Narrative:

While being vectored in a non-IFR GPS; non-autopilot equipped aircraft; in clear VFR conditions; I began to suspect that the dg was inaccurate due to a discrepancy between it; the compass; and a portable GPS. While at an assigned altitude of 6000 ft; with 6000 indicating on the altimeter; I began trouble shooting the suspected heading error. I first reset the dg to the compass; a routine IFR procedure. But the discrepancy reoccurred within a few minutes. At that point I began cross-checking among the dg; compass; and portable GPS. Since the GPS was not my own; and had recently been placed on the top of the glareshield next to the compass by the FBO that rented the aircraft; I was concerned that the GPS might be interfering with the compass. I turned my attention to removing the GPS from its mount; and in so doing noticed the compass did turn. The GPS interfered with the compass when the GPS was out of its cradle. I removed the GPS with some difficulty; from the glareshield and placed it on the co-pilot's seat. At that point ATC called to say I was at 6400 feet. Upon returning to 6000 ft the flight continued nominally. I did discoverer that the dg was precessing and the major dg drift may have been the original cause of heading uncertainty. It also appeared that when the GPS was firmly affixed in its cradle it did not interfere with the compass; only when dislodged and slid off the glareshield; immediately adjacent to the compass; as occurred when I was troubleshooting. There were no traffic conflicts during the altitude excursion. In retrospect; once the standard IFR procedure of resetting the dg to the compass failed; instead of attempting to troubleshoot a possible compass error caused by the portable GPS; I should have contacted ATC. While they could not have given me heading information; only track information; that at least would have confirmed or denied what the GPS was telling me. A secondary purpose would have been to alert ATC that the pilot was distracted by a navigation problem and may need early altitude alerts. This was all clear in hindsight; but not during the distraction.

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Original NASA ASRS Text

Title: C210 pilot reports altitude deviation while attempting to trouble shoot a directional gyro problem at 6000 feet.

Narrative: While being vectored in a non-IFR GPS; non-autopilot equipped aircraft; in clear VFR conditions; I began to suspect that the DG was inaccurate due to a discrepancy between it; the compass; and a portable GPS. While at an assigned altitude of 6000 ft; with 6000 indicating on the altimeter; I began trouble shooting the suspected heading error. I first reset the DG to the compass; a routine IFR procedure. But the discrepancy reoccurred within a few minutes. At that point I began cross-checking among the DG; compass; and portable GPS. Since the GPS was not my own; and had recently been placed on the top of the glareshield next to the compass by the FBO that rented the aircraft; I was concerned that the GPS might be interfering with the compass. I turned my attention to removing the GPS from its mount; and in so doing noticed the compass did turn. The GPS interfered with the compass when the GPS was out of its cradle. I removed the GPS with some difficulty; from the glareshield and placed it on the co-pilot's seat. At that point ATC called to say I was at 6400 feet. Upon returning to 6000 ft the flight continued nominally. I did discoverer that the DG was precessing and the major DG drift may have been the original cause of heading uncertainty. It also appeared that when the GPS was firmly affixed in its cradle it did not interfere with the compass; only when dislodged and slid off the glareshield; immediately adjacent to the compass; as occurred when I was troubleshooting. There were no traffic conflicts during the altitude excursion. In retrospect; once the standard IFR procedure of resetting the DG to the compass failed; instead of attempting to troubleshoot a possible compass error caused by the portable GPS; I should have contacted ATC. While they could not have given me heading information; only track information; that at least would have confirmed or denied what the GPS was telling me. A secondary purpose would have been to alert ATC that the pilot was distracted by a navigation problem and may need early altitude alerts. This was all clear in hindsight; but not during the distraction.

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.