An important technical and procedural issue in the 4-D contract concept is whether the contract applies locally (e.g., to a sector) or globally (to multiple sectors). The original concept envisages a negotiated contract from origin to destination, gate to gate. This is clearly what the pilot and the airlines would prefer, because it would be consistent with the capabilities that the FMS provides and would facilitate pilot flight planning. Controllers, however, may prefer to negotiate contracts sector by sector, because this would give them greater flexibility in management of the traffic pattern, particularly in response to unexpected events, weather disturbances, etc.

Of course, whether or not 4-D contracts are negotiated within or across sectors, the controller will still be responsible for the separation of aircraft. Also, controllers will be able to cancel a contract in response to unanticipated conditions at any time. When this is done, the aircraft will be under tactical control from the ground, as in current practice. Once the condition has passed, however, a new contract can be negotiated. A significant human factors concern is whether such negotiations can be undertaken efficiently and safely in a time-critical environment. Procedures for unambiguous and uninterruptable trajectory or clearance negotiation must be worked out. Thus, 4-D contracts will change aspects of the controller's job, but will not fundamentally alter responsibility for separation. The question is, will the changes affect the ability of controllers to maintain separation?

Only limited data are available to answer this question. On one hand, limited, routine clearances that are communicated to the pilot under the current system may be eliminated, so that controllers may be able to devote greater time to longer-range conflict prediction and planning. On the other hand, the current system is one in which the controller knows that the aircraft will follow a precise, specified path. This will be replaced by a system in which there will be some uncertainty about the aircraft's future position. The larger the 4-D tube, the greater the uncertainty. It is difficult to predict how controllers will react to such a system. One possibility is that they will attempt to reduce the uncertainty by querying pilots more frequently, which would tend to increase communications workload. Endsley (1996b) reported such an effect in an experimental evaluation of a free flight scenario. However, it is also possible that controllers will adapt to the system and attempt to calibrate their level of uncertainty in advance by negotiating narrower tubes for anticipated problem areas within a sector and larger tubes elsewhere.

The development of the 4-D contract concept has been accompanied by attention to human factors enhancements, particularly in the controller's tools and operating procedures. One product of the PHARE effort will be a set of integrated controller tools, known as the PHARE advanced tools, that incorporates the following capabilities:

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