ERA20LA341 · Tecnam P92

While in cruise flight at the conclusion of a flight lesson, the airplane suddenly began to vibrate severely. Despite the flight instructor’s efforts, the Rotax 900 series engine eventually lost total power, and he performed a forced landing to a soybean field. During the landing, as the airplane slowed, the nose landing gear dug into the soft earth and separated, and the airplane nosed over.

Examination of the engine revealed that the No. 1 cylinder’s exhaust valve spring retainer was broken, and the exhaust valve had fallen into the No. 1 combustion chamber, which resulted in the loss of power. This valve spring retainer failure was not the first one with a Rotax 900 series engine; in 2017, at the end of a cross country flight, an airplane powered with the same series engine experienced a total loss of engine power, and the pilot performed a forced landing during which the airplane sustained substantial damage. Examination of that engine revealed the presence of a broken valve spring retainer that had resulted in the loss of power. Additionally, it was discovered that the valve spring retainer displayed evidence of metal fatigue.

During 2019 and 2020, in addition to this accident, three more cases of broken valve spring retainers on the Rotax 900 engine series occurred in the United States. All the engines had differing hours of operation. Extensive metallurgical examination of the engine components from these four engines revealed that they met their specifications, and the fractured surfaces on the valve spring retainers revealed the presence of fatigue with pronounced vibration stripes, which was the same pattern observed on the valve spring retainer from the 2017 accident.

Review of the engine manufacturer’s published guidance revealed that air could be introduced into the oil lubrication system through several means, including exceedance of the maximum bank angle limitation of 40º, poorly or insufficiently vented hydraulic valve tappets, lack of proper oil system purging, spinning the propeller in the reverse direction from normal rotation, or opening portions of the oil system during maintenance or servicing. Testing an exemplar engine with air introduced into the lubrication system revealed that with air trapped in the hydraulic tappets, it took about 6.5 minutes of engine operation at 2,538 rpm for air to be purged from the tappets allowing them to work as designed. This indicated that with air trapped in the hydraulic tappets, the valve train could be overloaded, which could lead to a fatigue crack and breakage of a valve spring retainer; this was likely the reason for the fatigue cracking of the valve spring retainers in the 2017 accident, in this accident, and in the other four 2019-2020 engine failures.

During this investigation, the engine manufacturer reviewed its records and found a total of 18 production engine failures due to broken valve spring retainers. The engines were installed on multiple types of aircraft with a large spread in operating hours from as low as 7 hours to as high as 1,936.6 hours. All the components examined met their specifications, and not all the engines were affected by service bulletins that had been issued due to deviations in the manufacturing process of the valve push-rod assembly, which could result in partial wear on the rocker arm ball socket and initiate rocker arm cracking leading to a malfunction of the valve train. These engine failures indicated that valve train failure could occur for reasons other than the push-rod manufacturing issue such as air being introduced into the lubrication system. Additionally, after the engine manufacturer’s record review, an engine in an airplane that was produced in 2021, which should have had all changes included in Rotax guidance materials incorporated before it was placed into service, experienced a valve spring retainer failure, confirming that valve train failure could occur for reasons such as air being introduced into the lubrication system.

Verbatim NTSB analysis and probable cause from the NTSB dataset