On November 30, 2016, about 1730 mountain standard time, an Aero Commander 690, N9175N, was substantially damaged when it was involved in an accident at the Scottsdale Airport (SDL), Scottsdale, Arizona. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 maintenance relocation flight. The pilot reported that after a normal landing on runway 21 he began easing the power levers into reverse and applying light braking action to slow the speed. While reaching for the condition levers to bring the propellers back to low RPM, the airplane "darted to the right." The pilot applied left braking and adjusted the engines to low RPM settings to compensate for the veer. Despite his actions, the airplane continued to veer towards the right side of the runway and was approaching the A11 exit sign. The pilot reported that he could have either gotten more aggressive with the corrective actions, and hope to avoid striking the sign, or allow the airplane to exit the runway on what he thought was a hard smooth surface. The pilot chose to exit the runway surface which was the runway safety area (RSA). Once the airplane entered the RSA, the landing gear sunk deep into the rocks which quickly slowed the airplane to a stop. Security cameras captured the airplane during landing. The airplane touched down near the 1,000 ft runway distance markers and about 1,300 ft further, exited near taxiway A11. The airplane traveled about 50 ft into the RSA, creating deep troughs through the rock layer, and came to a stop about 2,500 ft from the approach end of runway 1. A postaccident examination of the airplane revealed impact damage to the left side of the fuselage. The impact damage consisted of multiple holes and dents in the fuselage cabin passenger area and window areas. A rock about 2 inches in length was found imbedded in the fuselage and could not be removed during the examination. Several rocks penetrated the passenger area, one impacting the right side window from the inside. The propeller blades had gouging on the leading edges from the midsections to the tips. The right landing gear brake rotor showed light gouging on the braking surface. The flight control system was examined, and all flight controls moved freely with unobstructed movement. The steering system was operated with the activation of the hydraulic system and the nose landing gear tire was positioned over a grease plate. Actuation of the brakes produced normal steering operation. No evidence of any mechanical malfunctions or failures were revealed that would have precluded normal operation. The RSA rock layer consisted of about 3-inch sized smoothed river rock. The depth of the rock layer varied from 4-inches to 12-inches deep through the area of the accident site. Accident Site and Runway Tire Marks According to the FAA, RSAs are not designed with a particular aircraft in mind. RSA's are designed to provide all aircraft types with the space and time needed to safely come to a stop while avoiding fixed obstacles on the airport that may damage the aircraft and potentially injure or kill the occupants. RSAs are designed to support the aircraft without sinking into the soil and causing an abrupt halt. RSAs can be made of many materials, including rocks. These rocks should be no larger than four inches in size. RSAs, like any other landing/runway ground surface, are not designed for contact by any part of the aircraft other than the landing gear. The design of an RSA does not take into consideration contact by propellers, engines, wings, etc. RSAs around the country have proven to be effective at mitigating damage, injury, and death when runway excursions and overruns occur. According to the Department of Transportation Advisory Circular (AC 150/5300-13A)- An RSA is defined as a surface surrounding the runway prepared or suitable for reducing the risk of damage to aircraft in the event of an undershoot, overshoot, or excursion from the runway. Also, FAA Order 5200.8, defines that RSA is intended to provide a measure of safety in the event of an aircraft's excursion from the runway by significantly reducing the extent of personal injury and aircraft damage during overruns, undershoots and veer-offs. SDL RSA design and specification drawings show a crushed aggregate depth of 4 inches.