Separation Standards

The desired increase in airspace capacity while maintaining or increasing safety, in combination with improvements in technology, necessitates that separation standards be reconsidered. However, existing separation standards were not developed based on a rigorous, cohesive analysis, but rather on a somewhat incongruous mix of practical considerations (e.g. "green between" on the controller display), experience, and analysis.

In addition, separation standards are large in comparison with navigation accuracy of current and future technology because of the need to consider large discrete errors ("blunders"). Since there are no reliable estimates of the probability density functions (PDFs) for blunders, nor is there a clear understanding of the factors influencing such PDFs, separation standards must be relatively large to accomodate blunders without the danger of aircraft collisions.

Furthermore, consider the inconsistencies in using the static, distance-based 5 nautical mile (nmi) separation standard for different conflict geometries in terms of time to collision for aircraft on an exact collision course. In a head-on conflict for two aircraft travelling at the same flight level and at 470 and 490 knots ground speed, the time to collision after violating the 5 nm separation is less than 19s. On the other hand, in a tail-chase situation for the same speeds, the two aircraft could be within 5 nm of each other and not collide for another 900s, a factor of 48 times longer than the head-on case. This suggets that the static, "hockey puck" shape separation standard used today for both the en route and terminal environments may be inefficient.

Figure. Current "Hockey puck" shape Separation Standard