A PARO PLT HIT THE WAKE OF A B747 AND DENTED THE OVERHEAD VENT DUCT AND COLLAPSED HIS SEAT SYS REQUIRING THE REPLACEMENT OF BOTH ITEMS.

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WHILE FLYING EBOUND; BELOW CLASS B AIRSPACE 2 MI E OF DOWNTOWN LOS ANGELES; I ENCOUNTERED SEVERE WAKE TURB GENERATED BY A B747 ON APCH TO LAX. THE TURB CAUSED MY HEAD TO STRIKE AND CRUSH THE OVERHEAD VENTILATOR AND COLLAPSE THE SEAT I WAS IN. I VISUALLY INSPECTED THE ACFT WINGS FOR WRINKLES OR STRESS MARKS: NONE WERE OBSERVED; SO CONTINUED MY FLT. WHEN I LANDED I CHKED FOR DAMAGE; NONE WAS FOUND. IN RETROSPECT; I SHOULD HAVE MADE A PRECAUTIONARY LNDG TO CHK FOR DAMAGE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PLT SAID THAT LAX HAD RECENTLY RAISED THE FLOOR OF THIS SECTION OF THE CLASS B AIRSPACE FROM 2000 FT TO 2500 FT; BUT THE APCH CTLRS STILL BRING THE ARRS DOWN TO 2500 FT EBOUND FROM SMO. THE WAKE OF THESE JETS THAT ARE OPERATING ON THE FLOOR OF THE CLASS B AIRSPACE IS; IN THE RPTR'S OPINION MORE OF A HAZARD NOW THAN BEFORE BECAUSE THE GA ACFT NOW OPERATE WITH LESS VERT CLRNC FROM THE WAKE GENERATING ACFT THAN BEFORE. THE RPTR WAS AT 2300 FT. HE SAID THAT HE FIRST SAW THE B747 AS IT DSNDED OUT OF THE 3000 FT OVCST JUST IN FRONT OF HIM AT ABOUT 11 O'CLOCK POS AND HE THOUGHT ABOUT THE POSSIBILITY OF A WAKE ENCOUNTER; BUT COULD NOT SEE ANY WAY TO AVOID ONE WITHOUT VIOLATING THE AIRSPACE'S RULES OR OTHER REGS. THE B747 TURNED R TO INTERCEPT THE ILS COURSE ABOUT 2-3 NM AHEAD OF HIM. AFTER THE B747 PASSED HIM; INBOUND; HE SUDDENLY FELT A SHARP JOLT. HE SAID THAT HE WAS MOMENTARILY IN SHOCK BECAUSE HIS HEAD HAD STRUCK THE OVERHEAD VENT DUCT SEVERELY DENTING IT AND ALSO COLLAPSING HIS HYD SEAT. BOTH ITEMS HAD TO BE REPLACED BY MAINT LATER. HE RPTED THE EVENT TO THE APCH CTLR AND TO AN APCH SUPVR. HE ALSO RPTED THE EVENT TO A PLT ORGANIZATION AND HE ALLEGED THAT THEY; IN TURN; MET WITH APCH AND DISCUSSED THE PROB. THE RPTR BELIEVES THAT THE JETS INBOUND FROM SMO SHOULD BE HELD AT 4000 FT AND TAKEN OUT A LITTLE FURTHER BEFORE TURNING BASE. THIS ALT WOULD; IN HIS ESTIMATION; PROVIDE MORE VERT SEPARATION AND ALLOW MORE TIME FOR THE WAKE TO DISSIPATE. THIS 4000 FT ALT WOULD STILL PROVIDE SEPARATION FROM THE 3500 FT ALT OF THE ACRS FROM THE S. THE RPTR FEELS THAT THE ONLY AVENUE OF ESCAPE AVAILABLE TO HIM DURING THIS ENCOUNTER WAS TO TURN AROUND AND BACK TRACK FROM HIS POS.