with arc transitions; was not viable due to their position well inside the transition arcs.

Narrative

I was conducting an enroute check ride for the flight crew. [Our destination] weather had dropped well below approach minimums. This was unforecasted and not known until the crew was running the descent and/or approach checklist(s). The flight crew had me call the company who said to proceed to the filed alternate; which was TWF. It was only about a 10 minute flight to the alternate and the crew was busy but managing effectively. The crew was instructed to descend to 7;000 FT and to expect a visual approach. At 7;000 FT we were in IMC conditions and about 5 NM from the TWF VOR. The crew notified ATC and they were assigned 'direct to the TWF VOR and cleared for the VOR 7 approach.' They selected the VOR7 approach in the FMS and were surprised when none of the transition fixes or IAFs that were on the approach plate were in the FMS. The VOR7 approach is the only VOR type approach in the FMS; although there is also a [published] VOR DME 7 approach at TWF. The two FMS database options available to the crew for transition fixes were ODCUY and D314J. D314J is a 10 DME arc. ODCUY does not exist on the VOR7 chart and there is no DME arc on the VOR7 chart. At this point we were IMC. The crew had the EGPWS display up and it was clear there was no immediate danger from terrain but there was plenty of terrain showing on the screen. The crew quickly started setting up raw data for the approach when we broke out into good VMC conditions and the airport was right in front of us; they made a normal visual approach from that point. After landing we did a thorough debrief of the flight and spent some time on this issue. The VOR DME Runway 7 approach has a 10 DME counterclockwise arc that begins at the 314 radial. This is D314J in a Collins box. On the SNAKO ONE departure ODCUY exists. It is at the intersection of the TWF 177 radial and the 10 DME arc. This same location is the beginning of a clockwise 10 DME arc on the VOR DME 7 approach; although it is not labeled as a fix. This same location is also HOLIT on the VOR Runway 7 approach. I notified safety and other operations employees about this issue and they have started working on it at the time I am writing this report.This safety issue was the result of a database error in the Collins AMS-5000 so there isn't much a crew can do to avoid this issue. The database was correct and current. Workload was high and the TWF Controller told them to expect a visual which didn't work out as well as expected. Then; this FMS issue further complicated matters. They didn't have the luxury of early planning. In a more normal scenario; the database issue would have been identified early and could have been dealt with. My recommendations in the short term are to identify if this is only a Collins issue. Identify if it applies to all Collins airplanes. There is a GPS 7 approach and an ILS25 approach; alert crews to try and utilize these approaches; as appropriate; and stay away from the VOR type approaches unless absolutely necessary. If flying a VOR type approach into TWF do not use the FMS. Fly the approach in raw data and possibly use the FMS and MFD for situational awareness.Without question this FMS coding issue started an error chain in a high workload environment. With continued IMC; closer proximity to terrain; and/or a less skilled crew this could have easily resulted in a CFIT accident.