FWP-149D N9145 accident near Killeen, Texas

Probable cause

The total loss of engine power due to a fatigue failure of a crankshaft counterweight mounting lobe, which resulted in a counterweight separation and catastrophic secondary damage to the engine drivetrain. Contributing to the accident was the mislabeled warning light, the pilot’s erroneous belief that the airplane was safe to operate after the metal contamination was observed in the oil filtration system, and his failure to maintain best glide airspeed during the forced landing, which resulted in the airplane exceeding its critical angle-of-attack and an inadvertent aerodynamic stall at a low altitude.

NTSB analysis

The pilot was conducting a cross-country flight when the airplane had a total loss of engine power while it approached the intended destination airport.

Based on witness accounts, the pilot was unable to glide the airplane to the intended destination airport and entered an inadvertent aerodynamic stall at a low altitude. One witness reported that the airplane’s wings rolled left-and-right 2 to 3 times before the airplane “stalled” with the left wing down. The airplane impacted the ground in a left-wing down roll attitude and was destroyed during a postimpact fire.

Postaccident examination of the engine determined that the loss of engine power was due to a fatigue failure of a crankshaft counterweight mounting lobe, which resulted in the separation of a counterweight and catastrophic secondary damage to the engine drivetrain.

A couple days before the accident, an aviation mechanic found metal contamination in the engine oil filtration system after an associated warning light illuminated during a post-maintenance engine run. The mechanic and pilot discussed the metal contamination and agreed to have it submitted to a laboratory for identification.

The oil filtration system was equipped with a ball-and-spring “bypass” switch, which when wired to a cockpit warning light offered a visual indication of when the filtration system was bypassing engine oil instead of flowing through the filter screen. A separate magnetic chip detector was offered as an optional feature for the oil filtration system; however, postaccident examination of the oil filtration system confirmed that the optional chip detector was not installed. As such, the warning light installed in the instrument panel was mislabeled “chip detector” instead of a label that conveyed the oil filtration system was in a bypass condition.

When the mechanic saw the warning light illuminated during his post-maintenance engine run, it was a visual indication that the engine had produced enough metal contamination to restrict oil flow in the filtration system, resulting in a bypass condition. The mechanic cleaned the filtration housing, filter screen, and bypass switch. He then added new oil to the engine and performed another engine run, during which the “chip light” did not illuminate. The mechanic told the pilot not to fly the airplane until the laboratory results were returned, but he did not ground the airplane to prevent additional flights.

The pilot conducted three flights, totaling at least 1.8 hours, following the maintenance. As such, it is likely he erroneously believed the airplane was safe to fly if the “chip detector” warning light was not illuminated, when, in fact, the engine was not in an airworthy condition due to the progressive failure of the crankshaft counterweight mounting lobe that likely produced the metal contamination found during the last maintenance.

A review of automatic dependent surveillance-broadcast track data revealed that the pilot did not maintain the airplane’s best glide airspeed during the final minutes of the flight. Based on the published glide performance, the airplane did not have sufficient altitude to reach the runway at any point during the approach. However, had the pilot maintained best glide airspeed the airplane would have retained a safe margin above the aerodynamic stall speed and, as such, might have resulted in a less severe off-field landing.

Text from the NTSB. Open the NTSB docket for CEN21FA304 for the full record.