History of Flight
On September 13, 2008, at 1000 Pacific daylight time, a Diamond DA-40, registration N419JS, departed from San Carlos Airport in San Carlos, California, under visual meteorological conditions. The pilot, who was also the registered owner, was operating the airplane under Part 91 and reported no issues during the pre-flight run-up, though he noted that the propeller RPM dropped slower than usual when cycled. During climb-out, engine RPM rose to 2,800; the pilot leveled off and reduced power, but the RPM did not decrease as expected. He cycled the propeller twice with no change. Deciding to return, he heard and felt a thump forward of the cockpit. The engine continued to run smoothly, and the pilot landed uneventfully. No injuries occurred, and the airplane sustained minor damage.
Aircraft and Propeller Governor Examination
The airplane, serial number 40.439, was manufactured in 2004 and powered by a Textron Lycoming IO-360-M1A engine with a Hartzell HC-C2YR-1BF propeller. At the time, the airframe and engine had 559 hours. The pilot contacted his maintenance facility, which later reported finding a blister in the engine casing and metal fragments in the oil. Disassembly of the engine revealed one ball bearing in the oil sump, damage to the case, and two damaged camshaft lifters. The propeller governor, an Ontic unit (part number C210776, serial number 0200), was removed for further examination.
At the manufacturer's facility, visual inspection showed the low-speed adjustment screw was installed backwards and safety wired incorrectly. The pilot valve plunger was loose and could be removed. Upon disassembly, a .125-inch ball bearing fell out of the governor oil drain hole. The retainer-thrust bearing and flyweight head assembly were found loose in the governor body. The thrust bearing retainer had 5 of its 11 ball bearings missing and showed scoring and deformation. The pilot valve race had a set screw protruding about .015 inch into its internal diameter, with flattening of the set screw tip, indicating misalignment during assembly. Metallurgical examination using SEM and EDX confirmed the set screw had been torqued against the plunger, causing flattening.
Maintenance Records and Service Bulletins
No maintenance had been performed on the adjustment screw; during aircraft assembly, the only governor adjustment involved the cable. Service difficulty report data indicated two similar events, all from a batch of 74 governor assemblies. As a result, the manufacturer issued mandatory service bulletin DES-353 on December 18, 2008, requiring inspection and repair of affected units.