In 1994, the UK Ministry of Defense ordered a suite of C-130J simulators for the Royal Air Force. The aircraft development effort was in the critical design phase - with First Flight still two years away. The aircraft design included a highly integrated, quad-redundant avionics architecture, with nine 1553 Busses linking six Bus Controllers and more than one hundred Remote Terminals. Nearly all the latest communication, navigation, surveillance, control and display systems were utilized. The suite of simulators was to include full flight, part task, and maintenance trainers. Producing these simulators concurrent with the aircraft development posed unique challenges for both the aircraft and simulator manufacturers. This paper describes those challenges, with primary emphasis on the concurrency issues surrounding the avionics systems development. It describes the decisions made, the logic behind those decisions, and lessons learned. This paper also discusses the need for industry guidance, and suggests a process for use on future concurrent development programs (new aircraft and major upgrades).