CEN12FA601

The pilot’s failure to maintain airspeed following a partial loss of engine power for reasons that could not be determined during postaccident examination, which resulted in an aerodynamic stall and loss of airplane control. Contributing to the severity of the pilot’s head injuries was the failure of the shoulder harness assembly.

Witnesses observed the airplane make a normal engine run-up before takeoff followed by a normal takeoff from the runway. Shortly after takeoff, the engine did not sound like it was developing full power, and the airplane was struggling to climb. Subsequently, the airplane made a 180-degree turn and then descended toward terrain in a nose-down attitude. Postaccident examination of the airplane revealed no evidence of any preimpact mechanical failures or anomalies that would have precluded normal operation. Although the weather conditions at the time of takeoff were conducive to the formation of carburetor ice at glide and cruise power, it could not be determined whether carburetor ice was a factor in the loss of engine power. It is likely that the pilot failed to maintain airspeed during the turn back to the airport, which resulted in a stall. Postaccident examination of the airframe revealed that the pilot's shoulder harness failed during the accident. Material examination of the shoulder harness webbing and stitching showed that they failed due to an overload event. The occupants' injuries suggest that both occupants were using the four-point shoulder harnesses at the time of the accident and were exposed to strong deceleration forces. If the pilot's shoulder harness had not failed, he likely would not have suffered the brain injury he received, which caused permanent disability. However, it is possible that an intact shoulder harness could have led to more severe chest, abdominal, or cervical injuries. The investigation could not determine whether the pilot would have died or sustained a permanent disability from some other injury if the shoulder harness had remained intact.

