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Gregory F. Zehner


Richard S. Meindl
Jeffrey A. Hudson



APRIL 1993

ABSTRACT: "Body size accommodation in USAF cockpits is still a significant problem despite all the years of experience and the many aircraft designs that have been developed. Adequate reach to controls, body clearances (particularly during escape) and vision (internal and external), are all functions of pilot body size and position in the cockpit.

One of the roots of this problem is the way cockpit accommodation is specified and tested. For many years the percentile pilot has been used. This paper describes the errors inherent in the "percentile man" approach, and presents a multivariate alternative for describing the body size variability existing in a given flying population. A number of body size "representative cases" are calculated which, when used properly in specifying, designing, and testing new aircraft, should ensure the desired level of accommodation.

The approach can be adapted to provide anthropometric descriptions of body size variability for a great many designs or for computer models of the human body by altering the measurements of interest and/or selecting different data sets describing the anthropometry of a user population."

[The Abstract is identical to that of the original version under the same title.] 

PREFACE: [The Preface does not, as one would expect, contain an explanation regarding the reason for issuing an abridged version of the original Technical Report of essentially the same title and authored by the same investigators.]

INTRODUCTION: "Military personnel of every size and shape much be able to operate complex equipment safely, effectively, and comfortably. Personnel charged with the specification and procurement of complex workstations and personal protective equipment are continually challenged by the need to accommodate and fit very large numbers of an increasingly heterogeneous population. In writing specifications, the goal is to ensure that the body size and proportions of most of the population will be accommodated in each item or system to be procured. Traditionally, this has been done by using percentiles to specify the portion of the population that must be accommodated. Typically, specifications read: "the system shall be designed to allow safe operation by the fifth percentile female pilot through the ninety-fifth percentile male pilot." What is not specified is how the 5th and 95th percentile pilots are defined.

The purpose of this report is to point out the drawbacks inherent in the percentile approach, and the present a more suitable method for describing variability in body size. The proposed method is based on the pioneering work of Bittner et al. (1986). For a detailed statistical description of the technique, see Meindl et al. (in press)."

[The work by Meindl, Hudson and Zehner, cited above as being "in press,"  has an Armstrong Laboratory technical report number of the 1993 series, thus appearing to have been published later than the abridged version. This is even more confusing since the "abridged" version, appropriately, has a later publication date.]

     Percentiles: ". . . while a 5th percentile Stature value can be accurately located, that value tells us little or nothing about the variability of other body dimensions of individuals with 5th percentile Stature. Consider Weight, for example. Individuals of 5th percentile Stature in the 1967 survey ranged from 125 lbs. (less than 1st percentile Weight) to 186 lbs. (74th percentile Weight). . . . [What might appear to be a] logical next step is to consider the fifth percentile for both measures. It is common for people to assume that the 5th percentile for both Stature and Weight represents a "5th percentile" person. In fact, only 1.3 percent of subjects in the 1967 survey were smaller than the 5th percentile for both measures, while 9% were smaller for one or the other. The problem is compounded with each additional measurement used to specify the size of the USAF individual. Thus, at worst, use of percentiles can mean that workspaces or equipment are not suitable for anyone. . . ."

"The pitfalls attendant upon the use of multiple percentiles can be illustrated by considering the body dimensions critical to cockpit design. . . . Sitting Height, Shoulder Breadth, Buttock-Knee Length, Knee Height Sitting, and Functional Reach. Generally a group of measures such as this is listed in a specification or standard along with 5th and 95th percentile values for each. This gives the impression that if these values are used as design criteria, 90% of the population will be accommodated. . . . There is no difficulty in identifying the individuals who constitute 90% of the population in Sitting Height. However, . . . when those same individuals are screened for 5th-95th percentile Buttock-Knee Length values, their numbers drop. With application of each additional cockpit dimension, the group diminishes until, finally, it represents only 67% of the population."

THE MULTIVARIATE ACCOMMODATION METHOD: "The multivariate accommodation method is an alternative to the percentile . . . It corrects the deficiencies of both while retaining the concept of accommodating a specific percentage of the population in the design. Briefly, the multivariate accommodation method is based on principal component analysis, which reduces a list of variables to a small manageable number, and then enables designers to select the desired percentage level of a population to be accommodated. This percentage level is accommodated in a way which takes into account not only size variance but proportional variability as well -- i.e. not only individuals who are uniformly large or small, but those whose measurements combine, for example, small torsos with long limbs, or vice versa.

A number of examples of the approach are given . . . beginning with a very simple two-measurement example, building to a basic cockpit layout, and concluding with a fairly complex 11-variable computer man-model."

DISCUSSION: "There are a number of multivariate statistical techniques which could be utilized to determine similar combinations of body size test cases. The technique described here, however, when combined with lists of minimum and maximum values, gives a much more accurate description of the body size and proportional variability existing in the population and, if used in designing workspaces, will greatly reduce the accommodation problems experienced by users. This assumes , of course, that the seat, rudder, and other adjustable components can be adjusted in sufficiently small increments. Without such adjustability, it may be necessary . . . to pick many more representative cases than the numbers suggested here to ensure the desired level of accommodation. However, for the purposes of writing anthropometric specifications, large numbers of representative cases may overwhelm the designer and thus, be counterproductive."


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