On August 24, 2003, a Mooney M20D (registration N6619U) was involved in an aviation accident near Hood River, OR. Investigators recorded the probable cause as: The number one cylinder intake seat had dropped down out of its normally seated position at an angle, preventing the valve from seating evenly during the compression stroke, which resulted in low engine compression and reduced power. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A commercial pilot and his son experienced reduced engine power during takeoff, leading to a forced landing into treetops and a bog after a cylinder intake valve seat failure was discovered.
Takeoff and Power Loss
During a takeoff from a 3,040-foot long asphalt runway, the flying pilot (in the left seat) noted that the takeoff roll seemed long and power appeared less than normal. The aircraft was rotated at 65 knots but settled back onto the runway. At that point, the pilot in the right seat—a commercial pilot with over 2,000 hours in the accident aircraft—took control and established a shallow climb toward rising terrain west of the airport.
Decision to Continue
Both pilots assessed that aborting the takeoff would result in an overrun into a grassy area containing vehicles and pedestrians. The right-seat pilot then banked to avoid a large tree, but the aircraft settled into the treetops and eventually came to rest inverted in a bog.
Post-Crash Examination
A post-crash examination and test run of the engine revealed that the number one cylinder could develop only 25-35 pounds of compression. Further inspection showed that the number one cylinder intake seat had dropped down out of its normally seated position at an angle, preventing the valve from seating evenly during the compression stroke.
Probable cause
The number one cylinder intake seat had dropped down out of its normally seated position at an angle, preventing the valve from seating evenly during the compression stroke, which resulted in low engine compression and reduced power.