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F-117A Stealth Fighter

 

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INTRODUCTION

This report is intended to serve as a guide to procedures for examining anthropometric accommodation offered in aircraft cockpits. The development of these examination procedures was an evolutionary process.  Between 1990 and 1995 we tested them in a variety of aircraft and cockpit mockups.  Included were the USAF F-16A, and C-141A aircraft at Wright-Patterson Air Force Base; the T-37B and T-38A at Wright-Patterson Air Force Base and Randolph Air Force Base;  the T-1A and F-22A at the contractors' facilities; the USN T-34C, T-44A, T-45A, and the TA-4J at Corpus Christi and Patuxent River Naval Air Stations; the eight Enhanced Flight Screener (EFS) competing aircraft at Wright-Patterson Air Force Base, contractors' facilities, and the Air Force Academy; and the ten Joint Primary Aircraft Training System (JPATS) competing aircraft and cockpit mockups at Wright-Patterson and at the contractors' facilities.

Appropriate body size accommodation in aircraft cockpits is still being sought in the military services in spite of the many years of experience gained by aircraft crewstation designers.  At the root of the problem are the methods traditionally used to specify and test new aircraft.  For many years, cockpit design was based on the concept of accommodating the 5th through 95th percentile for a limited number of critical anthropometric dimensions of the male pilot.  Within the aircraft industry, this concept was inappropriately extended as the "percentile man" concept and included an excessive number of dimensions.

As a result of the inherent restrictions of the 5th to 95th "percentile man" approach, considerable numbers of pilots have experienced difficulty operating or escaping from their aircraft.  To correct these deficiencies, multivariate alternatives to the percentile approach were developed to describe body size variability to be accommodated in new USAF aircraft.  An unwitting attempt at partial multivariate representation was incorporated in the two-dimensional drawing board manikins developed by the USAF in the mid 1970s.  With the move toward accommodating a greater percentage of potential women pilots, a much more sophisticated and complete multivariate analysis was developed in the late 1980s, again by the USAF, in which a number of body size combinations or "multivariate cases" are calculated.  These not only described small and large pilots, as the percentile approaches attempted to do, but take into detailed account the variability of body proportions found in many individuals who are not uniformly "large" or "small."  The multivariate models found in the table below are typical of those now used by the USAF to evaluate accommodation in aircraft cockpits.

 

ANTHROPOMETRIC MULTIVARIATE MODELS

 

 

 

Model 1

Model 2

Model 3

 

 

 

 

 

 

 

Generalized

Small Female

Male

 

 

Small Female

Short Reach

Short Torso

 

 

 

Higher Shldrs

Long Limbs

 

 

 

 

 

1.

Sitting Height

34.0

35.5

34.9

2.

Sitting Eye Height

28.9

30.7

30.2

3.

Sitting Acromion Height

21.3

22.7

22.6

4.

Sitting Knee Height

19.5

19.1

23.3

5.

Buttock-Knee Length

22.1

21.3

26.5

6.

Thumbtip Reach

28.3

27.6

33.9

 

 

 

Model 4

Model 5

Model 6

 

 

 

 

 

 

 

Generalized

Male

Male

 

 

Large Male

Longest

Long Torso

 

 

 

Limbs

Short Limbs

 

 

 

 

 

1.

Sitting Height

40.0

38.0

38.5

2.

Sitting Eye Height

35.0

32.9

33.4

3.

Sitting Acromion Height

26.9

25.0

25.2

4.

Sitting Knee Height

24.7

24.8

20.6

5.

Buttock-Knee Length

27.4

27.9

22.7

6.

Thumbtip Reach

35.6

36.0

29.7

 

This issue is more important than ever in today's Air Force because the demographics of the pilot population are changing.  In the 1950s and 1960s, when most current aircraft were being designed, the USAF pilot population was almost exclusively white and male.  Anthropometric databases reflected these demographics and, as a result, so did body size descriptions in aircraft specifications.  The current mix of males and females of all races has significantly changed the anthropometric profile of the population.

