On July 23, 1999, about 1500 eastern daylight time, a Cessna 210, N9712X, was substantially damaged when it impacted terrain near the Newton Airport, Newton, New Jersey. The certificated flight instructor and private pilot sustained serious injuries. Visual meteorological conditions prevailed for the local instructional flight that originated from Newton. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. The instructor stated that 2 days before the accident he flew the airplane for about an hour. He did not remember adding any oil to the engine prior to the flight, nor did he remember anyone else flying the airplane after the flight. According to the pilot, he arrived at the airplane about 1400, followed by the instructor approximately 20 minutes later. The pilot checked the oil, and then both the pilot and instructor repositioned the airplane for fuel. While at the fuel pumps, the pilot preflighted the airplane and added 1 1/2 quarts of oil. He remembered opening the oil cap because it was hot. He also remembered securing it and the cowling door. He added that prior to getting in the airplane, he checked both fuel caps, the air intake, and the oil cap for a second time. Both the instructor and pilot boarded the airplane, started the engine, and taxied to runway 6. While holding short of the runway, they preformed the engine run-up checks, and noticed no anomalies. They announced their intentions on UNICOM, taxied onto the runway, and the pilot executed a simulated "zero zero" departure. With 10 degrees of flaps selected, the pilot advanced the throttle, and the engine responded normally. Neither the pilot or instructor noticed any problems until about 100 feet agl. At that point, the instructor noticed oil spreading back onto the windscreen, and announced he had the controls. The pilot, unaware of the oil, removed his hood. Once able to see the windscreen, he immediately started looking for the source of the oil, but was unable to identify it. Because the engine was losing oil, the instructor reduced the power to prevent overheating the engine. Shortly afterwards, both he and pilot noticed a drop in propeller rpm. The instructor applied full throttle, and insured the mixture and propeller controls were full forward. Manifold pressure responded, and indicated maximum power, but propeller rpm continued to decrease. With only partial power, the instructor made a "shallow" left turn to avoid some obstacles, and return to Newton. Propeller rpm continued to decrease and the instructor could not maintain altitude. Prior to impacting trees, the instructor raised the nose of the airplane, and airspeed decreased to approximately 80 mph. The airplane entered the trees, impacted the ground, and came to rest upright. The pilot added that when the airplane impacted the trees the engine was producing maximum power, "it felt and sounded normal." According to the FAA Inspector, the engine oil filler cap was found removed and hanging by a chain. In addition, the oil that spread aft over the engine and cowling created a fan approximately 30 degrees wide, and started at the engine oil filler port. The inspector added that compression was obtained on all cylinders, and spark was confirmed to each cylinder. The fuel pump drive shaft was found intact, and fuel was observed at the fuel pump and fuel servo. According to the pilot's operating handbook, "When oil pressure to the piston in the propeller hub is relieved, centrifugal force, assisted by an internal spring, twists the blades towards low pitch (high RPM)." According to the NTSB Pilot/Operated Aircraft Accident Report the flight instructor had a total of 2,200 hours of flight experience, with 4 hours in the accident airplane's make and model. The pilot had a total of 292 hours of flight experience, with 13 hours in the accident airplane's make and model.