Narrative:

Performed a maintenance check for 4 control rod bolt replacement of the main rotor flight control bolts. This function requires that all of the flight control rod bolts be changed. The maintenance was performed by a crew of 5 technicians and 1 inspector. After the bolts were removed and replaced; they were inspected. The inspector made a sticker for the log sheet stating part numbers and serial numbers of the bolts that were changed. I signed the technician block and the inspector stamped the inspection block. The aircraft required a main rotor flight control rigging check be performed due to replacement of the control rods and bolts being changed. No defects were noted on the flight control rigging check. The aircraft was then flown for .7 flight hours for track and balance purposes and an operations check flight was performed. The pilot who performed the operations check flight reported no discrepancies. The aircraft was returned to service. The aircraft was flown from ZZZ airport to its home base. The aircraft was flown 11.9 flight hours before any discrepancy regarding the flight controls was written up. The assigned base technicians performed an inspection of all flight controls and no defects were found. A maintenance check flight of 3.4 flight hours was performed. The pilot reported the same discrepancy of the flight controls. The base technicians performed another inspection; and found no defects. It was then decided to truck the aircraft back to ZZZ for further investigation. The aircraft arrived in ZZZ and was inspected. The hydraulic pack was removed and replaced. A rigging check of the main rotor flight control system was performed. No defects were noted. The aircraft went on a .4 hour operations check flight. No defects were noted. The aircraft was released for service. It was then used for pilot training. Approximately 2.2 flight hours were put on the aircraft before the original flight control discrepancy was written up. The aircraft was inspected and 2 bolts that secure the hinged support to the swash plate were missing bushings. This was found by moving the main rotor blade in a twisting motion and feeling play somewhere in the main rotor control system. Upon further investigation the abovementioned bushings were discovered. After further investigation; it was determined that the hinged support was installed 180 degrees out of proper installation. While this has no bearing on the flight characteristics of the aircraft; it is believed that when the bolts were removed; this enabled the bushings to slide out with the bolts. New bolts were then installed; and the installing technician did not notice the bushings were missing; or did not know that the bushing was required to be installed. After reviewing the maintenance manual; the procedure for assembly of the mixing lever which also includes build-up of the control fork and rocker assembly; does not address the bushings that were left out. The only place that the bushings are found is in the ipc. I performed the inspection of the flight control rod bolt replacement. At no time did I see any defects. After further investigation; it is not possible to see the bushing missing. Also due to the preloading of the bearings no play were detected until the load was removed from the blades. The following procedural changes have been implemented to prevent this from occurring again. A discrepancy will be made for the flight control bolts in sections of the bolts that will be replaced instead of 1 discrepancy to cover all of the bolts being replaced. A quality control inspector will be present as all flight control bolts are being changed and must witness the removal; and installation to include torque procedures and cotter keys installed. The quality control inspectors have been instructed to physically check all of the flight control bolts for movement and just for cotter pins being installation. Callback conversation with reporter revealed the following information: reporter stated there are approximately 27 bolts that have to be replaced on a time controlled basis. The weight of the attached blades on the main rotor head create a pre-load stress that doesn't allow for noticing any loose bolts caused by; as in this case; missing bushings. Only by grabbing and twisting the T/east of each blade; were they ableto determine the area of play that caused the uncommanded porpoising. Reporter also stated he believes the changes they have made to their maintenance procedures; including adding references about the bushings with the bolts that secure the hinged support to the swashplate; will prevent any further discrepancies that result from the time control bolt replacement requirements.

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Original NASA ASRS Text

Title: AN MBB BK117 HELICOPTER EXPERIENCED UNCOMMANDED PORPOISING IN FLIGHT; NOT DRAMATIC; BUT NOTICEABLE; AFTER MAINT REPLACED (4 EA) CONTROL ROD BOLTS FOR THE MAIN ROTOR FLT CONTROL.

