Pilot Debrief

CESSNA 172 near Hernando, FL — 2024-06-21

Final reportERA24LA271
Sign in to save
Date
2024-06-21
Location
Hernando, FL, USA
Aircraft
CESSNA 172
Registration
N734XT
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Cruise

Probable cause

The improper installation of a cylinder rocker stud by maintenance personnel, which resulted in a partial loss of engine power and subsequent forced landing.

NTSB narrative

The flight instructor reported that, during an instructional flight, while maneuvering about 2,000 ft above ground level, the engine began to run rough and lost partial power. The airplane could not maintain altitude and the instructor was unable to restore engine power. He subsequently performed a forced landing on a road, during which the left wing impacted a tree and the airplane came to rest inverted. Subsequent examination of the engine revealed that the No. 3 exhaust valve rocker was loose and did not depress the exhaust valve when the pushrod was fully extended. The rocker and stud were removed and remnants of threads from the cylinder head were observed between the threads of the stud, consistent with over-tightening of the stud during installation. Review of maintenance records revealed that the No. 3 cylinder was replaced due to low compression about 13 months and 564 engine tachometer hours before the accident. Although the maintenance procedures specified that the stud be tightened to 150 ft-lbs of torque, the stud was likely over-tighten during installation, as was evident by cylinder thread material being pulled into the stud threads, which resulted in its looseness and inability of the rocker to open the exhaust valve.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Proper torque application during cylinder installation could have prevented engine failure.

NTSB coding

Evidence available

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

Docket documents8

Related mishaps