WPR18FA088 · Hastings Thomas G CIRRUS VK 30

The private pilot, who was the builder of the experimental amateur-built airplane, and three passengers departed on a cross-country flight. A witness about 2 miles from the accident site saw the airplane in cruise flight about 2,000 to 3,000 ft above the ground when the right wing rose, and he heard what he perceived as the engine advancing to full power as the airplane entered a vertical descent that continued to ground contact. The wreckage was mostly contained within an area about 100 ft in diameter except for the left horizontal stabilizer fairing, which was found about 2,500 ft west of the main wreckage.

The airplane's engine was mounted mid-fuselage and drove an aft-mounted, "pusher" configuration, constant-speed propeller. A torque tube (driveshaft) equipped with forward and aft elastomeric couplings was used to transfer power from the propeller flange located on the front of the engine crankshaft to the propeller. The driveshaft was found fractured about 2 ft forward of the propeller. A metallurgical examination revealed that the driveshaft fractured due to overstress loads in torsion and rotational bending. Deformation of the forward side of the fracture was consistent with continued rotation in the clockwise direction relative to the aft portion of the shaft, indicating that the shaft was being powered by the engine at the time of the fracture.

Further examination of the driveshaft revealed that one of eight nuts from the forward coupling and one of two nuts from the aft coupling were backed off (4 mm and 2.11 mm, respectively) of their respective bolts. It is likely that the bolts were under torqued and, at some point, the driveshaft developed a vibration which caused the nuts to back off, which lead to the catastrophic in-flight separation of the driveshaft. Review of the airplane's maintenance records revealed that, about 8 years and 300 flight hours before the accident, the entry indicated that an unspecified coupling was replaced, and the engine drive system was balanced. The entry did not detail the work accomplished. A further review of the records revealed that, 5.1 flight hours before the accident flight, the bolts on the flywheel and couplings were retorqued. The entry did not detail the work accomplished nor indicate the torque values, but it is likely that the nuts were not properly torqued at that time, allowing them to back off during flight.

Following the driveshaft failure, the engine likely began to overspeed, consistent with the increased engine sound reported by the witness. In addition, the forward portion of the driveshaft likely contacted the bottom of the horizontal stabilizer, and the horizontal fairing subsequently separated in flight, resulting in an inflight upset and loss of control.

The three-bladed propeller was found with two of the three blades broken, indicating that at the time of impact, there was no power to the propeller. The propeller spinner remained attached to the propeller and displayed little damage. The left elevator was found about 80 ft from the empennage and red paint transfer marks were found on the trailing edge of the elevator, consistent with propeller tip contact. Thus, it is likely that the propeller blade struck the elevator during the rapid descent, separating both from the airplane.

Although the level of diphenhydramine detected in specimens from the pilot's cavity blood (even accounting for post-mortem redistribution) may have been in a range where it may have resulted in psychoactive effects, there is no direct evidence of impaired judgment or decision-making and no evidence of psychomotor slowing. Therefore, it is unlikely that the pilot's use of diphenhydramine contributed to this accident. In addition, the pilot had moderate coronary artery disease without evidence of ever having an acute cardiac event. Given the circumstances of the accident, the pilot's heart disease was not a contributing factor.

Verbatim NTSB analysis and probable cause from the NTSB dataset