N562TU

HISTORY OF FLIGHT

On September 28, 2020, about 1800 eastern daylight time, a Tecnam P92 airplane, N562TU, was substantially damaged when it was involved in an accident near Centerville, Maryland. The flight instructor and student pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 instructional flight.

The airplane was being operated by Chesapeake Sport Pilot, a 14 CFR Part 91 flight school based at Bay Bridge Airport (W29), Stevensville, Maryland. According to the flight instructor, he and a student pilot were returning to W29 when the airplane suddenly began to vibrate severely. This was followed by a reduction in engine rpm from about 5,100 rpm to 4,500 rpm. Review of onboard recorded data indicated that the fuel pressure, cylinder head temperature, and oil temperature remained relatively steady until the loss of power occurred.

The flight instructor took over the flight controls from the student pilot and manipulated the throttle control to see if a different power setting would reduce the vibrations. Movement of the throttle control did not elicit a corresponding response from the engine. The flight instructor then ensured that the fuel valves were all on and turned on the electric fuel pump. There were no changes to the vibrations or power.

Assessing the situation as an impending engine failure, the flight instructor configured the airplane for best glide speed, turned towards the nearest field for a potential forced landing, and made a “Mayday” transmission. About 1 minute later, the engine lost total power. He attempted to restart the engine, but the engine would not crank when the starter was engaged, and all the avionics in the airplane shutdown. About 30 seconds later, all the displays came back on, and the flight instructor configured the airplane for landing and touched down uneventfully in the soybean field he had selected.

As the airplane slowed, the airplane’s nose dropped to the ground, and the nose landing gear dug into the soft earth. The nose landing gear separated, and the airplane nosed over.

AIRPLANE INFORMATION

The strut-braced, high-wing, two-seat, airplane was made of sheet and tubular aluminum. The design complied with Federation Aeronautique Internationale microlight rules and Federal Aviation Administration (FAA) light sport aircraft rules.

It was equipped with an American Society for Testing and Materials compliant, 4-cylinder, horizontally-opposed, 100-horsepower, Rotax 912 ULS 2 engine. The engine used a single central camshaft with hydraulic tappets. The cylinder heads were liquid cooled, and the cylinders were ram air cooled. The oil system was a dry sump, forced lubrication system. The engine used a reduction gearbox to drive the two-bladed, fixed-pitch Sensenich propeller.

According to FAA and airplane maintenance records, the airplane was manufactured in 2017. The airplane's most recent condition inspection was completed on March 3, 2020. At the time of the inspection, the airplane had accrued about 1,081 hours of operation, and the engine had accrued about 734 hours of operation.

FLIGHT RECORDERS

The airplane was not equipped with a flight data recorder nor was it required to be under CFR Part 91. It was equipped with two Garmin G3X flight displays that recorded historical information at a variable rate of about 10 Hertz to internal non-volatile memory.

Review of the data revealed a noticeable gap in the data toward the end of the flight. This was indicative of the power interruption to the displays as described by the flight instructor and resulted in the displays writing the remaining flight data to a separate file. This process resulted in a gap in the recorded data.

WRECKAGE AND IMPACT INFORMATION

Postaccident examination revealed that the airplane had sustained substantial damage to the fuselage and both wings.

Examination of the engine, serial number (S/N) 9569181, revealed that there were no anomalies with the oil system and that there was oil throughout the engine. Aluminum debris and engine oil were found in the carburetor for Nos. 1 and 3 cylinders.

The No. 1 cylinder’s exhaust valve spring retainer was broken, and the exhaust valve had fallen into the cylinder’s combustion chamber. A buildup of metallic material was found in the No. 1 intake manifold. The stem of the No. 1 exhaust valve was in place; however, the head of the exhaust valve was no longer attached.

After removal of the No. 1 cylinder head, damage to the cylinder head, piston, and valves was discovered. The No. 1 exhaust valve head was found imbedded in the No. 1 intake valve. The No. 1 piston had a large hole in the crown of the piston; the No. 1 cylinder displayed damage; and the No. 1 connecting rod was bent and twisted.

Additionally, the No. 3 cylinder head was removed, and metallic material was found in the No. 3 combustion chamber. The No. 3 piston displayed damage, and the No. 3 cylinder was damaged and displayed multiple impact marks.

