Piper PA-28-140 N261FC accident near Royal Kunia, Hawaii

Probable cause

Improper alteration of the crankshaft propeller flange at an unknown time, which resulted in an in-flight separation of the propeller assembly.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ANC25LA028 for the full record.