(Hecht and Handal, 2001; Hecht et al., 1998) provided data on the feasibility of stochastic modeling and outcome prediction in this domain. The enabling legislation for the current study specified that the resulting report to Congress be completed within 1 year; this time constraint precluded the committee from gaining hands-on experience with the two existing systems, as neither of them is currently in use or projected for future use.


The existing staffing models and the Grant Thornton approach are briefly described here as the basis for more detailed examination and assessment in subsequent sections.

WSSAS Staffing Model

The WSSAS staffing model, which was developed in the late 1980s and early 1990s, is an updated, Microsoft Windows–based version (using the Microsoft Access database management system) of an earlier MS-DOS-based program. The two factors driving staffing demand in the model are (1) the number and types of equipment maintained, and (2) the time required to maintain the equipment. The WSSAS model is based on a synthesis of the required maintenance times for each piece of equipment in the inventory (Grant Thornton, 2011). Original data on times required for both routine and nonroutine maintenance come from direct time studies conducted by the FAA or studies conducted by contractors as part of the development of each piece of equipment. Times for each task are totaled across each unit, with allowances added for travel time, nonavailable time, and indirect work. An overhead allowance is also added to cover “technical and program support.” Many reports from the model are available to managers, but the two most frequently discussed in committee meetings were the “Book 2A” and “Book 2B” reports. The Book 2A report gives overall staffing levels for each unit for current and previous years, with the added allowances. Book 2B provides a further staffing breakdown by ATSS specialties (FAA, 2012g). WSSAS is a complex model that requires considerable input data on a regular basis to provide managers with useful information regarding staffing levels. There were no data on reliability or validity of staffing predictions of this model, although users appeared to trust its outputs.

Tech Ops District Staffing Model

The Tech Ops District Model was developed in 2006 and updated in 2007 as an alternative to WSSAS.1 It is a regression model based on then-existing staffing levels and six high-level input variables, including a number of large Terminal Radar Approach Control (TRACON) facilities. Its basic premise is that the overall 6,100 staffing level (with an annual 2.5 percent reduction in head count based on “expected efficiencies”) is an exogenously determined (i.e., set by others without regard to the findings of a model) staffing level that is not subject to change through staffing model analyses. The model uses six predictors of district-level workload requirements, derived from a larger set of potential workload predictors. The regression was constructed based on 38 districts and accounted for more than 93 percent of workload variability in the dataset. All six coefficients were positive and statistically significant, which are necessary conditions for a structurally sound model as well as a valid one.2 Output from the Tech


1Tech Ops Models: WSSAS and District Model. Personal communication from the FAA to the Committee on Staffing Needs of Systems Specialists in Aviation, November 29, 2012.

2Rich McCormick, director, Labor Analysis, FAA, data from unpublished 2006 “Technical Workforce Staffing and Training Plan.”

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