Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 7.5
Situational Awareness; Fatigue; Physiological - Other
ATC Issue All Types; Inflight Event / Encounter CFTT / CFIT
Human Factors; Airspace Structure; Company Policy
I was notified by management that I assigned a Aircraft X an altitude below the MVA (Minimum Vectoring Altitude). I take complete responsibility for this; and to my knowledge; I have never done anything similar in the past. The weather was coming in. I vectored Aircraft X for an ILS Approach and descended him to 9;000 feet. The MVA in that area is 8;900 feet. While on a base turn and just about to be cleared for the approach; Aircraft X requested a descent to 8;700 feet; which is the crossing altitude at the final approach fix. I approved it; turned and cleared him for the ILS at 8;700 feet. Aircraft X then turned faster than expected and I had to cancel his approach clearance and fix his intercept. Again; I assigned him 8;700 feet to maintain. I then cleared him again; he intercepted the localizer; and he landed without incident. I'm not really sure why I approved 8;700 feet initially. That is never an altitude that I assign to anyone. Aircraft X requested it; and for some reason I approved it. The only rationale I have is that I was extremely fatigued; and I believe that Aircraft X was the first aircraft I had talked to in hours. Again; this was my fault; and I will make sure that this never happens again under my watch. Return to prior mid shift procedures; and perhaps require 9 hours off between day/mid shifts. Every other shift has a 9 hour requirement; but the mid only has an 8 hour requirement. The new mid procedures also significantly increase fatigue. In this case; I have to attribute sleep deprivation as the main contributing cause.
COS TRACON Controller reported they descended an aircraft to an altitude below the Minimum Vectoring Altitude.