Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Flight Crew Commercial; Flight Crew Instrument
Flight Crew Last 90 Days 40; Flight Crew Total 1320; Flight Crew Type 709
Situational Awareness
Inflight Event / Encounter Wake Vortex Encounter
General None Reported / Taken
Environment - Non Weather Related; Human Factors; Procedure
In cruise heading north on flight following with Travis Air Force Base RAPCON at 5;500 feet at 167 KTAS.  Travis called out a descending C17 on approach into Travis with a caution for wake turbulence.  Assumed I was above their trailing wake but flew into the turbulence and experience rough turbulence as we crossed the wake.  Hit my head on the ceiling and the contents of the cockpit got tossed up in the air. Lesson learned:  Don't try to gauge the altitude of wake turbulence.  It's too hard to pinpoint where the risk is.  Either climb well above it or turn around go the other way.  I could have easily turned away and climbed to avoid it completely.  I misjudged the location of the wake turbulence. Appreciate Travis RAPCON calling it out.  I just didn't adjust enough.
General Aviation pilot reported encountering wake turbulence from a military transport in the vicinity of SUU airport.