Orion (P3)
Air Traffic Control Fully Certified
Situational Awareness; Communication Breakdown; Confusion
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Human Factors; Procedure
A P-3 Orion was in route; he flew through Sector X and into Sector Y. I asked what his approach request was for [their destination]. He indicated vectors to the ILS26. I declined vectors due to potential traffic volume that was occurring throughout the day because of the terrain in the vicinity of [their destination] vectors to final approach are difficult and time consuming. I offered him approach via IAF [at the VOR] and PT or via IAF [at the intersection] and the DME ARC. He elected to do the ARC. I cleared him to [their destination] via [the intersection] and to cross [the intersection] at or above 060 and cleared ILS Runway 26 Approach. About 6 minutes later I noticed he had crossed [the intersection] and descended to 055 and was heading approximately 180 off course. I called out that he was off route and say intentions; no answer. Repeated and no answer. He was now about 1 minute from a 060 MIA and increasing to 084 eventually. I called on Guard. No response. I issued a low altitude alert on regular frequency and on Guard. No response. I realized that he may be on an adjacent sector frequency still so I enlisted another aircraft to relay a change to the correct frequency. He came up 30 seconds later. I issued a climb and heading to lower terrain and re established his intentions and assigned an ILS approach via PT and the VOR. I was in error in forgetting to switch the aircraft to the correct control frequency. That would have speed up communications when I discovered the navigational error. However; the main issue was poor navigation by the pilot who was in deviation to his clearance and flying the approach incorrectly.
Enroute Controller described a loss of terrain separation event when traffic cleared via conventional navigation failed to navigate properly and was off course causing the terrain issue.