On June 14, 2008, about 1215 eastern daylight time, a twin-engine Piper PA-34 airplane, N11HR, sustained substantial damage during a hard landing when the left propeller feathered on short final, at Ocala International Airport-Jim Taylor Field, Ocala, Florida. The airplane was being operated by the pilot as a instrument flight rules (IFR) personal cross-country flight under Title 14, CFR Part 91, when the accident occurred. The private certificated pilot and the three passengers were not injured. Visual meteorological conditions prevailed during the final approach, and an instrument flight plan was filed. The airplane departed Houston, Texas, about 0630. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on June 18, the pilot said about 6-10 feet above the runway during landing, the left engine's propeller went to the feathered position uncommanded. He said the airplane dropped hard onto the runway, collapsing the nose landing gear. The pilot reported that during the past several months the left propeller would feather uncommanded when taxiing from landing. He said that on the advice of his mechanic, he had the propeller overhauled, which did not fix the problem, and then replaced the engine. The pilot discussed the continuing problem with his mechanic, and determined that the propeller would probably not feather uncommanded in-flight. For the accident flight, the pilot said he planned to land with extra power, but found himself high and long on final approach. He said when he reduced engine rpm to idle, about 6-10 feet above the runway, the left propeller feathered. He reported that the airplane struck the runway hard, driving the nose landing gear through the top of the airplane's nose, and breaking the windshield. On June 20, the propeller and propeller governor were removed and taken to an FAA certified propeller repair station. An examination of the propeller and governor found no evidence of mechanical failure or anomaly. Additional Information/Research During a discussion with a certificated propeller shop owner in Tucson, AZ, he said the problem of propellers feathering uncommanded on light twin-engine airplanes was quite common in their area. He said the local college aeronautical training center had the same problem with the Beech 76 Duchess' used for training. Their investigation, in conjunction with the FAA, revealed that the problem was that of engine oil viscosity. He said the problem only occurs during the summer months when temperatures are high. He said there are numerous contributing factors. 1. Ambient temperature in conjunction with engine operating temperature 2. Oil viscosity/grade i.e. 15w50 vs. 5w40 3. Oil cooler size/capacity 4. Engine wear/tolerances 5. Propeller governor volume 6. Propeller cylinder spring pressure According to the propeller shop owner, depending on circumstances, one or more of the above may cause or contribute to the problem. He said there are several fixes, and that any one or a combination of the following could potentially solve the problem, depending on the combination of factors creating the problem for the specific engine and propeller. 1. Use a higher oil viscosity/grade of oil to raise the viscosity at higher temperature. 2. Larger oil cooler 3. Replace/rebuild the engine 4. Change propeller governor to a higher volume governor 5. Reduce the propeller cylinder spring pressure In the college aeronautical training center case, the propeller manufacturer authorized the reduction of the propeller cylinder operating pressure through the use of lighter springs, and reduced the pressure from 41 psi to 22 psi, and eliminated the problem. The propeller manufacturer maintains a list of airplanes approved for the reduced operating pressures. According to the propeller shop, there are no ill effects due to the reduced operating pressure, however the manufacturer does not include an airplane on the list unless a problem is reported for the specific make and model. A canvas of east coast and west coast propeller shops by the IIC revealed that during summer months the shops receive complaints of propellers going into feather during shutdown. They attributed this to low oil viscosity also, and advise pilots to shutdown at a lower rpm so the mechanical stops will engage before the oil pressure bleeds off and the propeller feathers. The east, west, and central US propeller shops contacted were not aware of each other's findings, and according to propeller shop managers, mechanics are not familiar with the issues associated with oil viscosity either. All the shops reported mechanics sending airworthy propellers and governors in for unnecessary repairs.