The refueling boom on the KC-10 tanker is controlled by a digital fly-by-wire system. Use of a three-level verification procedure was begun early in the development cycle to detect design and coding errors. The software code was exercised in a hardware/software interface which became more complete with each succeeding level. The supplier’s validation facility was used to provide first-level code checks. A multipurpose man-in-loop simulator which contained most of the flight-type operator’s station components was used for second-level checks. The KC-10 ground and flight test activities provided the final verification. In-service history (nearing two years) indicates a completely successful validation program.