3. 4D contracts

3.1 4D trajectories versus 4D contracts

As mentioned in the previous section, 4D trajectories offer many advantages for all air traffic stakeholders. However, defining a 4D trajectory for each flight does not guarantee that, in reality, each aircraft will be where it is supposed to be at any given time. It is therefore necessary to constantly monitor the position of each aircraft, so as to be able to detect any deviation from its agreed 4D trajectory as early as possible. If a deviation is identified, and no action is taken to correct it, many of the benefits of using 4D trajectories are undermined, notably the assurance that two aircraft will pass each other with sufficient separation distance.

To alleviate this problem, we need to find a way of ensuring...