Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 7.5
Confusion
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Human Factors; Procedure
Aircraft X was released for departure off of JAC airport via the GEYSER Departure Procedure. The aircraft departed Runway 01 at JAC and reported on frequency. I radar identified Aircraft X 2 miles north of JAC and asked the pilot to confirm his altitude. I gave a control instruction to the pilot; leaving 14;000 feet MSL cleared right turn direct a fix south of Aircraft X's position. The pilot read back the clearance correctly. Continuing my scan I began to ship another Aircraft to ZDV and while doing so noticed a radar hit showing a right turn of Aircraft X at about 10;200 feet MSL. The minimum IFR altitude in that area was just over 11;000 feet MSL. The departure procedure requires a right turn to intercept the outbound course from the JAC VOR. At times a quick turn by an aircraft is translated as a bigger turn by the radar system; therefore I couldn't tell initially if that turn was due to the aircraft or the radar. I finished shipping the aircraft to ZDV and watched Aircraft X for the second radar return. The second radar return showed a continuation of the right turn southbound. I immediately restated the clearance to not turn on course until leaving 14;000 feet MSL. The pilot again responded with the correct clearance given. I informed my manager of what was happening while I continued to monitor the aircraft's climb rate. Aircraft X had a climb rate that was fast enough to stay out of high terrain so no low altitude alert was issued. The manager left and I then shipped Aircraft X to the next sector in ZLC after a handoff was accepted. Sector 08 received Aircraft X and continued their climb. The data block was dropped leaving R16's airspace. The response by the pilot was that the fix NALSI is a fly by point instead of a fly over fix. The FMS of the Aircraft X is being stated as the reason for the early turn. I have never witnessed an FMS system make a 'smart turn' 10 miles from a fix to stay on course. Not all aircraft make that early of a turn including Aircraft X flown by company to many destinations from JAC. I believe that fix needs to be made a fly over fix to mitigate the ability of an FMS system making that error.
ZLC Center Controller reported an aircraft turned early leading to a climb below the minimum IFR altitude.