Narrative:

I was [training a developmental] on a radar session. The developmental is around 70 hours training on position and our level of traffic this session was below his ability level so I was allowing him to work the session with as little input from me as possible. We had a slow stream of arrivals with very few departures; at the time of the event we had; I believe; 1 aircraft on final already switched to tower and 2 other aircraft being vectored for the approach. There may have possible been a departure as well; but no aircraft were a factor with one another.aircraft X was a harei arrival and was on a 330 vector for the downwind initially descended to 16000 ft. I heard the developmental descend the aircraft to 13000 ft and the pilot readback the clearance more as a question than a statement. The developmental confirmed the 13000 ft altitude and the pilot readback the altitude with certainty. At this point aircraft X was over a 13700 ft MVA. I noticed aircraft X at 13700 ft and pointed this out to my developmental asking if he saw anything very wrong happening. He saw aircraft X descending below 13700 ft and issued a low altitude alert and a climb to 14000 ft. Aircraft X ticked down to 13100 ft before showing a quick climb and level at 14000 ft.even though I heard the 13000 ft altitude issued I never visually correlated the control instruction with aircraft X at the time the instruction was given. As this was a slow easy session I wasn't expecting the developmental to do anything; that easy; wrong. We normally descend aircraft in that position to 14000 ft and once on the base to 13000 ft; but since I didn't visually correlate the instruction with aircraft X verbally hearing 13000 ft didn't raise a mental flag to me as 13000 ft is a commonly used altitude. When I saw aircraft X at 13700 ft I assumed he had been assigned 14000 ft and was blowing through his assigned altitude. That's the point at which I realized that he was the aircraft that had been assigned 13000 ft.due to the low level of traffic and ability of the developmental I was complacent in how much attention I was giving the session. I also had an expectation bias that the developmental could easily handle the traffic with no problems and at this particular point of the session; with all traffic conflicts resolved; was not expecting this type of mistake.

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Original NASA ASRS Text

Title: ASE TRACON Controller reported that during training a Developmental descended an aircraft below the MVA.

Narrative: I was [training a developmental] on a Radar session. The developmental is around 70 hours training on position and our level of traffic this session was below his ability level so I was allowing him to work the session with as little input from me as possible. We had a slow stream of arrivals with very few departures; at the time of the event we had; I believe; 1 aircraft on final already switched to tower and 2 other aircraft being vectored for the approach. There may have possible been a departure as well; but no aircraft were a factor with one another.Aircraft X was a HAREI arrival and was on a 330 vector for the downwind initially descended to 16000 ft. I heard the developmental descend the aircraft to 13000 ft and the pilot readback the clearance more as a question than a statement. The developmental confirmed the 13000 ft altitude and the pilot readback the altitude with certainty. At this point Aircraft X was over a 13700 ft MVA. I noticed Aircraft X at 13700 ft and pointed this out to my developmental asking if he saw anything very wrong happening. He saw Aircraft X descending below 13700 ft and issued a low altitude alert and a climb to 14000 ft. Aircraft X ticked down to 13100 ft before showing a quick climb and level at 14000 ft.Even though I heard the 13000 ft altitude issued I never visually correlated the control instruction with Aircraft X at the time the instruction was given. As this was a slow easy session I wasn't expecting the developmental to do anything; that easy; wrong. We normally descend aircraft in that position to 14000 ft and once on the base to 13000 ft; but since I didn't visually correlate the instruction with Aircraft X verbally hearing 13000 ft didn't raise a mental flag to me as 13000 ft is a commonly used altitude. When I saw Aircraft X at 13700 ft I assumed he had been assigned 14000 ft and was blowing through his assigned altitude. That's the point at which I realized that he was the aircraft that had been assigned 13000 ft.Due to the low level of traffic and ability of the developmental I was complacent in how much attention I was giving the session. I also had an expectation bias that the developmental could easily handle the traffic with no problems and at this particular point of the session; with all traffic conflicts resolved; was not expecting this type of mistake.

Data retrieved from NASA's ASRS site 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.