Date: 202004
Local time of day: 0001-0600
Location: GJT.Airport, CO
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class E ZDV
ATC / advisory: Center ZDV
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Airspace Structure; Airport; Chart Or Publication; Procedure
Primary problem: Airspace Structure

Narrative: Aircraft X checked on to my frequency. I asked the pilot to advise when they had automated weather at GJT airport and what approach they were requesting. They advised they were requesting the RNAV 29 approach to GJT starting over the waypoint CESBA. Aircraft X was tracking from the Southeast to the Northwest. With giving them direct to CESBA; they must be vectored out as they get closer to the IAF due to the requirements of no more than a 90 degree turn. I advised the pilot to expect vectors to get set up for the approach; and also advised them that I could only cross them at CESBA at 12000 feet due to the terrain. The approach shows them crossing CESBA at or above 8500 feet. The pilot responded with 'Ok'. As Aircraft X got closer to the airport; I had them vectored 10 degrees left for the approach; and descending to 13000 feet. As they cleared the higher terrain; I descended them to 12000 feet; and watched to make sure that when given the approach clearance to CESBA they would stay clear of higher terrain. When I gave the approach clearance direct to CESBA; maintain 12000 feet until CESBA cleared RNAV 29 GJT; they read it back except for the 'maintain 12000 until CESBA' which I missed in the readback. As I watched Aircraft X track towards CESBA I noticed that their altitude dropped below 12000 feet. At 11;200 feet I issued a low altitude alert; and to maintain 12000 feet until CESBA. Aircraft X rogered the low altitude alert and continued the descent before beginning a climb back up. They then asked me to clarify if they were cleared for the approach or not. It was my fault that I missed the readback; but the issue with the approaches for Runway 29 is that their IAF start in much higher terrain; causing a very steep descent profile for the pilots. We try to have the conversation with the pilots about this instead of denying the procedure. I have witnessed aircraft go missed approach on the RNAV 29 approach because they claimed they were unable to descend properly; even though the discussion was had with the pilots. I would recommend that this information be forwarded to the airlines to advise their pilots if at all possible; to not utilize RNAV 29 or LDA 29 approaches due to having to be vectored out and starting the approach at higher altitudes; not allowing for a stabilized approach/descent. The ILS/LOC RWY 11 Approach is a good alternative as it allows for a circle to land; as well as a lower starting altitude at 10;000 feet.




Synopsis: A Center Controller reported an air carrier descended without complying with a crossing restriction and flew below the Minimum Vectoring Altitude.