Piper PA-28R-200 N41997 accident near Boscobel, Wisconsin

Probable cause

The ice obstruction of the engine oil breather tube and alternate air outlet, which resulted in a failure of the crankcase oil seal and loss of engine oil. Contributing to the accident was the incorrect engine oil breather tube installed in the airplane by maintenance personnel.

NTSB analysis

During cruise on the return leg of a night cross-country instructional flight at 9,000 ft mean sea level (msl), the flight instructor and student pilot noticed an odor in the cockpit. The flight instructor then noticed an opaque windscreen and immediately thought they had encountered icing conditions. The flight instructor contacted air traffic control (ATC) and requested a lower altitude. Using a flashlight, the flight instructor illuminated the windscreen and noticed a “brown river” completely obscuring their forward visibility. The cockpit engine oil pressure gauge indicated little to no oil pressure.

The flight instructor declared an emergency with ATC and assumed control of the airplane from the pilot receiving instruction. The flight instructor located the nearest airport to attempt a forced landing. However, due to restricted visibility due to the oil-covered windscreen, the airplane landed hard adjacent to the runway, bounced, and then came to rest upright on the ground. The airplane sustained substantial damage to the right horizontal stabilator, and the airframe contained engine oil on the top engine cowling, windscreen, and empennage.

During recovery of the airplane, recovery personnel noted that ice obstructed the engine oil breather tube and the alternate air outlet. The obstructed breather tube created increased crankcase pressure and caused the extrusion of the solid-ring crankshaft oil seal from the crankcase and subsequent loss of engine oil.

Airplane records showed that the oil engine breather tube was replaced about 18 years before the accident. The design, fabrication, and installation of the accident airplane’s oil breather tube did not comply with the airplane manufacturer’s original design criteria. Specifically, the oil breather tube protruded about 3” outside the engine cowling when it should only extend 0.5”. The additional length of oil breather tube outside of the engine cowling would be more susceptible to ice accretion during flight. Additionally, the replaced oil breather tube was not fabricated with alternate air outlets that were positioned within the engine compartment, which would have provided an unobstructed vent after the primary outlet became contaminated with ice during the flight.

Text from the NTSB. Open the NTSB docket for CEN25LA053 for the full record.