Date: 201706
Local time of day: 1201-1800
Location: ZOA.ARTCC, CA
Flight conditions: VMC
Light: Daylight
Aircraft: Medium Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E ZOA
ATC / advisory: Center ZOA
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness; Confusion; Troubleshooting
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert
Contributing factors: Aircraft; Airspace Structure; Procedure
Primary problem: Procedure

Narrative: Aircraft X was inbound from the south. Sector Controller called me and said Aircraft X wanted to do the approach from [fix] which was 'new' to me since they always do it from the south. Aircraft X checked on and requested to start his approach from [fix] that is 22 miles to the northeast of the Airport. He was 30 miles south and the normal approach for the RNAV (RNP) is over [fix]. This approach is only 'specific' to this company's flights and doesn't follow our traditional RNAV Approach. Furthermore; we as a work force in my area weren't briefed when the aircraft was added.As I said it is common and standard for this flight from the south to start the approach over [fix]. After some conversation this pilot still wanted to start his approach over [fix]. I was reviewing the approach in between transmissions and talking to others in the area about the strange request. I continued to review the approach and the [fix] area is actually under all the missed approach wording and is not visible. There is a box that I surmised is a brief of the procedure over [fix] that shows what I depicted was a hold/procedure turn and expected the pilot to do this on his own since he was due south and there were no intermediary fixes to get a 90 degree turn to intercept.About 20 miles south of [fix] I asked the aircraft if he had airport in sight and he said 'affirmative' but wanted to continue north to the [fix]. So; with the aircraft heading northbound away from the airport I cleared Aircraft X to maintain 15;700 (my MVA) until [fix] and cleared for the approach. About 1 minute later I noticed Aircraft X turning right and started a descent and my mind was trying to understand what I was seeing. I quizzed Aircraft X about what he was doing and stated; 'I didn't see you hit [fix]' and asked if he could see the ground and maintain his own separation. I panicked and didn't say the right thing. Aircraft X assured me he was on the arc and that the airplane made a 'smart turn' and all was okay and I let him proceed on the approach. I terminated his radar even though it was lost as we were talking or maybe before....not sure! Looking back; I believe the pilot put in a different fix or the FMS turned the aircraft all by itself and started a perilous descent. Fortunately it was VFR and I am not sure what happened but it scared the daylights out of me as I was trying to figure out the data I was seeing and making sense of it. I am not a Pilot and not sure what he did. I ran this scenario by other veteran controllers here and they agreed he should have not started descent and the sharp intercept angle until he was established over the fix he requested. We should have verbal briefings about [company] special approaches and if that what I saw about the fix is a holding pattern or procedure turn. There were several of us in disagreement. Company needs to be alerted that this pilot did not establish himself on a portion of the approach before descending. In hindsight; I should have cancelled the clearance and climbed the aircraft to stay at or above the MVA since the pilot missed the requested fix. I just need some clarification on this whole thing and what I can do to ensure I don't ever go through this again; nor any of my colleagues.




Synopsis: ZOA Center Controller reported clearing an aircraft for a company specific approach procedure but the aircraft did not comply with the procedure as depicted.