On June 12, 1999, at 1335 eastern daylight time, an Aero Commander 500S, N33PG, was substantially damaged while landing at the Bradley International Airport (BDL), Windsor Locks, Connecticut. The two certificated airline transport pilots were not injured. Visual meteorological conditions prevailed for the proficiency flight. No flight plan had been filed for the flight that was conducted under 14 CFR Part 91. The purpose of the flight was pilot proficiency. The pilot-in-command (PIC) and airplane owner occupied the left seat, while the second pilot (safety pilot) occupied the right seat. The PIC reported that the flight originated from Westchester County Airport (HPN), White Plains, New York, and was planned to terminate there. During one of the approaches the landing gear selector was placed in the down position, and the nose landing gear extended; however, the main landing gear failed to extend. The pilots elected to retract the landing gear and diverted to BDL due to its better weather. At the request of the PIC, the runway was foamed for the landing. The safety pilot stated: "...Prior to being cleared for the approach, I asked the owner if he wanted to make the landing. He said he felt that I had more experience and asked that I do it...." The safety pilot further stated: "...At approximately 1130 EDT we were cleared, and I made the approach at a slightly lower airspeed than I normally would, and touched down on the foam in a slightly nose-high attitude. The aircraft came to rest straight ahead, right side up...." An inspector from the Federal Aviation Administration (FAA) reported that both engines had been shut down and the propellers feathered when the airplane was on short final. He interviewed witnesses who reported that the airplane appeared to drop in from about 50 feet. Examination of the airplane revealed that the lower aft fuselage was crushed upward. The crushing was visible on the lower 1/2 of the aft fuselage. Examination of the landing gear extension sequence revealed that during landing gear retraction, the air in the emergency extension air cylinder was compressed, and the landing gear was held retracted by the uplocks. Normal landing gear extension ported hydraulic pressure to remove the uplocks, after which the landing gear is extended by the pressure in the emergency air cylinder. If the uplocks cannot be removed with hydraulic pressure, there is no alternate method of landing gear extension. After the accident, the airplane hydraulic system was examined and found to be full of fluid. The airplane was placed on jacks and hydraulic power applied. The landing gear, which had been previously extended, was retracted with no problems noted. The landing gear selector was then placed in the down position, and the nose landing gear extended; however, the main landing gear failed to extend. Examination of the landing gear wing flap control valve found contamination in the system. According to a letter from the FAA, the technician who examined the airplane reported, "...the contamination caused the main gear uplock cylinder not to release and allow the gear to extend...."