Medium Large Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (mon) 7
Situational Awareness; Confusion; Distraction
ATC Issue All Types; Airspace Violation All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Procedure; Human Factors; ATC Equipment / Nav Facility / Buildings
Aircraft X was inbound to GTF from the south. We were in CENRAP; and I had the appropriate CENRAP map selected. I descended Aircraft X to 6000 ft in an 8500 ft MVA (that is 6000 ft in normal operations). This occurred approximately 19 miles from the airport. 12 miles from the airport; the aircraft reported the field in sight; and I cleared them for the Visual Approach. I do not know the altitude the aircraft was at when I cleared them; but a minute or two later; the local controller noticed Aircraft X indicated 7000 ft on their Mode C; which could not be validated due to CENRAP. It was then I realized I had issued the descent to 6000 ft in the 8500 ft MVA.Improved radar to maintain standard operations at all times. Also; MVA's that do not contradict one another based on CENRAP or standard operations.
GTF TRACON Controller describes where he descended an aircraft below the MVA while working under CENRAP conditions. Under normal radar conditions he would have been alright with the altitude descent.