WPR13LA184

The improper overhaul of the propeller, which resulted in the installation of an incorrect propeller actuating pin and deformation of the ferrule hole and led to the pin’s fatigue failure. Contributing to the accident was the failure of multiple owners and maintenance technicians to comply with the propeller manufacturer's recommended overhaul interval.

The pilot reported that, shortly after he began a letdown of the airplane from 9,500 ft mean sea level for landing at the airport, the airplane experienced a "very sudden" onset, high-frequency vibration. The pilot unsuccessfully attempted to diagnose and correct the problem. Although the pilot initially believed that the airplane could reach the airport, it was unable to maintain altitude. When the airplane was about 2 miles from the runway and over a lake, the pilot began a right 180-degree turn to land on an unobstructed portion of the shoreline. During the turn, the right wing struck the lake, and the airplane then came to rest inverted in about 2 ft of water. Postaccident examination of the airframe and engine did not reveal any preimpact mechanical problems that would have precluded normal operation and continued flight. However, detailed examination of the three-bladed, controllable-pitch propeller revealed that one steel actuating pin, located inside the propeller hub and which controlled the pitch of one blade, had failed in fatigue. The pin's failure caused the pins to be torn from the other two propeller blades, which allowed the blades to rotate freely in the propeller hub and resulted in a loss of propeller thrust and the vibration that was felt by the pilot. About 7 years before the airplane was manufactured, the propeller manufacturer changed the design of the component hole (the "propeller ferrule") from a through hole to a blind hole that only penetrated one surface of the ferrule. The ferrule accepted the threaded end of the pin, and the corresponding pin thread length was reduced so that the pin would not contact the bottom of the hole when installed. The postaccident examination indicated that the failed pin was likely the longer pin associated with the previous design as evidenced by the contact and deformation of the bottom of the ferrule hole, which resulted in visible ferrule damage. The manufacturer's guidance explicitly addressed and prohibited installation of the longer pins in the blind ferrule hole. Bottoming of the pin in ferrule hole and the hole's subsequent deformation during installation prevented the proper seating of the pin and permitted operational bending loads to be transmitted to the threaded portion of the pin, which was not designed for such bending loads. The bending loads on the threaded portion resulted in crack initiation in the pin thread valley and the eventual fatigue failure of the pin. Review of the maintenance records indicated that multiple people had owned the airplane and that it had been continuously maintained in an airworthy condition through annual inspections and scheduled and unscheduled maintenance since at least 1998. However, the propeller was last overhauled more than 15 years before the accident, which exceeded the propeller manufacturer's recommended overhaul interval by a factor of three. The investigation could not determine why the owners or maintenance personnel did not comply with the propeller manufacturer's recommended overhaul interval. Further, the maintenance records did not provide sufficient detail to determine why the incorrect part was used or why its incorrect use and resulting damage were not detected during the overhaul. If the propeller had been overhauled in accordance with the propeller manufacturer's guidance, the improper pin installation likely would have been detected and the damaged, nonairworthy propeller would have been replaced with an airworthy unit.

