Narrative:

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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 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.