ZLC Controller described an altitude deviation; reportedly due to aircraft equipment problems.
Synopsis
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.
Second reporter narrative
On climb out from GPI; assigned to 10;000 FT; ATC reported that aircraft had deviated in altitude climbing past assigned altitude of 10;000 FT. On climb out; the aircraft had two warning lights; one warning light indicated that the primary and secondary Baro systems were mismatched. The other indicator showed an aircraft primary computer system caution (probably related to mismatch in the Baro systems). The primary system is on the pilot side; the secondary is on the copilot side; and the backup is in between. On seeing the Baro mismatch; primary reference falls to the pilot side until it is established which Baro is more accurate. The primary means of determining which one is more accurate is to compare the primary and secondary to the backup. Whichever is closer in altitude to the backup will be the one relied upon. After level off; ATC repeatedly asked for altitude readout without implying that the aircraft had passed the assigned altitude of 10;000 FT. During this time the aircraft was leveling off over terrain that tops 6;700 FT. ATC finally indicated that they believed the aircraft was above assigned altitude. An immediate descent was initiated while simultaneously attempting to troubleshoot the system through use of the pocket checklist. Memory items from the checklist were completed during climb out; including use of the pilot Baro until it could be established which of the Baro systems was more accurate. During descent; it was determined that the copilot Baro system was closer to the backup system. After leveling off using the copilot system; ATC seemed to be happy with the aircraft altitude. After the Baro system electronic circuit breakers were reset; the systems matched within 20 FT. Shortly thereafter; ATC made notification of a suspected Pilot Deviation and asked that they be contacted upon arrival at destination. I believe it would have been a helpful for ATC to announce that they disagreed with the altitude the aircraft was reporting. This was by far the largest altitude mismatch in this aircraft. As with many new aircraft there are often unexplained anomalies. This is something that needs to be addressed. The problem seemed to disappear as quickly as it appeared. Lastly; flying single pilot IFR is difficult enough as it is. It would have been wise to report a system malfunction to ATC before it became an issue.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.