RETRACTOR TYPE SEAT BELT SHOULDER HARNESS IN THE 2001 CESSNA 172S ACFT ALLOWS BOTH PLT POSITION OCCUPANTS TO HIT THE CEILING DURING AN ENCOUNTER WITH AN INSTANTANEOUS VERTICAL WIND SHEAR.

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2001 MODEL YR C-172 S ACFT. NEW AC TOTAL TACH TIME 60 HRS. WHILE FLYING IN CRUISE CONDITION AT 3500 FT MSL; LIGHT TURB WAS EXPERIENCED. A SINGLE INSTANCE OF CLR AIR DOWNDRAFT TURB WAS EXPERIENCED. DURING THIS INSTANCE; BOTH FRONT SEAT OCCUPANTS WERE JARRED SUCH THAT THEIR HEADS STRUCK THE CEILING OF THE COCKPIT. RETRACTOR TYPE SEAT BELTS (BOTH SHOULDER HARNESS AND LAP BELT RETRACT) IN THIS 2001 MODEL YEAR C-172 DID NOT RESTRAIN FRONT SEAT CREW MEMBERS FROM RISING VERTICALLY TO THE POINT WHERE THEIR HEADS STRUCK THE CAB CEILING. THIS MOMENTARY SINGLE INSTANCE OF TURB ALSO CAUSED OTHER NON-RESTRAINED OBJECTS IN THE CABIN (CHKLIST BOOK AND POH) TO ALSO STRIKE THE CEILING. NO INJURIES WERE ENCOUNTERED. NO LOSS OF ACFT CTL WAS ENCOUNTERED. NO SIGNIFICANT LOSS OF ALTITUDE WAS EXPERIENCED. PLT ADVISED ATC OF ENCOUNTER AND REQUESTED DEVIATION TO UCA ARPT 12 MI AHEAD. FELLOW COMPANY ACFT APPROX 15 MILES AHEAD RESPONDED THAT THEY WERE IN SMOOTH CONDITIONS. A SECOND ACFT ALSO INDICATED SAME. PLT ELECTED TO CONTINUE FLT TO INTENDED DESTINATION AND NO ADDITIONAL EVENTS ENCOUNTERED. PRE-ENG START; PRE-TKOF CHKLIST ITEMS HAD ALL CREW MEMBERS CONFIRM SEAT-BACKS IN UPRIGHT POSITION AND SEAT BELTS SECURE. DURING EVENT; REAR SEAT PAX INDICATED THAT LAP RESTRAINT DID STOP INDIVIDUAL HALF WAY UP FROM MAKING CONTACT WITH COCKPIT CEILING. THIS SEAT BELT DESIGN DIFFERS FROM OLDER MODEL C-172 IN THAT THE OLDER MODEL ACFT HAD A LAP AND SHOULDER BELT THAT LOOPED THROUGH THE BUCKLE TO SECURE. CONCERN IS RAISED THAT RETRACTION TYPE LAP MECHANISM RESPONSE TO SHORT DURATION TURB IN THE FORM OF A DOWN DRAFT MAY BE INSUFFICIENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE BELTS ARE CONNECTED TOGETHER AND BOTH HAVE RETRACTORS INSTALLED. THEY APPEAR TO TAKE UP THE SLACK IN NORMAL OP BUT DID NOT RESTRAIN OCCUPANT DURING A VERTICAL DOWNDRAFT SIT. THERE IS NO PROVISION FOR MANUALLY LOCKING THE BELTS IN A FIXED POSITION. THE HEAD TO CEILING CLRNC IN THIS INSTANCE WAS 3 TO 4 INCHES.