Lockheed worked on modifying the AH-56 design to address the vibration and other issues. As a precaution, Cheyenne #9 (s/n 66-8834) was fitted with an ejection seat for the pilot after the March accident. The downward firing ejection seat was placed in the forward seat in place of the gunner's station. This prototype would be used for all remaining envelope expansion flights. Prototype #9 also received an upgraded transmission and drivetrain, and a hinged rear canopy in place of the original sliding canopy around 1970. The new transmission allowed the T64-GE-16 turboshaft engine output to be increased from a derated 3,435 horsepower (2,561 kW) to 3,925 horsepower (2,927 kW). The new canopy eliminated the canopy vibrations.