On November 12, 1999, at 1738 hours Pacific standard time, an American General AG5B, N1195L, was substantially damaged by an encounter with in-flight turbulence while on downwind leg for landing at the Van Nuys, California, municipal airport. The airline transport pilot and two passengers were not injured. Visual meteorological conditions prevailed for the local area sightseeing flight, which was operated by D&S Aviation under the provisions of 14 CFR Part 135. The flight departed from Van Nuys about 1645, and no flight plan was filed. The pilot reported that the flight was an evening scenic flight of the Los Angeles area. It was dark when the flight returned to land and entered on left downwind for landing on runway 16L (northbound). The downwind leg was flown east of the 405 (San Diego) freeway and west of Sepulveda Boulevard at 1800 feet msl, and approximately 115 knots IAS with flaps up. As they were abeam the control tower on downwind (midfield), there was an instantaneous jolt of sudden severe turbulence that ended before he could take any action. His left hand was on the flight controls and his right hand was on the throttle. The pilot said the flight control in his left hand felt "like the handle of a sledge hammer when you strike a steel pipe." The passenger, who was not a pilot, reported that they took off about 1645 and flew for about an hour. As they were returning to land and were halfway down the airport on the right side to land (the passenger's description), they flew through one hard bump. It was "so fast, like an air pocket." It felt like they dropped 10 feet and stopped; then the turbulence was gone and the landing routine. He felt that the speed was normal, there was no shrieking or wind noise, and the pilot made no control input before or after the turbulence. The pilot estimated the operating weight of the aircraft was 2,121 pounds. The certificated maneuvering speed (Va) is 113 knots IAS at 2,400 pounds. One of the aileron mass balance weights was located on November 16, 1999, in the bedroom of an unoccupied apartment where it had penetrated the roof and ceiling. The apartment is located beneath the downwind leg flight path on the east side of the airport approximately midfield. According to a Van Nuys Flight Standards District Office inspector, the location was 50 yards north of Sherman Way (an east-west street which bisects the airport), and 150 yards west of Sepulveda Boulevard (a north-south street which is approximately under the downwind leg). The Burbank-Glendale-Pasadena (Burbank) airport, at Burbank, California, is located 6 miles east of the Van Nuys airport, and the instrument landing system (ILS) final approach course to runway 8 passes over the midfield of Van Nuys airport at glide slope altitude of 2,752 feet (msl). Data obtained from the Van Nuys and Burbank airport noise abatement offices showed that a heavy Federal Express Airbus A-300-B4-600, Flight 2216, passed overhead the Van Nuys airport at 1735:54, at 2,675 feet, and passed over the Grumman's downwind flight path location east of the 405 freeway at 1736:33, at 2297 feet (msl). The same data shows an aircraft on a VFR transponder code flying northbound and crossing under the Airbus' flight path at 1738:56, at 1,800 feet msl. The intersection of the two flight paths is approximately the location where the aileron mass balance was later located. A recording of Van Nuys Air Traffic Control Tower communications on the frequency used to control traffic on the east side of the airport revealed that the tower broadcast a wake turbulence cautionary advisory to all aircraft. The accident aircraft made initial contact with the tower approximately 30 seconds later. The wake turbulence advisory was not repeated. The Safety Board investigator examined the aircraft on November 16, 1999. Each wing is assembled in three sections, which are spliced together. The tip section of the left wing, outboard of the second splice, was deformed upward about 5 degrees with respect to the inboard section of the wing. The upper wing skin exhibited compression buckling. In comparing the two damage areas, the buckling was substantial on the outboard wing section and modest on the upper wing skin inboard of the tip splice. The right wing exhibited modest compression buckling over its entire span but was not visibly deformed. There was no visible damage to the fuselage, empennage, or landing gear. The mass balance weights on both ailerons were absent. The steel tubes supporting the mass balances separated immediately forward of the weld attachment to the aileron torque tube in the vicinity of the heat affected region of the weld. Both failures exhibited a shear lip on the top portion and downward bending on the bottom portion. There was no damage to control stops or evidence of over-travel.