ANC25LA028

During an instructional flight, the flight instructor detected a vibration; shortly thereafter he heard a loud bang, the engine lost total power, and the nose dropped. He performed a forced landing to a heavily vegetated field. After landing, the flight instructor noticed that the propeller was missing. It was later found in a nearby residence; no one was injured. The airplane sustained substantial damage to both wings, the fuselage, and the empennage. Examination of the separated propeller revealed that the engine crankshaft had fractured and separated just aft of the crankshaft propeller flange (CPF). This flange is an integral part of the crankshaft and is where the propeller attaches to the engine; however, the CPF was found attached to the separated propeller and not the crankshaft where it belonged. The CPF was removed from the propeller and examined by an NTSB metallurgist to determine why the crankshaft had fractured and separated. The examination revealed that the CPF had been improperly modified by the drilling and installation of additional bolts. Investigators were unable to determine the purpose for the modification, when it was made, or by whom. Available maintenance records did not include any entries noting that the propeller assembly had been removed or altered since initial installation. Additionally, there are no Lycoming maintenance instructions, repair instructions, or service bulletins that permit hole drilling and filling of the CPF.

On March 27, 2025, about 1200 Hawaii-Aleutian standard time, a Piper PA-28-140, N261FC, was substantially damaged when it was involved in an accident near Royal Kunia, Hawaii. The flight instructor and student pilot reported minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor and student departed Honolulu Airport (HNL), Honolulu, Hawaii, for the student’s first instructional flight. As the airplane climbed to about 1,500 ft mean sea level the instructor reported feeling a slight vibration, which he had not felt on a previous flight in the accident airplane earlier that day. The flight instructor initiated a climb and turned back to HNL. He then heard a loud bang, the engine lost total power, and the nose of the airplane dropped. The flight instructor subsequently initiated an off-airport emergency landing to a field. The airplane impacted several small trees and vegetation during landing, resulting in substantial damage to the fuselage, wings, and empennage. Upon landing, the flight instructor noticed that the propeller was missing. It was later found in a nearby residence; no one was injured. Examination of the separated propeller revealed that the engine crankshaft had fractured and separated just aft of the CPF. The separated CPF, mating flywheel starter ring gear, and six attaching bolts with washers were sent to the NTSB Materials Laboratory for examination. The examination revealed that the CPF had separated from the crankshaft through the webs between the lightening holes at roughly the same radial location. The separation was due to multiple fatigue fractures through the flange webs. The fracture surfaces on two diametrically opposite webs were perpendicular to embedded cylindrical-shaped items, consistent with bolt shanks. The fatigue fractures initiated at these bolt pieces in two of the crankshaft propeller flange webs and propagated over a long period of time. Maintenance records indicated that the Lycoming O-320-62A engine and the Sensenich propeller were installed on the airplane on September 15, 2019. Available maintenance records did not include any entries noting that the propeller assembly had been removed or altered since its initial installation. The most recent annual inspection was completed on February 5, 2025, and indicated 1,805.1 hours on the engine since overhaul. The most recent 100-hour inspection of the propeller was completed on March 7, 2025, and records indicate that it had about 3,975.6 hours since new. There is no record of a crankshaft modification, and no part markings were observed on the outer diameter of the propeller flange as specified on the manufacturer’s drawing; therefore, the exact age and model of the crankshaft could not be determined. Historical engine records indicating the date and details of the last engine overhaul could not be located.