Pilot Debrief

COSTRUZIONI AERONAUTICHE TECNA P92 Eaglet near Stevensville, MD — 2017-07-15

Final reportERA17LA246
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Date
2017-07-15
Location
Stevensville, MD, USA
Airport
W29
Aircraft
COSTRUZIONI AERONAUTICHE TECNA P92 Eaglet
Registration
N561TU
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Approach

Probable cause

The fatigue failure of an exhaust valve spring retainer due to air trapped in the lubrication system, which resulted in a total loss of engine power.

NTSB narrative

**This report was modified on December 4, 2022. Please see the public docket for this accident to view the original report.** While in the airport traffic pattern for landing at the conclusion of a cross-country flight, the airplane experienced a total loss of engine power, and the pilot performed a forced landing during which the airplane sustained substantial damage. The airplane had recently been purchased and the Rotax 912 ULS 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 substantially 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. The hydraulic lifter for the exhaust valve displayed a small indentation on the edge of the lifter, and when the hydraulic lifters were manually depressed, the lifter for the exhaust valve was easier to depress than the lifter for the intake valve. The pushrod for the exhaust valve was straight, but displayed a ridge on the rocker arm side of the pushrod, and the rocker arm displayed impact damage on the valve connection face. The exhaust valve was found in the combustion chamber. It was chipped, and bent, and deformed into an “S” shape. A hole was visible in the piston as the result of the piston face striking the exhaust valve after it dropped into the cylinder. A small amount of oil captured from the hydraulic tappets indicated that the oil contained significantly elevated levels of nickel, which could have come from manganese-containing alloys, as they occur in high-alloy hardened steels, e.g. for camshafts, valves or valve shafts. 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. Between 2 and 3 years after this accident, four more cases of broken valve spring retainers on 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 that was observed on the valve spring retainer from this 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 of 40°, 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 of 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 this accident and in the other four failures identified.

Analysis

Primary failure mode
Engine power loss
First missed decision gate
The pilots could have aborted the landing earlier after the engine started to run rough.

NTSB coding

Evidence available

  • Photos
  • 15 docket documents
View NTSB final reportView NTSB docket

Docket documents15

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