The paper introduces a dimensionless alternative to the International Civil Aviation Organization (ICAO) target level of safety (TLS) of collision frequency less than
per flight hour. The method used is to calculate the collision probability ensuring that the ICAO TLS is met, and to find an alternative dimensionless metric that gives the same result. The collision probability depends on separation and position accuracy, so that improving the latter increases capacity for the same level of safety. Thus, the dimensionless alternative to the ICAO TLS provides an alternative safety metric to allow a trade-off between airspace capacity and navigation accuracy. The new metric is applied to aircraft flying in parallel paths along air corridors, as in the North Atlantic. The aim is to enable the growth of air transport that requires more capacity, hence smaller separation, while preserving safety, by increasing position accuracy. The distinguishing feature is that the new dimensionless safety metric is equivalent to the ICAO TLS, while being independent of the unit measurement, such as time or distance.
