A meaningful feature of the helicopter industry compared to the other aeronautical industries is the production of transmissions: the essential components for driving the power from the engines to the rotor blades. Transmissions are composed of gears most of which are vital for flight safe; hence gears are requested to be designed and manufactured with a very high quality standard. One of the widely diffused processes to manufacture helicopter gears is electron beam welding (EBW), whose nondestructive testing must assure high sensitivity and defect detection capability, independently by the defect position or orientation in the joint. It was just to answer to this requirement that a special application of the ultrasonic scattering ('delta' technique) has been introduced, starting from the early seventies, in the automatic scanning inspection of welded joints. This technical paper shows the results of research and development activities recently completed in our laboratories for the application of the delta technique aimed at improving results understanding and to get reliable knowledge support to make better effective the requirements management, the compliance with application limits and the correct use of the results itself. To explain the way to inspect proficiently a welded joint by this scattering technique, both theoretical and phenomenological aspects, which are basis for interpretation of this special testing process, are described. Moreover some experimental highlights, reached by research activities that validate the technique application, its limits and advantages, are illustrated. Specifically the paper describes how the excellent sensitivity to little discontinuities, compared to traditional ultrasonic techniques, cannot be simply evaluated and interpreted by conventional criteria as, for example, the classical correlation between size (diameter) and echo amplitude to estimate the dimension of a discontinuity.