American AA-1 N6116L accident near Fort Meade, Maryland

Probable cause

The mechanic’s failure to detect the incorrect dry valve tappet clearances while troubleshooting ongoing engine issues, which resulted in partial loss of engine power during takeoff. Contributing to the partial loss of engine power was the mechanic's failure to inspect the dry tappet clearances in accordance with the engine operator's manual and a referenced service instruction.

NTSB analysis

The commercial pilot reported that, during the 4 months before the accident, the airplane had experienced ongoing engine issues, including engine roughness. Maintenance personnel were informed about the engine roughness, and a mechanic attributed the problem to the ignition system and subsequently replaced components of the system.

The pilot reported that the purpose of the flight was to verify that the previous repairs made to eliminate the engine roughness had resolved the problem. The pilot performed a thorough preflight inspection and a full-power engine run-up and noted no discrepancies. During the initial climb, when the airplane was about 250 ft above ground level, the engine rpm decreased from 2,500 to 1,500 and then increased to 1,900. The pilot's attempts to restore engine power were unsuccessful. The airplane impacted treetops and then came to rest on the ground inverted.

Postaccident examination of the engine revealed that all of the dry valve tappet clearances were less than prescribed by indications on the engine data plate. The reported loss of engine power and engine roughness was consistent with the valve clearances being incorrectly adjusted.

The engine operator's manual and a nonmandatory service instruction (SI) referenced in the manual specify that the tappet clearances should be checked every 100 hours of operation. A review of the maintenance logbooks revealed that the last check of the tappet clearances was performed in accordance with the SI about 224 hours, or about 7 years 7 months, before the accident by the same mechanic who had been troubleshooting the recent engine issues. If the mechanic had checked the dry tappet clearances while troubleshooting the ongoing engine issues, it is likely that he would have detected the incorrect dry tappet clearances and properly resolved the ongoing engine issues.

Text from the NTSB. Open the NTSB docket for ERA15LA123 for the full record.