Narrative:

Cruising at 6;000 ft MSL west; there were scattered clouds at my altitude. Just beginning to enter IMC when artificial horizon began rolling erratically. Requested a 180-degree turn from ATC to exit IMC; said that I had a problem and wanted to leave IMC. ATC asked what my intentions would be after the 180 degree turn. I asked ATC to stand by; because I was concentrating on controlling the airplane without my artificial horizon. I asked for a turn again; and ATC directed me to turn right to 360 and descend to 4;000 feet. I exited IMC; acknowledged the turn to 360 and requested to remain at 6;000; because there were more clouds below me. ATC asked why I had filed an IFR flight plan if I was unwilling to fly in clouds. I explained that I had a failure of my artificial horizon. ATC asked if I wanted to declare an emergency; and I said not yet; I was back in VMC and wanted a chance to figure out what I wanted to do. About that time; my directional gyro also began to precess and I realized that it was a vacuum pump failure. At that point; I declared an emergency; maintained VMC and descended and landed safely. In retrospect; when I lost one of my flight instruments in IMC; I should have immediately declared an emergency; explained my problem; and requested vectors to the nearest airport so that ATC would quickly understand my problems and intentions and provide the assistance I needed. It also would have forced ATC to immediately allow my 180 degree turn - the delay between my request and ATC's amended clearance could have been a serious problem here.

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Original NASA ASRS Text

Title: A C172 pilot experienced loss of vacuum pump and associated instruments while in IMC. He declared an emergency and diverted to a VFR airport.

Narrative: Cruising at 6;000 FT MSL west; there were scattered clouds at my altitude. Just beginning to enter IMC when artificial horizon began rolling erratically. Requested a 180-degree turn from ATC to exit IMC; said that I had a problem and wanted to leave IMC. ATC asked what my intentions would be after the 180 degree turn. I asked ATC to stand by; because I was concentrating on controlling the airplane without my artificial horizon. I asked for a turn again; and ATC directed me to turn right to 360 and descend to 4;000 feet. I exited IMC; acknowledged the turn to 360 and requested to remain at 6;000; because there were more clouds below me. ATC asked why I had filed an IFR flight plan if I was unwilling to fly in clouds. I explained that I had a failure of my artificial horizon. ATC asked if I wanted to declare an emergency; and I said not yet; I was back in VMC and wanted a chance to figure out what I wanted to do. About that time; my directional gyro also began to precess and I realized that it was a vacuum pump failure. At that point; I declared an emergency; maintained VMC and descended and landed safely. In retrospect; when I lost one of my flight instruments in IMC; I should have immediately declared an emergency; explained my problem; and requested vectors to the nearest airport so that ATC would quickly understand my problems and intentions and provide the assistance I needed. It also would have forced ATC to immediately allow my 180 degree turn - the delay between my request and ATC's amended clearance could have been a serious problem here.

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.