TESTS AND RESEARCH

Accident with N561TU

The National Transportation Safety Board (NTSB) first became aware of valve spring retainer fracturing issues with Rotax 900 series engines in 2017 due to an accident that occurred in Stevensville, Maryland, with another Tecnam P92 airplane, N561TU, that was also operated by Chesapeake Sport Pilot. (NTSB Case No. ERA17LA246). The airplane was powered by a Rotax 912 ULS2-01 engine, S/N 9569084. In this accident, the airplane experienced a total loss of engine power at the end of a cross country flight, and the pilot performed a forced landing during which the airplane sustained substantial damage.

The airplane had recently been purchased, and the engine had 13.2 hours total operating time. Review of onboard data indicated that the fuel pressure, cylinder head temperature, and oil temperature remained relatively steady until the loss of power occurred, which indicated that the engine failure likely did not involve the fuel system, cooling system, or lubrication system.

Examination of the engine revealed that there was no oil in the oil line between the oil thermostat and oil pump. The oil pump drive pin also displayed excessive wear in relation to the operating hours of the engine, and the magnetic plug was covered in metallic particles, although the oil filter was clean. Further examination of the engine revealed that the No. 1 cylinder was damaged, and evidence of bluing was present. The cylinder’s exhaust valve spring retainer was fractured in half, and one half of the cotter was fractured. A small ridge could be felt on the exhaust valve spring retainer and galling (a rough surface) was visible on the exhaust valve bore in the cylinder head.

Examination of the fractured surface on the exhaust valve spring retainer revealed the presence of fatigue with pronounced vibration stripes when viewed with an electron microscope; however, the heat treatment corresponded to the target specifications, as did the statistical process control value. According to the NTSB’s final report on the accident, the root cause of the failure could not be determined based on the available information.

Additional Valve Spring Retainer Fractures

In 2019 and 2020, another four valve spring retainer fractures occurred in the United States involving the following aircraft: N1PJ, N204BF (NTSB Case No. WPR20LA012), N117BF, and N562TU (this case).

Examinations of the damaged engines revealed:

o S/N 4421750 (N1PJ), intake valve failure, broken valve spring retainer cylinder No. 2

o S/N 9569290 (N204BF), intake valve failure, broken valve spring retainer, cylinder No. 2

o S/N 9569271 (N117BF), intake valve failure, broken valve spring retainer, cylinder No. 2

o S/N 9569181 (N562TU), exhaust valve failure, broken valve spring retainer, cylinder No .1

All the engines had differing hours of operation; however, all experienced a valve spring retainer failure during engine operation. At the request of the NTSB, numerous components from the four engines were shipped by Rotech Flight Safety to the Austrian Federal Safety Investigations Authority (BMK) for examination and testing at the engine manufacturer’s factory in Gunskirchen, Austria. Extensive metallurgical examination of the intake and exhaust valves, valve spring retainers, valve springs, valve tappets, pushrod assemblies, pistons, cylinder heads, valve cotters, and camshafts was conducted. The results of the examinations were similar, to those from the examination of the engine components from the 2017 accident with N561TU. All of the parts met their specifications, and the fractured surfaces on the exhaust valve spring retainers revealed the presence of fatigue with pronounced vibration stripes.

Review of Published Guidance

Review of Rotax 900 series operators manuals indicated that the dry sump lubrication system would provide sufficient lubrication up to a maximum bank angle of 40°. The engines were also limited to a maximum of 5 seconds of operation at -0.5 G.

A limited review revealed that about 463 aircraft models used Rotax 900 series engines. These included plans-built aircraft, kit aircraft, and certificated manufactured aircraft. Review of published guidance materials from some of these manufacturers revealed however that the Rotax engine bank angle and G limitations were not published in the flight manuals or pilot’s operating handbooks, and in many cases, the maximum published bank angle limitation for the aircraft was 60°, which exceeded the Rotax published limitation.

Review of the Rotax 912 Heavy Maintenance Manual 72-00-00, Edition 1, Revision 4, page 69, revealed that wear of “the valve spring support can indicate a malfunction of the valve train as a result of badly or insufficiently vented hydraulic valve tappets.” Figure 1 shows the components of the engine valvetrain.

Figure 1. Illustration of components of the engine valvetrain.

Review of Rotax Service Instruction SI-916 i B-003 / SI-915 i-003R1 / SI-912 i-004R2 /...