Propagation of Airborne Spacing Errors in Merging Traffic Streams

Research results from domains such as road highway traffic and military vehicle platoons conclude that string stability cannot be obtained when vehicles use only relative spacing information to maintain constant distance behind a predecessor. Air traffic studies confirm these results but such instability has not been shown for constant time delay based airborne spacing. Unlike constant distance based, constant time delay based spacing has the potential to enhance stability by anticipating changes in spacing using the preceding aircraft’s history. This simulation based study analysed the merging of aircraft by constant time delay based spacing over the period of the order of an hour to observe any build up of error propagation effects. Aircraft descended from 12,000 feet to 4,000 feet, each trying to achieve ninety seconds spacing with respect to its predecessor. The spacing anticipation time for each trailing aircraft to react to the preceding aircraft’s time history was varied from 0 to 20s. Without anticipation, a time spacing error was observed to propagate at about 20 knots groundspeed in a forwards direction (towards runway) growing to about -3.5s (trail aircraft too early and too close). This compression wave was avoided by increasing spacing anticipation to 10s. Values larger than 10s reversed the error and moved it upstream. A tuned scenario was repeated for 5,400 random values of initial time spacing error and top of descent with automatic and manual airborne spacing modes. Time spacing accuracy and pilot activity were measured to be within required values previously derived for a pair of aircraft but the corresponding cost in speed variation was higher.