Narrative:

At 6000 ft I was skimming the tops proceeding direct sat VORTAC on a 270 degree heading. I received clearance to 4000 ft. At 5900 ft I was in IMC. At approximately 5800 ft, I flew through the first of 2 periods of similar violent chop that began and ended suddenly with smooth air between. I immediately felt a g-force sensation, as if I was in a steep right bank trying to maintain altitude, after I came through the second series of chop. Because of occasional breaks in the overcast, the ground lights appeared to be in an unusual position. This was the first of a series of quick and sudden unusual flight attitudes as evidenced by instrument indications and visual references. Because of the sudden occurrence of the unusual attitudes I instantly assumed that I had had a vacuum failure and subsequent attitude gyro failure. Therefore, since I was in IMC I declared an emergency. It is now my belief, based on all events, indications, and my experience that I flew through wake turbulence which caused the gyros to tumble. The 2 periods of turbulence were very violent, jolting and symmetrical and could have been produced by one of the several planes flying at sat at that time. The surrounding air was smooth and stable which, as I understand it, will allow wake vortices to linger. After stabilizing the aircraft at 3000 ft, I was offered and accepted no gyro vectors to the ILS course and completed a partial panel approach to 100 ft above minimums with a safe landing. All events took place in IMC so I am not sure of what specific attitudes (flight attitudes) the airplane were in. No tower report was required. Note: there were 2 glimpses of ground lights and one of the sky because of overcast breaks during the period of unusual attitudes but they did not provide a very sure means of knowing the attitude of the airplane.

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Original NASA ASRS Text

Title: PLT OF A PA34 SENECA IN IMC HAD BRIEF LOSS OF ACFT CTL APPARENTLY DUE TO WAKE TURB. GYROS TUMBLED AND HE RECEIVED A NO GYRO APCH.

Narrative: AT 6000 FT I WAS SKIMMING THE TOPS PROCEEDING DIRECT SAT VORTAC ON A 270 DEG HDG. I RECEIVED CLRNC TO 4000 FT. AT 5900 FT I WAS IN IMC. AT APPROX 5800 FT, I FLEW THROUGH THE FIRST OF 2 PERIODS OF SIMILAR VIOLENT CHOP THAT BEGAN AND ENDED SUDDENLY WITH SMOOTH AIR BTWN. I IMMEDIATELY FELT A G-FORCE SENSATION, AS IF I WAS IN A STEEP R BANK TRYING TO MAINTAIN ALT, AFTER I CAME THROUGH THE SECOND SERIES OF CHOP. BECAUSE OF OCCASIONAL BREAKS IN THE OVCST, THE GND LIGHTS APPEARED TO BE IN AN UNUSUAL POS. THIS WAS THE FIRST OF A SERIES OF QUICK AND SUDDEN UNUSUAL FLT ATTITUDES AS EVIDENCED BY INST INDICATIONS AND VISUAL REFS. BECAUSE OF THE SUDDEN OCCURRENCE OF THE UNUSUAL ATTITUDES I INSTANTLY ASSUMED THAT I HAD HAD A VACUUM FAILURE AND SUBSEQUENT ATTITUDE GYRO FAILURE. THEREFORE, SINCE I WAS IN IMC I DECLARED AN EMER. IT IS NOW MY BELIEF, BASED ON ALL EVENTS, INDICATIONS, AND MY EXPERIENCE THAT I FLEW THROUGH WAKE TURB WHICH CAUSED THE GYROS TO TUMBLE. THE 2 PERIODS OF TURB WERE VERY VIOLENT, JOLTING AND SYMMETRICAL AND COULD HAVE BEEN PRODUCED BY ONE OF THE SEVERAL PLANES FLYING AT SAT AT THAT TIME. THE SURROUNDING AIR WAS SMOOTH AND STABLE WHICH, AS I UNDERSTAND IT, WILL ALLOW WAKE VORTICES TO LINGER. AFTER STABILIZING THE ACFT AT 3000 FT, I WAS OFFERED AND ACCEPTED NO GYRO VECTORS TO THE ILS COURSE AND COMPLETED A PARTIAL PANEL APCH TO 100 FT ABOVE MINIMUMS WITH A SAFE LNDG. ALL EVENTS TOOK PLACE IN IMC SO I AM NOT SURE OF WHAT SPECIFIC ATTITUDES (FLT ATTITUDES) THE AIRPLANE WERE IN. NO TWR RPT WAS REQUIRED. NOTE: THERE WERE 2 GLIMPSES OF GND LIGHTS AND ONE OF THE SKY BECAUSE OF OVCST BREAKS DURING THE PERIOD OF UNUSUAL ATTITUDES BUT THEY DID NOT PROVIDE A VERY SURE MEANS OF KNOWING THE ATTITUDE OF THE AIRPLANE.

Data retrieved from NASA's ASRS site as of July 2007 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.