Narrative: PERFORMED A MAINT CHK FOR 4 CTL ROD BOLT REPLACEMENT OF THE MAIN ROTOR FLT CTL BOLTS. THIS FUNCTION REQUIRES THAT ALL OF THE FLT CTL ROD BOLTS BE CHANGED. THE MAINT WAS PERFORMED BY A CREW OF 5 TECHNICIANS AND 1 INSPECTOR. AFTER THE BOLTS WERE REMOVED AND REPLACED; THEY WERE INSPECTED. THE INSPECTOR MADE A STICKER FOR THE LOG SHEET STATING PART NUMBERS AND SERIAL NUMBERS OF THE BOLTS THAT WERE CHANGED. I SIGNED THE TECHNICIAN BLOCK AND THE INSPECTOR STAMPED THE INSPECTION BLOCK. THE ACFT REQUIRED A MAIN ROTOR FLT CTL RIGGING CHK BE PERFORMED DUE TO REPLACEMENT OF THE CTL RODS AND BOLTS BEING CHANGED. NO DEFECTS WERE NOTED ON THE FLT CTL RIGGING CHK. THE ACFT WAS THEN FLOWN FOR .7 FLT HRS FOR TRACK AND BAL PURPOSES AND AN OPS CHK FLT WAS PERFORMED. THE PLT WHO PERFORMED THE OPS CHK FLT RPTED NO DISCREPANCIES. THE ACFT WAS RETURNED TO SVC. THE ACFT WAS FLOWN FROM ZZZ ARPT TO ITS HOME BASE. THE ACFT WAS FLOWN 11.9 FLT HRS BEFORE ANY DISCREPANCY REGARDING THE FLT CTLS WAS WRITTEN UP. THE ASSIGNED BASE TECHNICIANS PERFORMED AN INSPECTION OF ALL FLT CTLS AND NO DEFECTS WERE FOUND. A MAINT CHK FLT OF 3.4 FLT HRS WAS PERFORMED. THE PLT RPTED THE SAME DISCREPANCY OF THE FLT CTLS. THE BASE TECHNICIANS PERFORMED ANOTHER INSPECTION; AND FOUND NO DEFECTS. IT WAS THEN DECIDED TO TRUCK THE ACFT BACK TO ZZZ FOR FURTHER INVESTIGATION. THE ACFT ARRIVED IN ZZZ AND WAS INSPECTED. THE HYD PACK WAS REMOVED AND REPLACED. A RIGGING CHK OF THE MAIN ROTOR FLT CTL SYS WAS PERFORMED. NO DEFECTS WERE NOTED. THE ACFT WENT ON A .4 HR OPS CHK FLT. NO DEFECTS WERE NOTED. THE ACFT WAS RELEASED FOR SVC. IT WAS THEN USED FOR PLT TRAINING. APPROX 2.2 FLT HRS WERE PUT ON THE ACFT BEFORE THE ORIGINAL FLT CTL DISCREPANCY WAS WRITTEN UP. THE ACFT WAS INSPECTED AND 2 BOLTS THAT SECURE THE HINGED SUPPORT TO THE SWASH PLATE WERE MISSING BUSHINGS. THIS WAS FOUND BY MOVING THE MAIN ROTOR BLADE IN A TWISTING MOTION AND FEELING PLAY SOMEWHERE IN THE MAIN ROTOR CTL SYS. UPON FURTHER INVESTIGATION THE ABOVEMENTIONED BUSHINGS WERE DISCOVERED. AFTER FURTHER INVESTIGATION; IT WAS DETERMINED THAT THE HINGED SUPPORT WAS INSTALLED 180 DEGS OUT OF PROPER INSTALLATION. WHILE THIS HAS NO BEARING ON THE FLT CHARACTERISTICS OF THE ACFT; IT IS BELIEVED THAT WHEN THE BOLTS WERE REMOVED; THIS ENABLED THE BUSHINGS TO SLIDE OUT WITH THE BOLTS. NEW BOLTS WERE THEN INSTALLED; AND THE INSTALLING TECHNICIAN DID NOT NOTICE THE BUSHINGS WERE MISSING; OR DID NOT KNOW THAT THE BUSHING WAS REQUIRED TO BE INSTALLED. AFTER REVIEWING THE MAINT MANUAL; THE PROC FOR ASSEMBLY OF THE MIXING LEVER WHICH ALSO INCLUDES BUILD-UP OF THE CTL FORK AND ROCKER ASSEMBLY; DOES NOT ADDRESS THE BUSHINGS THAT WERE LEFT OUT. THE ONLY PLACE THAT THE BUSHINGS ARE FOUND IS IN THE IPC. I PERFORMED THE INSPECTION OF THE FLT CTL ROD BOLT REPLACEMENT. AT NO TIME DID I SEE ANY DEFECTS. AFTER FURTHER INVESTIGATION; IT IS NOT POSSIBLE TO SEE THE BUSHING MISSING. ALSO DUE TO THE PRELOADING OF THE BEARINGS NO PLAY WERE DETECTED UNTIL THE LOAD WAS REMOVED FROM THE BLADES. THE FOLLOWING PROCEDURAL CHANGES HAVE BEEN IMPLEMENTED TO PREVENT THIS FROM OCCURRING AGAIN. A DISCREPANCY WILL BE MADE FOR THE FLT CTL BOLTS IN SECTIONS OF THE BOLTS THAT WILL BE REPLACED INSTEAD OF 1 DISCREPANCY TO COVER ALL OF THE BOLTS BEING REPLACED. A QUALITY CTL INSPECTOR WILL BE PRESENT AS ALL FLT CTL BOLTS ARE BEING CHANGED AND MUST WITNESS THE REMOVAL; AND INSTALLATION TO INCLUDE TORQUE PROCS AND COTTER KEYS INSTALLED. THE QUALITY CTL INSPECTORS HAVE BEEN INSTRUCTED TO PHYSICALLY CHK ALL OF THE FLT CTL BOLTS FOR MOVEMENT AND JUST FOR COTTER PINS BEING INSTALLATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THERE ARE APPROX 27 BOLTS THAT HAVE TO BE REPLACED ON A TIME CONTROLLED BASIS. THE WEIGHT OF THE ATTACHED BLADES ON THE MAIN ROTOR HEAD CREATE A PRE-LOAD STRESS THAT DOESN'T ALLOW FOR NOTICING ANY LOOSE BOLTS CAUSED BY; AS IN THIS CASE; MISSING BUSHINGS. ONLY BY GRABBING AND TWISTING THE T/E OF EACH BLADE; WERE THEY ABLETO DETERMINE THE AREA OF PLAY THAT CAUSED THE UNCOMMANDED PORPOISING. REPORTER ALSO STATED HE BELIEVES THE CHANGES THEY HAVE MADE TO THEIR MAINT PROCEDURES; INCLUDING ADDING REFERENCES ABOUT THE BUSHINGS WITH THE BOLTS THAT SECURE THE HINGED SUPPORT TO THE SWASHPLATE; WILL PREVENT ANY FURTHER DISCREPANCIES THAT RESULT FROM THE TIME CONTROL BOLT REPLACEMENT REQUIREMENTS.

Data retrieved from NASA's ASRS site as of May 2009 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.