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In an effort to achieve consistent, low variance spacing between aircraft pairs during arrival operations and to reduce aircraft maneuvering, noise, fuel burn, and controller workload, the Federal Aviation Administration (FAA) is developing, and UPS has implemented an Automatic Dependent Surveillance-Broadcast (ADS-B) concept termed Merging and Spacing (M&S). M&S has two phases: a strategic set-up by a ground operator followed by tactical Flight Deck-Based Merging and Spacing (FDMS). In the initial implementation, both phases, involve pilots being requested to fly speeds from sources other than Air Traffic Control (ATC). In FDMS, the speeds are generated and displayed on-board the aircraft via a Cockpit Display of Traffic Information (CDTI) or other displays. The flight crew follows those speeds to achieve and maintain a desired time interval from a lead aircraft. This paper focuses on FDMS and presents the subjective and objective results of a human-in-the-loop simulation that examined the concept from the flight crew perspective during an in-trail operation in the en route and terminal environments, from a Continuous Descent Arrival (CDA) through to landing. Termed FDMS 3, the simulation was conducted in February and March of 2007 and is part of a development and maturation process that is underway for FDMS. The simulation examined the impact of FDMS on: concept and display acceptability; workload and situation awareness; and procedures for non-normal situations. Nine airline-qualified pilots flew a series of scenarios while acting as the pilot flying. Results indicated general acceptability and improvements over current-day operations under normal and non normal conditions. Pilots in general reported that FDMS: was acceptable, was compatible with current operations, was similar in terms of workload as compared to operations without FDMS, allowed for a reduction in communications with ATC, and allowed for acceptable situation awareness. Pilots were able - - to use the displays to implement speed changes to manage normal and non-normal situations such that the desired spacing interval was maintained with minimal variation. Some pilots reported issues with the CDTI retrofit location and reported increased acceptability of FDMS when the CDTI location was moved to the primary field of view. The simulation results related to the concept were used to further refine FDMS and to focus future simulations. They also supported an implementation that was certified and operationally approved.