Narrative:

A C182 was inbound to gpi; level at 12;000 ft. There was an aircraft on missed approach proceeding to killy intersection to hold at 11;000 ft. A pilatus had departed gpi VFR climbing to 12;000 ft toward mso when the C182 was north of gpi. Another aircraft was released southbound to 10;000 ft. This aircraft was a factor with the pilatus in that it was a jet and requested altitude was above the pilatus. As the C182 was abeam (west) gpi; an EA50 was released to 10;000 ft on course westbound. This route intersected the C182 approximately 8 miles west of gpi. During this time; the two southbound aircraft were maneuvered to clear each other and the aircraft at killy. The aircraft at killy was cleared to mso (southbound) behind the two preceding southbound departures. Its requested routing crossed the pilatus; both inbound to mso. Since this aircraft was a jet behind a prop; but behind; I choose to climb the jet to 12;000 ft and descend the pilatus to 11;000 ft. Altitudes were coordinated with geg approach at mso. The C182 was cleared to 11;000 ft. The EA50 departed gpi and reported climbing to 10;000 ft. As it turned westbound and passed under the C182; I saw it's altitude at 10;300 ft. I called the EA50 to verify 10;000 ft altitude. There was a response of; 'out of 9;900 for 10;000 ft.' before that response was done; the mode C indicated 10;400 ft. I called the traffic. The altitude now showed 10;800. I asked the pilot to say altitude. He said; 'just above 10;000 ft;' then [said] something I could not understand; but was something about a navigation system and the pilot was making some kind of adjustment; and back to another system. The usual communication with management ensued. I then read the pilot deviation verbiage to the pilot. While this was happening; the pilatus was heading toward tav depicted as 11;800 ft. I called geg approach at mso about this terrain. They depict 8;600 ft at that location and to keep the aircraft coming; they were good with 11;000 ft. I verified with the flm that if geg took radar on the aircraft that their depicted altitude would be usable. As the aircraft came upon the tav; I called geg to see if they could take radar. Of course; they could not. I then turned the pilatus out of the tav. By the time this was done; the pilatus had been in the box for 4 or so radar updates. It entered it 1.3 miles outside geg approach airspace. While this was going on; there was communication/coordination with gpi; geg; hln; zeg; and sector 6. In come cased multiple calls. Also with other aircraft on frequency; some in areas of weak/questionable radio coverage areas. Recommendation; more closely align these tavs with those of adjacent airspace.

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Original NASA ASRS Text

Title: ZLC Controller described an altitude deviation; reportedly due to aircraft equipment problems.

Narrative: A C182 was inbound to GPI; level at 12;000 FT. There was an aircraft on missed approach proceeding to KILLY intersection to hold at 11;000 FT. A Pilatus had departed GPI VFR climbing to 12;000 FT toward MSO when the C182 was north of GPI. Another aircraft was released southbound to 10;000 FT. This aircraft was a factor with the Pilatus in that it was a jet and requested altitude was above the Pilatus. As the C182 was abeam (west) GPI; an EA50 was released to 10;000 FT on course westbound. This route intersected the C182 approximately 8 miles west of GPI. During this time; the two southbound aircraft were maneuvered to clear each other and the aircraft at KILLY. The aircraft at KILLY was cleared to MSO (southbound) behind the two preceding southbound departures. Its requested routing crossed the Pilatus; both inbound to MSO. Since this aircraft was a jet behind a prop; but behind; I choose to climb the jet to 12;000 FT and descend the Pilatus to 11;000 FT. Altitudes were coordinated with GEG Approach at MSO. The C182 was cleared to 11;000 FT. The EA50 departed GPI and reported climbing to 10;000 FT. As it turned westbound and passed under the C182; I saw it's altitude at 10;300 FT. I called the EA50 to verify 10;000 FT altitude. There was a response of; 'Out of 9;900 for 10;000 FT.' Before that response was done; the Mode C indicated 10;400 FT. I called the traffic. The altitude now showed 10;800. I asked the pilot to say altitude. He said; 'Just above 10;000 FT;' then [said] something I could not understand; but was something about a NAV system and the pilot was making some kind of adjustment; and back to another system. The usual communication with management ensued. I then read the Pilot Deviation verbiage to the pilot. While this was happening; the Pilatus was heading toward TAV depicted as 11;800 FT. I called GEG Approach at MSO about this terrain. They depict 8;600 FT at that location and to keep the aircraft coming; they were good with 11;000 FT. I verified with the FLM that if GEG took RADAR on the aircraft that their depicted altitude would be usable. As the aircraft came upon the TAV; I called GEG to see if they could take RADAR. Of course; they could not. I then turned the Pilatus out of the TAV. By the time this was done; the Pilatus had been in the box for 4 or so RADAR updates. IT entered it 1.3 miles outside GEG Approach airspace. While this was going on; there was communication/coordination with GPI; GEG; HLN; ZEG; and Sector 6. In come cased multiple calls. Also with other aircraft on frequency; some in areas of weak/questionable radio coverage areas. Recommendation; more closely align these TAVs with those of adjacent airspace.

Data retrieved from NASA's ASRS site as of April 2012 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.