An optics-based longitudinal tip-path-plane measurement system has been designed and flown onboard a Bell 206B helicopter. The system is composed of cameras that measure the longitudinal tip-path-plane angle with respect to the fuselage and an air data boom fitted with a horizontal wind vane that measures the free-stream velocity in the longitudinal plane with respect to the fuselage. A global positioning system and inertial mass unit were also included to compare the measurements of the imaging system with the theoretical longitudinal tip-path-plane for a given drag-to-weight ratio, flight path angle, and acceleration. In general, the longitudinal tip-path-plane angle measurements compare well with theory and prove highly repeatable for steady-state flight maneuvers for the Bell 206B helicopter.