Pilot Debrief

Mooney M20K near Paramus, NJ — 2018-12-09

Final reportERA19LA062
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Date
2018-12-09
Location
Paramus, NJ, USA
Aircraft
Mooney M20K
Registration
N5756W
Category
Airplane
Highest injury
Minor
Fatalities
0
Phase of flight
Cruise

Probable cause

Maintenance personnel's improper assembly of the engine during overhaul, which resulted in lubrication restriction due to a bearing shift and the subsequent engine failure.

NTSB narrative

During cruise flight, the pilot noticed an increase in cylinder head temperature on two cylinders and unsuccessfully attempted to reduce the temperature. He reported that he soon heard "an explosion" and noted a hole in the engine cowling. He performed a forced landing to a golf course, during which the landing gear collapsed, which resulted in substantial damage to the right wing. Postaccident examination of the engine revealed that the Nos. 1 and 2 connecting rods exhibited thermal discoloration consistent with a loss of lubrication. Additionally, the breakaway torques measured at 8 of the 12 through-bolts showed that they were below the manufacturer's specifications. The No. 2 main bearing saddles exhibited impingement damage, and the No. 2 bearing was partially rotated clockwise, which blocked the oil passages leading to the connecting rod bearings. The mating surfaces of the main bearing saddles had remnants of silk thread around the through-bolt holes, which was contrary to manufacturer guidance that prohibited placing silk thread in these areas. Given this information, it is likely that maintenance personnel applied silk thread to a prohibited area during an engine overhaul that occurred about 255.8 flight hours before the accident. This resulted in a loss of torque on the crankcase through-bolts, which allowed the No. 2 main bearings to shift, resulting in restriction of lubrication to that area and subsequent engine failure.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Proper torque and assembly of engine components during overhaul could have prevented failure.

NTSB coding

Evidence available

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

Docket documents9

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