What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
Pilot was on special flight permit due to aircraft out of annual. In cruise at 5,500', 1.5 hours into the flight, the pilot heard loud popping noise from the engine with subsequent uncommanded reduction in power to 1900 RPM and then 1200. The pilot, unable to maintain altitude, declared an emergency and landed on a highway.
Investigation revealed valve guides had some carbon buildup possibly leading to stuck exhaust valve(s) at operating temperature and power. Additionally, two cylinders had low compression as they were bored to .10 over and paired with standard size (mis-matched) cylinders.
As the FAA filed it
PILOT WAS ON SPECIAL FLIGHT PERMIT DUE TO AIRCRAFT OUT OF ANNUAL. IN CRUISE AT 5,500', 1.5 HOURS INTO THE FLIGHT, THE PILOT HEARD LOUD POPPING NOISE FROM THE ENGINE WITH SUBSEQUENT UNCOMMANDED REDUCTION IN POWER TO 1900 RPM AND THEN 1200. THE PILOT, UNABLE TO MAINTAIN ALTITUDE, DECLARED AN EMERGENCY AND LANDED ON A HIGHWAY.
INVESTIGATION REVEALED VALVE GUIDES HAD SOME CARBON BUILDUP POSSIBLY LEADING TO STUCK EXHAUST VALVE(S) AT OPERATING TEMPERATURE AND POWER. ADDITIONALLY, TWO CYLINDERS HAD LOW COMPRESSION AS THEY WERE BORED TO .10 OVER AND PAIRED WITH STANDARD SIZE (MIS-MATCHED) CYLINDERS.
Other reports on N28314 · 1
Matched to the same aircraft by N-number and date.