On April 24, 1998, at 1405 mountain daylight time, a Rockwell International 690B, N690ES, was substantially damaged during landing at Broomfield, Colorado. The private certificated pilot receiving instruction and the airline transport rated flight instructor were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the instructional flight being conducted under Title 14 CFR Part 91. The flight originated at Boulder, Colorado, approximately 1330. According to the pilot-owner, he had recently purchased the airplane and had contracted with the instructor pilot to give him instruction and a checkout in the airplane. Although he was multiengine rated and had owned other multiengine airplanes, this was his first experience with a turboprop airplane. The two pilots flew the airplane from Oklahoma City, Oklahoma, to Boulder, Colorado, where they landed and had lunch. After lunch, they flew to the Jefferson County (Jeffco) Airport, Broomfield, Colorado, to make some full stop landings. The first two landings were without incident. The pilot-owner said that after the third landing, he brought the condition levers from ground idle to reverse. The airplane veered to the right and departed the runway. The instructor pilot took control and attempted to straighten the airplane, but it struck a concrete culvert. The pilot-owner said that it appeared that "one engine was developing more power than the other. This could be the result of a faulty propeller governor, the pitch controller, or possibly the Bendix controller." According to an FAA inspector who examined the airplane, the left landing gear collapsed, and the tip from one of the left propeller blades penetrated the pressure vessel. The right propeller and right and nose landing gears also sustained damage. The airplane was ferried back to Commander Aircraft at Downtown Airpark in Oklahoma City, Oklahoma, for repairs. According to the company's director of maintenance, no abnormalities were found with either the propeller, pitch, or Bendix controllers.