On May 6, 1999, at 1030 hours Pacific daylight time, a WSK-PZL Warzawa-Okecie PZL-104 Wilga 80, N4UU, (a Polish manufactured airplane), ground looped while landing at the Reno-Stead Airport, Reno, Nevada. The experimental aircraft, owned and operated by the private pilot, sustained substantial damage. Neither the airline transport licensed flight instructor nor the private pilot trainee were injured. The local area instructional flight was conducted under the provisions of 14 CFR Part 91 and originated about 0830 from the Reno-Stead Airport. Visual meteorological conditions prevailed and no flight plan was filed. The flight instructor reported that they were landing on runway 26 and the aircraft touched down on the runway centerline with a slight crab to the left. The flight instructor stated that the pilot trainee "overcorrected with right rudder input causing a swerve to the right." The pilot trainee applied left rudder but the aircraft continued to veer to the right. The flight instructor reported that he took the flight controls in an effort to correct the drift, but realized that the "swerve had become uncontrollable." He stated that he felt it was too late to recover by applying engine power, so he began to apply control inputs for a ground loop. As the aircraft began to ground loop, the left wing contacted the runway surface. The flight instructor reported that the tail of the aircraft began to rise, and "the combination of a slight tail wind, approximately 8 gallon fuel imbalance (left wing heavy), and possible excessive braking resulted in the aircraft pitching over and coming to rest inverted on the runway." The flight instructor reported that the automated weather observation service (AWOS) at the airport indicated that the winds were from the east at 4 knots at the time of the accident.