C182 HAD A ROUGH RUNNING ENG ON START UP.

Narrative

MATERIAL DEFICIENCY. FAILED COMPONENT: INTAKE MANIFOLD BALANCE TUBE ASSEMBLY; TELEDYNE CONTINENTAL MOTORS PART NUMBER #630628. INSTALLED ON: RECIPROCATING ENG; TCM 0-470-U; 783.2 HRS TSMOH (INSTALLED JAN/XA/95). END ITEM: CESSNA C182Q (1979); 2463.5 HRS TTAF. BACKGROUND: AFTER ENG START PLT EXPERIENCED A 'ROUGH-RUNNING ENG' AND SHUT DOWN PRIOR TO TAXI. RPTR IS UNIT'S MAINT OFFICER; WHO ARRANGED TO HAVE THE ENG RUN FOR TROUBLESHOOTING BY A LICENSED AMT (AVIATION MAINT TECHNICIAN). AMT DUPLICATED THE MALFUNCTION AND DETERMINED THE #6 CYLINDER WAS NOT FIRING. VISUAL INSPECTION DISCOVERED THE INTAKE MANIFOLD BAL TUBE ASSEMBLY HAD CRACKED AND SEPARATED AT 1 OF ITS 2 WELDS. THE FAILED WELD WAS AT THE L FORWARD SIDE OF THE INTAKE MANIFOLD; IN IMMEDIATE VICINITY OF THE #6 CYLINDER INTAKE VALVE. (THE AMT DETERMINED THAT OUTSIDE AIR WAS BEING DRAWN INTO THIS CYLINDER; DILUTING THE FUEL-AIR MIXTURE TO THE POINT WHERE COMBUSTION WAS NOT POSSIBLE). CAUSE: THE BAL TUBE HAD SETTLED ONTO THE FORWARD LORD MOUNT INSTALLED ON THE LOWER ENG COWLING; CAUSING A RESISTANT FORCE TO BE APPLIED AT A SINGLE POINT AT THE MIDDLE OF THE BAL TUBE. (THE BAL TUBE HAD SETTLED BECAUSE THE ENTIRE ENG HAD SETTLED ON ITS MAIN MOTOR MOUNTS -- DUE TO COMPRESSION OF THE RUBBER IN THE MOUNTS. BUT THAT'S ANOTHER STORY.) DEFICIENCY: WITH THE FULL WT OF THE ENG BEARING ON THE BAL TUBE AT THIS SINGLE POINT (THE LORD MOUNT) THE STRESS WAS RELIEVED NOT BY THE TUBE BENDING AT ITS MIDPOINT BUT RATHER BY ONE OF ITS END WELDS FAILING. WHEN THIS BAL TUBE WAS REMOVED AND INSPECTED THE AMT REMARKED THAT THE BEAD OF THE FAILED WELD APPEARED LESS SUBSTANTIAL THAN THE OTHER BEAD. IT APPEARS THE FAILED WELD WAS THE WEAKEST POINT OF THIS PARTICULAR TUBE. SUSPECT THIS IS NOT HOW (AND WHERE) THIS COMPONENT WAS EXPECTED TO GIVE OUT.