Salt lake city center controller trainee and their instructor reported they did not notice an aircra...

Narrative:

Aircraft X enroute from the southeast. Shortly after check-in; the pilot requested direct to a waypoint direct eko; which [was] about 10 miles east of eko. I cleared him as requested; and confirmed that the pilot wanted to conduct a visual approach. A few minutes later; I cleared the pilot to descend to 14;000 ft; which the pilot read back. Soon after; I gave the pilot the airport's position and advised to 'report in sight for a visual approach.' the pilot did not read it back correctly. I still do not know exactly what he said; but it sounded wrong; so I repeated myself to the aircraft; 'just want to confirm; report airport in sight for a visual approach.' this time; the pilot responded 'wilco.' at this point the aircraft was still well above the mia (minimum IFR altitude); at around 16;000 ft; if I remember right. I went back to my scan; checking up on the rest of the sector. A short time later; aircraft X came up on the radio asking to confirm that I cleared him for a visual approach but wanted him to report the airport in sight. At that moment his altitude indicated 13;400 ft; which [matched] the mia where the plane was positioned. I immediately responded 'negative; maintain one three thousand four hundred; report the airport in sight.' the next update to the aircraft's datablock showed 13;100 ft.; three hundred ft. Below the mia. The only thing I feel I could have done differently would have been to re-clear the aircraft to 14;000 ft after the bad readback.

Original NASA ASRS Text

Title: Salt Lake City Center Controller trainee and their instructor reported they did not notice an aircraft leave its assigned altitude and descend below the Minimum IFR Altitude.

Narrative: Aircraft X enroute from the southeast. Shortly after check-in; the pilot requested direct to a waypoint direct EKO; which [was] about 10 miles east of EKO. I cleared him as requested; and confirmed that the pilot wanted to conduct a visual approach. A few minutes later; I cleared the pilot to descend to 14;000 ft; which the pilot read back. Soon after; I gave the pilot the airport's position and advised to 'report in sight for a visual approach.' The pilot did not read it back correctly. I still do not know exactly what he said; but it sounded wrong; so I repeated myself to the aircraft; 'Just want to confirm; report airport in sight for a visual approach.' This time; the pilot responded 'wilco.' At this point the aircraft was still well above the MIA (Minimum IFR Altitude); at around 16;000 ft; if I remember right. I went back to my scan; checking up on the rest of the sector. A short time later; Aircraft X came up on the radio asking to confirm that I cleared him for a visual approach but wanted him to report the airport in sight. At that moment his altitude indicated 13;400 ft; which [matched] the MIA where the plane was positioned. I immediately responded 'Negative; maintain one three thousand four hundred; report the airport in sight.' The next update to the aircraft's datablock showed 13;100 ft.; three hundred ft. below the MIA. The only thing I feel I could have done differently would have been to re-clear the aircraft to 14;000 ft after the bad readback.

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.

Attributes
ACN	1683248
Time
Date	201909
Local Time Of Day	1201-1800
Place
Locale Reference	ZLC.ARTCC
State Reference	UT
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	Medium Transport Low Wing 2 Turbojet Eng
Flight Phase	Descent
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Trainee
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Salt lake city center controller trainee and their instructor reported they did not notice an aircraft leave its assigned altitude and descend below the minimum IFR altitude.