On March 19, 2015, about 1630 Pacific daylight time, a Piper PA-30 multiengine airplane, N8403Y, was substantially damaged following a loss of control on landing roll, which resulted in a runway excursion and impact with terrain at the Red Bluff Municipal Airport (RBL), Red Bluff, California. The private pilot and two passengers received minor injures, and one passenger was seriously injured. Visual meteorological conditions prevailed for the personal cross-country flight, which was operated in accordance with 14 Code of Federal Regulations Part 91, and a flight plan was not filed. The flight originated from the Shelter Cover Airport (0Q5), Shelter Cove, California, at 1530, with RBL as its destination. In a report submitted to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot, who was not multiengine land rated and listed 3 hours in make and model, reported that after landing on Runaway 15 and applying the brakes, the airplane pulled hard 90 degrees to the right. The pilot stated that he then released the brakes, but the airplane continued to turn to the right, and slid sideways down the runway; rudder and aileron inputs had no effect in correcting the control loss. The pilot opined that the airplane continued sliding until it went off the right side of the runway, into the dirt, and then over the side of a berm and down a 45-degree embankment. The excursion resulted in the collapse of the landing gear and substantial damage to the fuselage. The airplane continued downhill for about 40 feet where it came to rest in an upright position. At the request of the NTSB IIC, and under the supervision of a Federal Aviation Administration (FAA) aviation safety inspector, a postaccident inspection of the airplane's wheel and brake system was performed on April 20, 2015, by a licensed FAA airframe and powerplant mechanic. The mechanic reported that upon inspection of both [left and right] wheel and brake assemblies, no fluid leakage was present on either brake caliper, and that both brake discs were in good [condition]. The mechanic also noted that both main wheel tires had flat spots. The mechanic stated that when he actuated both brake pedals, they were firm and had equal travel. Additionally, a second FAA mechanic stationed outside of the airplane attempted to spin both wheels with the brakes applied, however, neither wheel would not spin. Subsequent to the brakes being released, both wheels were observed to rotate normally. The mechanic concluded that based on his examination of the airplane's wheel and brake system, he found no abnormality with either system. Reported wind at RBL about 35 minutes prior to the accident was 330 degrees at 4 knots, and about 25 minutes after the accident, was report to be 170 degrees at 5 knots. The pilot reported that prior to landing he obtained the Automatic Terminal Information Service report (ATIS), which indicated the wind was calm.