The actual incidence of midair collisions and near midair collisions in terminal areas between controtied and uncontrolled traffic cen be expected to depend on the number of aircraft operations, the relative location and utilization of airports and runways in each terminal, tharestrictions on the use of the terminal airspace imposed by topographical features. and arrival/departura routes, the incidence of marginal visibility, and the effectiveness dk traffic advisories warning of potentially conflicting traffic. This paper shows that the relative incidence of collision exposure in the various terminals, as measured by the number of hazardous near midair collisions reported in these terminals during the 1968 FAA Near Midair Collision Study, is accounted for to a remarkable extent by the relative number of operations and the relative degree to which controlled and uncontrolled aircraft share the same airports. Only a very few specific terminals, where the unequal influence of the other factors cited is believed t o be significant, depart somewhat from this finding.

Using the 1968 FAA near midair collision data and Howell's flight test data on visual detection of aircraft from other aircraft, an estimate is made of the effectiveness of the see-and-avoid doctrine as a function of closing speed in today's air traffic environment. It is estimated that at present about 95 percent of potential collisions are avoided at low closing speeds; this effectiveness ofsee and avoid falls t o about 50 percem above 400 knot closing speed.Midair collisions are found to be primarily attributable to the failure by pilots to see the other aircraft in time to maneuver, and an estimate is given of the potential reduction in anticipated collision rates achievable with the use of pilot warning indicator systems of various degrees of effectiveness. The probability that a pilot reports an NMAC is found t o increase as the.closing speed of the encounter increases, but it also appears that pilots tendto underestimate the miss distance at the high closing speeds. The estimates made of the probability of successful maneuvers are i n good agreement with the independent results of simulation experiments. The expected number of midair collisions in thevarious closing speed ranges, as predicted.from the NMAC and Howell data, are in reasonable agreement-with experience.

Second language learners may merge similar sounds from their native (L1) and second (L2) languages into a single phonetic category, neutralizing subphonemic differences in these similar sounds. This study investigates whether Dutch speakers produce phonetically distinct variants of /s/ in their L1 Dutch and L2 English, and whether and how this phonetic categorization develops over time. Target /s/ sounds in matching words in L1 and L2 were compared in their centre of spectral gravity. Speakers varied in their individual learning curves in the categorization of produced /s/ sounds, both in starting points and in longitudinal trajectories. After 3 years, however, all speakers had converged in producing their /s/ variants in L1 and L2 as two similar but different sounds.