On June 29, 1995, at 0925 hours Pacific daylight time, the crew of Casino Express, a Boeing 737-200, N457TM, from Portland, Oregon, experienced an unusual vibration during a descent through flight level 250 into Elko, Nevada. A flight attendant reported to the pilots that part of the right wing was missing. The first officer confirmed that a section of the right aileron had separated from the aircraft. The captain declared an emergency and the aircraft subsequently landed without further incident. There were no injuries to the 78 passengers or 6 crewmembers onboard. Visual meteorological conditions existed at the time with no in-flight turbulence reported. After experiencing the vibration, the captain slowed the aircraft from .76 Mach to 250 knots indicated airspeed and the vibration decreased. He determined that the aircraft was controllable and advised the cabin crew to prepare for a precautionary emergency landing. Examination of the aircraft afterwards revealed that 1.5 feet of the right outboard aileron had separated at the hinge point to the front spar. The aileron trim tab remained attached. Boeing has reported no previous occurrences similar to this to their knowledge. Casino Air reported that the aileron was last inspected on the daily walk-around, which is performed each day before flight, and no defects were noted at that time. The investigation revealed that the aileron underwent a major repair late in 1990 involving skin and core replacement. The aileron was installed on the aircraft in September, 1992. According to the operator, the aileron has not been repaired since the installation. Boeing examined the part and they indicated that there was very little "filleting" or flow of the adhesive between the skin and core. They believe one possible cause of the insufficient "filleting" could be the use of adhesive beyond it's shelf life. No other examinations were conducted.