Narrative:

Had departed pao on an IFR flight. After an uneventful vectors departure; we were cleared to join; and proceed via our filed route. We were established at 6000 ft MSL; on a northerly heading. About 20 NM w-sw of sacramento; we were advised that a B737 (belonging to a 121 operator) would fly 1000 ft over us; same heading. We looked for them; and a few seconds later; they appear 12 O'clock; directly over us. We took precautions for a wake encounter; since we were on the same heading; and wake tends to sink. We waited for approximately 2 mins; and nothing happened. By this time the aircraft was very far (probably 8-10 NM) 12 O'clock; and we considered it to be no longer a factor. Just as we lowered our guard; there was a 30 degree roll to the right; followed immediately by a violent left roll tendency. At this point our aircraft had entered a 90 degree left roll; and the nose started to drop. The PF; followed normal unusual attitude recovery procedures; and we returned to level flight with only a 50 foot altitude loss. It took us several mins to realize what had just occurred; after thinking of every possible cause for the event. At no point were we issued a wake turbulence warning (only a traffic warning). It may be a good idea to make sure controllers are aware of wake turbulence risks even when not in landing-takeoff operations; particularly; when 2 aircraft of substantially different size/weight are involved; and issue corresponding alerts; or better yet; take preventive measures; such as avoiding having a heavy aircraft fly 1000 foot above a light aircraft; when on the same heading and course line.

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Original NASA ASRS Text

Title: C172 ENCOUNTERED WAKE TURB FROM B737 AFTER IT PASSED 1000 FT OVERHEAD ON THE SAME COURSE.

Narrative: HAD DEPARTED PAO ON AN IFR FLT. AFTER AN UNEVENTFUL VECTORS DEP; WE WERE CLRED TO JOIN; AND PROCEED VIA OUR FILED RTE. WE WERE ESTABLISHED AT 6000 FT MSL; ON A NORTHERLY HEADING. ABOUT 20 NM W-SW OF SACRAMENTO; WE WERE ADVISED THAT A B737 (BELONGING TO A 121 OPERATOR) WOULD FLY 1000 FT OVER US; SAME HDG. WE LOOKED FOR THEM; AND A FEW SECONDS LATER; THEY APPEAR 12 O'CLOCK; DIRECTLY OVER US. WE TOOK PRECAUTIONS FOR A WAKE ENCOUNTER; SINCE WE WERE ON THE SAME HDG; AND WAKE TENDS TO SINK. WE WAITED FOR APPROX 2 MINS; AND NOTHING HAPPENED. BY THIS TIME THE ACFT WAS VERY FAR (PROBABLY 8-10 NM) 12 O'CLOCK; AND WE CONSIDERED IT TO BE NO LONGER A FACTOR. JUST AS WE LOWERED OUR GUARD; THERE WAS A 30 DEG ROLL TO THE R; FOLLOWED IMMEDIATELY BY A VIOLENT L ROLL TENDENCY. AT THIS POINT OUR ACFT HAD ENTERED A 90 DEG L ROLL; AND THE NOSE STARTED TO DROP. THE PF; FOLLOWED NORMAL UNUSUAL ATTITUDE RECOVERY PROCS; AND WE RETURNED TO LEVEL FLT WITH ONLY A 50 FOOT ALTITUDE LOSS. IT TOOK US SEVERAL MINS TO REALIZE WHAT HAD JUST OCCURRED; AFTER THINKING OF EVERY POSSIBLE CAUSE FOR THE EVENT. AT NO POINT WERE WE ISSUED A WAKE TURBULENCE WARNING (ONLY A TFC WARNING). IT MAY BE A GOOD IDEA TO MAKE SURE CTLRS ARE AWARE OF WAKE TURBULENCE RISKS EVEN WHEN NOT IN LNDG-TAKEOFF OPS; PARTICULARLY; WHEN 2 ACFT OF SUBSTANTIALLY DIFFERENT SIZE/WEIGHT ARE INVOLVED; AND ISSUE CORRESPONDING ALERTS; OR BETTER YET; TAKE PREVENTIVE MEASURES; SUCH AS AVOIDING HAVING A HEAVY ACFT FLY 1000 FOOT ABOVE A LIGHT ACFT; WHEN ON THE SAME HDG AND COURSE LINE.

Data retrieved from NASA's ASRS site as of January 2009 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.