Cessna 152 N4886B accident near Bell Glade, Florida

Probable cause

A partial loss of engine power due to the No. 1 cylinder's valve keeper becoming dislodged, which ultimately resulted in the intake valve becoming stuck in the valve guide and in the loss of compression to the cylinder.

NTSB analysis

The private pilot reported that, while conducting a personal, cross-country flight and descending from cruise level, the engine began gradually losing power, so he applied carburetor heat, which initially restored cruise power. Several minutes later, the engine again lost partial power and began to vibrate. The engine continued to lose power, and the airplane was unable to maintain altitude. The pilot chose to execute a forced landing on a field, during which the nosewheel dug into the ground, and the airplane subsequently nosed over.

Examination of the engine revealed that the No. 1 intake valve was stuck in the valve guide and that the upper valve spring seat on the No. 1 cylinder was broken. Detailed examination revealed contact marks on the interior and end faces of the valve keeper and pitting and radial cracks in the intake valve head. The valve springs' strength was found to be within serviceable limits. The rocker arm contact wear pattern on the valve tip was abnormal.

It is likely that, during operation, the intake valve keeper was dislodged and then became wedged in the valve spring seat until the valve spring seat fractured, at which time the intake valve became stuck in the valve guide. This resulted in a loss of compression to that cylinder and the subsequent partial loss of engine power. The contact marks on the interior and end faces of the valve keeper indicate that the keeper had been dislodged for some time while the engine operated and that the valve spring seat was intact. The pitting and radial cracks in the intake valve head indicate that the engine was operated while the valve was not seating properly, which is also consistent with operation with a dislodged valve keeper. The lack of a normal rocker arm contact wear pattern on the valve tip indicates that the valve keeper likely became dislodged soon after the valve was placed into service.

The engine manufacturer's Operator's Manual recommended periodic inspections (every 400 flight hours) of the intake valve. During a review of the engine maintenance records, no records were found indicating whether the periodic valve inspections were completed. If these inspections had been completed, the abnormal wear pattern on the valve and the dislodged valve keeper would likely have been detected.

Text from the NTSB. Open the NTSB docket for ERA19LA069 for the full record.