ANC24FA008 · Piper PA-18-150

The pilot was conducting a cross-country personal flight with one passenger. After takeoff, the airplane climbed to altitude to overfly a large body of water. After crossing over the body of water the airplane began a descent. Subsequently, a vehicle dash-mounted camera captured about 7 seconds of video of the airplane falling vertically in an inverted flat spin with the left wing’s outboard section partially separated from the airplane, folded in a V-shape, before impact. The recorded video did not capture the initiating event. Review of flight track data from the airplane’s GPS indicated the pilot initiated a descent of about 1,000 ft per minute (fpm) as the airplane neared the shoreline. The descent rate remained steady until the airplane crossed over the shoreline at an altitude of about 4,500 ft mean sea level (msl), when the descent rate increased to about 4,000 fpm. The airplane came to rest in low brush and trees in an inverted attitude. All major airframe and engine components were recovered at the impact site. Control continuity was established between the cockpit flight controls and their respective control surfaces. Examination of the engine revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation. Examination of the left wing revealed a portion of the leading edge skin was flattened and fractured into two pieces. There was mechanical tearing on the upper portion of the aluminum leading edge skin and the leading edge ribs were crushed against the forward spar in this area. The fabric skin covering the aluminum leading edge skin in this area was torn and ripped, coincident with the mechanical damage. Multiple linear, parallel gouges, cuts, and scuffs were oriented aft and inboard in the wing fabric around the area that was torn. There was no evidence of transferred material in these marks, and no evidence of tree debris around the leading edge damage. The mechanical damage to the left wing’s leading edge and the linear, parallel witness marks were consistent with impact by an object. The examined fracture surfaces had an angled, dull, grainy appearance consistent with overstress separation. The left wing had no obvious corrosion, and it was assembled correctly and was intact before the initiating event, with no pre-existing conditions that would compromise its strength. The deformation of the left wing’s leading edge and spars and the fracture of the left wing spars were not consistent with normal in-flight loads. The separation of the propeller tip from overstress was consistent with impact separation while under power. Based on physical evidence, it is likely that the leading edge of the left wing was impacted, at an undetermined time, which deformed the forward and aft spars and fractured the forward spar. Aerodynamic loads then twisted the left wing’s outboard section leading edge down and fractured the rear spar, partially separating the left wing from the fuselage. The left wing’s outboard section remained attached to the lift struts, and aerodynamic loads deformed it into a V-shape about the strut attach points. The location and orientation of silver paint transfer marks indicate the upper surface of the left wing’s outboard section likely impacted the airplane’s left ski as the wing fractured and partially separated. This sequence rendered the airplane uncontrollable and resulted in it entering an inverted flat spin. Examination of the wing showed impact signatures consistent with a glancing impact, but the timing of the contact could not be determined. Based on the available evidence, investigators could not establish whether the damage occurred on the ground, in flight, or immediately before the accident. If a second aircraft had contacted the accident airplane, it is possible that the damage would have been minor enough for continued flight and a normal landing. However, aside from an unverified radio transmission or phone call, the investigation found no evidence that another aircraft was near the accident airplane at the time of the event.

Verbatim NTSB analysis and probable cause from the NTSB dataset