While en route the pilot canceled his IFR flight plan, but continued to communicate with SoCal Approach for flight following services. All communications, including those during the IFR portion of the flight, were normal until about 1033. At that time the pilot informed the air traffic controller that he had a "serious vibration," but that he was still planning to continue to his destination, L35. The controller asked the pilot whether he knew what the cause of the vibration was. The pilot responded in the negative, and described some of his instrument indications. This was followed by a brief exchange about the number of persons and amount of fuel on board. About 1035, the controller asked the pilot whether he was able to maintain his altitude, and the pilot responded that he believed he could. The controller then advised the pilot to hold his altitude as long as possible, and to "circle down" over the airport once he reached it. The pilot concurred with that strategy. The controller then provided a telephone number and requested that the pilot call him to advise that he had landed safely, which the pilot agreed to do. About 1036, when the airplane was about 7 miles southwest of the airport, the pilot queried the controller as to when he thought he should switch to the L35 "CTAF" (common traffic advisory frequency). The controller provided the traffic that he observed to the pilot, and then instructed the pilot to contact CTAF. The pilot agreed, and no further communications occurred between the pilot and SoCal Approach. The L35 CTAF communications were not recorded, but the radio operator on the ground at L35 provided a written statement of his recollections. According to the radio operator's statement, the pilot contacted CTAF and advised that his airplane had a serious vibration problem. The radio operator "cleared" him to land on runway 8, and also broadcast an announcement for all other aircraft to "stay clear" to allow the subject airplane to land. The pilot then announced that he was unable to make the airport, and was planning to land in a field. No other communications were received from the airplane. L35 was situated at the east end of Big Bear Lake, which is located in an east-west valley, in mountainous terrain. The lake and the airport are at a similar elevation; L35 elevation is 6,752 feet above mean sea level. L35 is equipped with a single runway 08/26, which measured 5,850 by 75 feet. Propeller Design The propeller was a constant-speed design which permitted the blades to change pitch in flight to maintain a preset rpm selected by the pilot. Pitch change commands were effected by pressurized engine oil, metered by the propeller speed governor, that drove a piston fore and aft inside the hub, which, through a series of links, converted the piston translation into blade rotation. Each propeller blade had ring-shaped steel ferrule attached to its blade butt. The ferrule was oriented so that its circular plane was perpendicular to the span of the blade. Once installed, the ferrule did not move relative to the blade. Each ferrule measured about 5 inches in outside diameter, and about 1 inch thick. The face that was oriented towards the propeller axis of rotation (center of the hub, away from the propeller blade) was referred to as the "forward" face; this is not a reference to its orientation in the airplane axis system. Each propeller blade was also equipped with a steel actuating pin. Each pin measured about 3 1/2 inches in length. The "upper" two-thirds of the overall pin length consisted of a constant- diameter rod that terminated in a smooth cone at the top. The lower one-quarter of the overall pin was threaded, and a hexagonal set of wrenching flats was situated between the threaded section and the upper smooth rod. The wrenching flat section was approximately twice the diameter of the other two sections, and each of those sections was smoothly faired into the wrenching flat. An actuating pin was threaded into the forward face of each ferrule. The pin extended parallel to, but in the opposite direction of, the blade. A special washer served as part of the interface between the pin and the ferrule. One side of the washer was planar with concentric grooves. The center hole was chamfered on the opposite side to accommodate the stress relief fillet of the pin. The planar side of the washer abutted the ferrule face, and the chamfered side of the washer accommodated the stress relief fillet of the pin that was situated between the pin threads and the wrenching flats. When properly installed, there was no relative motion between the pin, washer, and ferrule. The wrenching flats and the ferrule had accommodations for safety-wiring the pin to the ferrule to prevent the pin from backing out of the ferrule. The actuating pins were primary components of the propeller blade pitch control linkage. They converted the linear piston motion into rotary motion of the propeller blades in the hub, and also served to hold the blades at the commanded pitch setting. Due to the combination of operating airloads on the propeller blades, and the positioning loads of the propeller hub piston, the actuating pins were continuously loaded in bending. Load magnitudes and directions were not constant. The pin, washer, and ferrule design was such that, when properly installed, the bending loads were resisted by the ferrule, washer, and the portion of the pin "above" (protruding from) the ferrule face; no bending loads were transmitted to the (primarily threaded) portion of the pin "below" the ferrule face. Failure to properly seat the pin in the washer and seat the washer against the ferrule face would permit bending loads to be transferred to any portion of the pin not engaged in the ferrule threads. Ferrule and Actuating Pin Design Changes The original design of the ferrules utilized through-holes for the actuating pin installations; the threaded hole for the pin extended completely through the ferrule. In 1972, due to ferrule failures at the through-holes, McCauley issued Service Bulletin (SB) 99. SB99 was considered "necessary and required" by McCauley, and was to be accomplished at the next propeller overhaul. SB99 mandated the use of modified or new (redesigned) ferrules and actuating pins across a wide range of propeller models, including the accident propeller model. The SB eliminated the use of ferrule through-holes, and instead mandated the use of blind holes that only penetrated one surface of the ferrule. In conjunction with the shallower blind hole, the threaded section of the actuating pin was reduced by approximately 0.085 inches. For the accident propeller model, the superseded (through-hole) ferrule part number (PN) was C-3111, and was replaced by the blind-hole ferrule PN C-4451. In addition to a lack of through-holes, new PN ferrules bore a machined circumferential groove on their outer diameter. Superseded ferrules with through-holes could be modified to the new blind hole configuration by disabling or removing the threads in the through hole, and drilling and threading a new blind hole. The ferrule face opposite the forward face was referred to as the "aft" face. SB99 explicitly warned against the creation of bulges on the aft face as a result of the drilling, and prohibited the use of any ferrules that incurred or exhibited such bulging. The superseded (longer) actuating pin PN was B-3491, and the new (shorter) actuating pin PN was B-4460. In addition, although manufactured of the same material, the required hardness of the new actuating pin was increased. SB99 permitted the use of superseded ferrules with new blind holes, and the use of superseded actuating pins with threaded portions reduced in length by cutting. Once the new pin design was implemented, the superseded pins were no longer produced; the new and old pins were not produced concurrently. The engineering drawings for both the superseded a