Pilot Seat The pilot seat was found with its four seat posts attached to the aircraft floor, and the seat frame was bent down, forward, and to the right. Pilot Restraint The pilot restraint was manufactured by Aero Fabricators (model number H-702-300) and installed on the accident airplane on November 11, 2010. The restraint was manufactured on May 27, 2010. The shoulder harness label stated the restraint system was a FAA-PMA part with a "rated strength of assembly [of] 1,500 pounds." The lap belt was attached to the seat at the back seat posts. The lap belt fitting was pinned between the installation bolt and the seat frame, which prevented the lap belt from swiveling forward and aft. The lap belt webbing was pinned between the lap belt fitting and the seat frame, which prevented the seat belt from being able to be adjusted for proper fitting. The left side lap belt had been cut by rescue personnel, and the tongue end of the lap belt was missing from the wreckage. The shoulder harness had been removed from the airplane by FAA inspectors. The shoulder harness was separated at the stitched "Y" junction that connected the two shoulder straps and the fuselage attach strap behind the occupants head. Passenger Seat The passenger seat was found with its four seat posts attached to the aircraft floor, and the front frame tube was creased down approximately 10 inches from the forward left seat post. Passenger Restraint The passenger restraint was manufactured by Aero Fabricators (model number H-702-300) and installed on the accident airplane on November 11, 2010. The shoulder harness portion of the restraint was manufactured on May 27, 2010, and the lap belt portion was manufactured on August 25, 2010. The shoulder harness label stated the restraint system was a FAA-PMA part with a "rated strength of assembly [of] 1,500 pounds." The lap belt was attached to the seat at the back seat posts. The lap belt fitting was pinned between the installation bolt and the seat frame, which prevented the lap belt from swiveling forward and aft. The lap belt webbing was pinned between the lap belt fitting and the seat frame, which prevented the seat belt from being able to be adjusted for proper fitting. The shoulder harness had been removed from the airplane by FAA inspectors. The shoulder harness was intact. Seat Restraint Installation The NTSB Survival Factors Group inquired to the mechanic who installed the seat belts and shoulder harnesses into the airplane about the pinned lap belts found in the accident airplane. He explained that he installed bushings into the attachment fittings for the lap belts. The bushings would allow the bolt at the attach point to be fully tightened while also allowing the seat belt to be adjusted and swivel forward and aft. The bushings are not provided by Aero Fabricators in the installation kit for a new restraint system or detailed as required parts in the installation instructions. During the examination of the airplane, the bushings were not found in the lap belt attachment fittings, thus pinning the hardware and lap belt webbing in one position. Shoulder Harness Restraint System Testing Webbing Breaking Strength Testing On March 18, 2013, an independent research lab conducted a breaking strength test using an exemplar webbing sample provided by Aero Fabricators. In accordance with the Society of Automotive Engineers Aerospace Standard (SAE AS) 8043B, the breaking strength of the webbing was measured approximately 20.9 kilonewtons (kN), which met the breaking strength requirement for upper torso of 17.8 kN, but did not meet the breaking strength requirement for the pelvic of 22.2 kN. Technical Standard Order (TSO) C22f, dated January 1, 1990, was effective at the time Aero Fabricators applied for the seat belt STC. TSO C22f stated "new models of safety belts that are to be identified with applicable TSO markings and that are manufactured after May 1, 1972, must meet the standards set forth in National Aerospace Standard (NAS) Specification 802 revised May 15, 1950. NAS 802 stated that the rated minimum breaking strength of the complete belt assembly, i.e. 1,500 pounds (lbs). Therefore, the pelvic breaking strength requirement at the time of Aero Fabricators STC application was 2,250 lbs (10kN). Although the breaking strength of the webbing did not meet the TSO pelvic requirements currently effective as detailed in SAE AS 8043B, the webbing breaking strength met TSO C22f standards in effect at the time of STC approval. Thread Testing On March 8, 2013, and April 8, 2013, an independent research lab conducted thread tensile strength and elongation testing using a black and white thread sample provided by Aero Fabricators. In accordance with the test standard, both the black and white threads met the minimum breaking strength and minimum elongation per the specifications. March 2013 Shoulder Harness Static Testing On March 20, 2013, an independent research lab conducted a series of 3 static tests, under the supervision of NTSB and FAA personnel. Two exemplar should harness restraint assemblies manufactured by Aero Fabricators, and the aft occupant shoulder harness from the accident airplane were tested. The shoulder harness restraint was installed on a loading block per the SAE AS 8043B, and load was applied through three attachment points to obtain the load profile described in the standard. The attachment points were one on each side of the lap belt fitting and the third on the shoulder harness fitting. The loading profile included evaluation of the restraint system at 612 lbs (equivalent load at 9g's), 1,500 lbs (rated strength depicted on restraint label), 2,500 lbs (per SAE AS 8043B), and to restraint failure. The first test conducted used an exemplar shoulder harness restraint system ordered from Aero Fabricators in November 2012. This restraint system reached a maximum load of 1,239 pounds at the point of failure of the restraint. The restraint experienced a stitching failure at the "Y" shoulder harness junction at the maximum load. The second test conducted used the aft occupant shoulder harness from the accident airplane. This restraint system reached a maximum load of 1,202 pounds at the point of failure of the restraint. The restraint also experienced a stitching failure at the "Y" junction at the maximum load. The third test conducted used a second exemplar shoulder harness restraint system ordered from Aero Fabricators in March 2013. This restraint system reached a maximum load of 1,281 pounds at the point of failure of the restraint. The restraint also experienced a stitching failure at the "Y" junction at the maximum load. August 2013 Shoulder Harness Static Testing On August 29, 2013, an independent research lab conducted another test, under the supervision of NTSB and FAA personnel. This testing was conducted using a mock-up of the accident airplane seat structure obtained from measurements taken during a postaccident examination and from an exemplar Aeronca airplane. The seat was constructed to achieve the proper seat pan angle to closely mimic the seat installed in an Aeronca airplane. This test was conducted similar to the test conducted during the original STC certification for the Aero Fabricators shoulder harness conducted in March 1992, the loads were calculated using 40 percent of a 9g load of a 170 lb person. A limit load of 408 lbs and an ultimate load of 612 lbs were tested. The test blocks used by Aero Fabricators during STC certification testing were borrowed for this testing and were used as described in FAA Advisory Circular (AC) 23-4, Static Strength Substantiation of Attachment Points for Occupant Restraint System Installations. The lap and torso blocks were placed on the rigid seat and the shoulder harness restraint assembly was installed. The load was applied through the torso and lap locations, and then eval