Aircraft mix (i.e., the mix of aircraft with different performance characteristics in a sector) has been repeatedly cited as a complexity factor in en route air traffic control. However, scant attention has been directed to a statistical examination of this relationship. The present study is the third in a series of investigations designed to define, quantify, and assess the validity of aircraft mix as a contributor to traffic complexity. Eighteen 30-minute samples of System Analysis Recording data were collected from the Fort Worth and Atlanta en route centers. Performance and Objective Workload Evaluation Research (POWER) measures and the Aircraft Mix Index (Pfleiderer, 2003a) were computed in 6-minute intervals for each of the 36 samples. Principal Components Analysis of the combined data sets produced four components with eigenvalues>1 accounting for approximately 71% of the variance. The Aircraft Mix Index was most closely associated with Component 1, which was composed of variables generally associated with traffic complexity. These variables were used as predictors against a criterion of controllers' subjective "Complexity" ratings in multiple regression analyses of low- and high-altitude sector samples. The Aircraft Mix Index failed to contribute significantly to the explained variance in the both the low-altitude (R=.69; R2=.47) and high-altitude (R=.57; R2=.33) sector models. In the aggregate, the results suggest that although aircraft mix appears to be associated with traffic complexity, it may not be as influential as other complexity factors in the en route environment.

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