The 98,000 lb. thrust Pratt & Whitney PW4098 high-bypass turbofan engine recently completed a flying test-bed program on the Boeing 777-300 airplane. The purpose of the one-month program was to validate engine operability and to gather data that can be used for upcoming engine certification to the standards of Federal Aviation Regulations part 33. Testing included engine transient operation, steady-state performance, in-flight starting, component cooling, and inlet compatibility. When engine certification is complete, an airplane certification program will be conducted for the 777-300/PW4098, a combination of the world's largest twin engine airplane and the world's largest turbofan engine yet to fly.