Fatigue crack results in total loss of engine power — General Aviation News

Jun 03, 2023

By General Aviation News Staff · March 27, 2023 · 19 Comments

According to the pilot receiving instruction they departed the airport in Lodi, California, uneventfully and climbed to their cruise altitude, 3,000 feet mean sea level. The pilot receiving instruction donned an instrument instruction tool at 200 feet above ground level.

Once they reached cruise altitude, he reduced power to 2,450 rpm, applied carburetor heat, and leaned the fuel/air mixture to achieve best rpm. He followed the airplane’s cruise checklist, scanned the engine instruments and did not observe any anomalies as each instrument was reporting normal operation.

Approximately five minutes later, he heard a sound that resembled a gun shot, and the propeller stop rotating. The instructor took control of the Cessna 150J and announced “my controls” while the pilot searched for a suitable place to land and attempted to restart the engine. He was unsuccessful.

Although they were surrounded by fields, their options for suitable landing sites were limited due to trees or other obstacles. They were able to glide the airplane to the end of an almond tree field where the airplane hit trees before coming to rest in the field.

Photographs of the airplane taken by local law enforcement showed substantial damage to the wings and empennage.

A post-accident examination of the engine revealed that the top of the engine case was fractured with a hole that extended about seven inches in diameter. Additionally, the lower case had fractured around the circumference of the No. 4 cylinder. Continuity of the throttle and mixture controls was confirmed from the cockpit to their respective arms at the carburetor.

The No. 3 cylinder connecting rod was fractured at the connecting rod shank and was mechanically damaged. The No. 4 cylinder barrel rim was bent outboard towards the crankcase. A borescope inspection of No. 3 cylinder revealed that the connecting rod bushing end was still attached to the piston pin, but the piston ring seal was fractured, and part of a piston ring was exposed. No evidence of oil starvation or thermal damage was observed.

Metallurgical analysis of the No. 3 cylinder connecting rod revealed that the fractured face exhibited crack arrest marks consistent with fatigue cracking. The fatigue crack emanated from the outer surface at one corner of the arm. A portion of the fatigue crack also displayed mechanical damage that destroyed some of the fatigue crack features. However, the origin of the fatigue crack did not exhibit any indications of mechanical damage such as a gouge. The No. 3 cylinder piston contained evidence of heavy combustion deposits at the outer crown. Similar deposits were also observed near the spark plug ports and the intake and exhaust valves.

Post-accident examination of the airplane revealed that the carburetor input fuel line contained a smell that resembled automotive gasoline. The owner stated that he had only used 100 low lead aviation grade gasoline (100LL) in the 60 total flight hours he had accumulated in the airplane since he purchased it. Fuel receipts showed that the pilot purchased 12.3 gallons of 100LL the day before the accident and 17 gallons of 100LL on the day of the accident. According to the owner, the previous owner had implied that he only ever used 100LL during their correspondence. The fuel from the accident site was not tested.

The pilot stated that he regularly used carburetor heat due to a “serious issue with carb ice” as he had that day when he reached cruise altitude. He would have adjusted the carburetor heat until the carburetor heat temperature gauge reached a certain temperature and then leaned the mixture out to achieve best rpm before enrichening the mixture about three full turns.

The previous owner who flew the airplane from the engine’s most recent overhaul had passed away and was not available for a statement.

According to the FAA Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B), detonation is defined as “an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber. It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves.”

The section also provides several causes of detonation, including the use of a lower grade fuel than specified by the manufacturer and operating the engine at high power settings with an excessively lean mixture.

According to the PHAK, preignition occurs when “the fuel-air mixture ignites prior to the engine’s normal ignition event. Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part of heat sufficiently to ignite the fuel-air charge. Preignition causes the engine to lose power and produces high operating temperatures. As with detonation, preignition may also cause severe engine damage because the expanding gases exert excessive pressure on the piston while still on its compression stroke.”

It should also be noted that detonation and preignition can occur simultaneously and/or one may be caused by the other.

Probable Cause: A total loss of engine power during cruise flight due to a fatigue crack in the No. 3 cylinder connecting rod shaft resulting from either preignition or detonation, which resulted in a forced landing and impact with terrain.

NTSB Identification: 102703

To download the final report. Click here. This will trigger a PDF download to your device.

This March 2021 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.