Human factors issues related to the retrofit of data link in commercial transport aircraft are discussed. Topics that must be considered for data link implementation include, the loss of the “party line,” (i.e., the availability to all aircraft of information transmitted on a common voice frequency), and the “scheduling” of information to the flight crew. This paper focuses primarily on the human factors issues related to retrofit of Mode S. Retrofit is a difficult task because panel space accessible to flight crew members is limited. As with all cockpit equipment, data link implementation will have to comply with Federal Aviation Regulation 25.1523, which requires the manufacturer to address the conspicuity and ease of use of the data link device, and to assess the impact on crew workload. Operational sequence diagrams are provided to illustrate a methodology that can be used to decompose the flight crew body channel utilization of candidate avionics configurations in order to optimize the pilot-vehicle interface.