Narrative:

I was on a VFR flight from gnoss (dvo) to salinas (sns) via oakland and san jose, using flight following. At the time of the incident, I had been switched to san jose tower for the transition of their airspace, and given a clearance 'to cross midfield at or above 2000 ft.' I continued on the same heading towards the field. However, a jet aircraft taking off turned directly towards me and would pass close enough overhead to make me very concerned about wake turbulence. So I took evasive action by using aft cyclic and lowered the collective to reduce the helicopter to maneuvering speed. I lost about 500 ft in altitude. There was no time to get permission to take action with the busy radio traffic. Shortly afterwards, a different controller said that 'I had been given a clearance to maintain VFR and cross midfield at or above 2000 ft.' and I 'had descended into the approach path.' I replied that 'I needed to take evasive action to avoid the wake turbulence.' (nevertheless, I still received a strong jolt of turbulence.) also it was my understanding that I was cleared to 'cross midfield at or above 2000 ft,' not 'to maintain 2000 ft or above and cross midfield.' the wording of the clearance is confusing as to the altitude requirement immediately prior to reaching midfield. More worrying, is that controllers give the usual warning about wake turbulence to departing aircraft but do not seem to be concerned about turbulence from overflt of transitioning aircraft. I experienced extreme turbulence at sfo when a heavy slow jet was allowed to take off right over me while transitioning their airspace on a previous occasion. (Banged my head on the ceiling!)

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Original NASA ASRS Text

Title: ROBINSON R44 HELI PLT DSNDS TO AVOID THE WAKE TURB OF A JET DEPARTING FROM THE SJC ARPT OVER WHICH HE WAS ABOUT TO TRANSITION AT 2000 FT MSL. HE EXPERIENCED SOME TURB EVEN THOUGH HE DSNDED 500 FT AND COMPLAINS THAT ATC DOES NOT TAKE INTO CONSIDERATION THE WAKE OF OVERFLYING JETS DURING OP TO AND FROM LARGE ARPTS.

Narrative: I WAS ON A VFR FLT FROM GNOSS (DVO) TO SALINAS (SNS) VIA OAKLAND AND SAN JOSE, USING FLT FOLLOWING. AT THE TIME OF THE INCIDENT, I HAD BEEN SWITCHED TO SAN JOSE TWR FOR THE TRANSITION OF THEIR AIRSPACE, AND GIVEN A CLRNC 'TO CROSS MIDFIELD AT OR ABOVE 2000 FT.' I CONTINUED ON THE SAME HDG TOWARDS THE FIELD. HOWEVER, A JET ACFT TAKING OFF TURNED DIRECTLY TOWARDS ME AND WOULD PASS CLOSE ENOUGH OVERHEAD TO MAKE ME VERY CONCERNED ABOUT WAKE TURB. SO I TOOK EVASIVE ACTION BY USING AFT CYCLIC AND LOWERED THE COLLECTIVE TO REDUCE THE HELI TO MANEUVERING SPD. I LOST ABOUT 500 FT IN ALT. THERE WAS NO TIME TO GET PERMISSION TO TAKE ACTION WITH THE BUSY RADIO TFC. SHORTLY AFTERWARDS, A DIFFERENT CTLR SAID THAT 'I HAD BEEN GIVEN A CLRNC TO MAINTAIN VFR AND CROSS MIDFIELD AT OR ABOVE 2000 FT.' AND I 'HAD DSNDED INTO THE APCH PATH.' I REPLIED THAT 'I NEEDED TO TAKE EVASIVE ACTION TO AVOID THE WAKE TURB.' (NEVERTHELESS, I STILL RECEIVED A STRONG JOLT OF TURB.) ALSO IT WAS MY UNDERSTANDING THAT I WAS CLRED TO 'CROSS MIDFIELD AT OR ABOVE 2000 FT,' NOT 'TO MAINTAIN 2000 FT OR ABOVE AND CROSS MIDFIELD.' THE WORDING OF THE CLRNC IS CONFUSING AS TO THE ALT REQUIREMENT IMMEDIATELY PRIOR TO REACHING MIDFIELD. MORE WORRYING, IS THAT CTLRS GIVE THE USUAL WARNING ABOUT WAKE TURB TO DEPARTING ACFT BUT DO NOT SEEM TO BE CONCERNED ABOUT TURB FROM OVERFLT OF TRANSITIONING ACFT. I EXPERIENCED EXTREME TURB AT SFO WHEN A HVY SLOW JET WAS ALLOWED TO TAKE OFF RIGHT OVER ME WHILE TRANSITIONING THEIR AIRSPACE ON A PREVIOUS OCCASION. (BANGED MY HEAD ON THE CEILING!)

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.