In addition, the Air Force body size restrictions for entry into undergraduate flight training have changed.  More large pilots are being admitted than ever before - and consideration is being given to changing body size restrictions to allow smaller people into pilot training as well.  These changes, however, should not be made before carefully assessing the consequences of allowing individuals to fly aircraft not designed to accommodate their particular body sizes.  The only reasonable way to make these decisions is through the use of data that describe the anthropometric limits a given cockpit imposes on the flying population.  If there is a high probability, for example,  that the long-legged pilot will strike the canopy bow during ejection, or that the short-legged pilot will not be able to reach full rudder throw, then consideration should be given to disallowing persons in those size categories to fly specific aircraft.

Describing anthropometric accommodation in cockpits is far from an exact undertaking.  It is well known, for example, that there are important differences between the body postures required by anthropometrists to ensure repeatable body measurements and the actual postures and the ways in which pilots position themselves in the seat to operate their aircraft.  The most common discrepancies occur in determining Sitting Height and Sitting Eye Height, for which the anthropometrist requires that the subject sit very erect and look straight ahead.  The head is positioned in the Frankfurt Plane. *  Few tasks, if any, require that the body be so positioned.  However, we need reliably measured Sitting Heights and Sitting Eye Heights to determine accommodated under the cockpit overhead and lines of sight over the nose of the aircraft.  Similarly, for an understanding of operational knee and shin clearances, interference with control stick movement, knee clearance during ejection, leg reach to rudder pedals, and hand reach to controls, we must concern ourselves with such dimensions as Buttock-Knee Length, Sitting Knee Height, Sitting Shoulder Height, Thumbtip Reach, Thigh Circumference, and Sitting Abdominal Depth.  

*  Frankfurt Plane: The Frankfurt Plane is a standard plane of reference of the head, realized when the lowest point on the bony margin of the eye socket (orbit) and the left tragion (top of the tragus or “flap” which forms the forward margin of the “ear-hole” are in a common horizontal plane.

The approach taken in developing these procedures is to use a number of test subjects representing as well as possible the body sizes found within the potential flying population, as represented by the multivariate cases.  Since it is next to impossible to find subjects whose body sizes duplicate the cases, we were required to develop techniques of analysis by which we could predict the accommodation of the appropriate cases.  In a very real sense we use the subjects as human "tools" to establish the upper and lower limits of body size accommodation.

In this effort we concerned ourselves with the seven aspects of anthropometric accommodation listed below.  They are arranged in increasing order of complexity.

1.  Maximum Sitting Height accommodation.

2.  Vision from the cockpit to the outside and toward the instrument panel.

3.  Static ejection clearances of the knee, leg, and torso with cockpit structures.

4.  Operational leg clearances with the main instrument panel.

5. Operational leg clearance with control stick/wheel motion envelope.

6. Rudder pedal operation.

7. Hand reach to and actuation of controls.

In some aspects of accommodation, overhead and ejection clearances and vision for example, anthropometric relationships are rather straightforward.  Overhead clearances are directly related to Sitting Height.  Ejection clearances are related to Buttock-Knee Length, Shoulder Breadth, and Elbow to Elbow Breadth, separately.  Vision out of the aircraft, primarily vision over-the-nose, is directly related to Sitting Eye Height.

Other aspects of accommodation are more complex.  Operational leg clearances, for example, are influenced not only by measures of leg length, especially Buttock-Knee Length, but also frequently by seat position.  If interference is found, it is usually between the areas around the knees and the main instrument panel or side consoles as well as hand controls that are mounted on these surfaces.  Since it is usually the large pilot who experiences these interferences, the seat is usually at or near the full down position.  Relief can sometimes be gained by raising or further lowering the seat.  Whether or not the pilot can raise the seat, of course, depends on the existence of sufficient head room.  If the top of the helmet quickly encounters the underside of the canopy or other overhead, or if visual access to critical displays is lost under the glare shield, it may be unwise to raise the seat.

Operational leg clearance with the control stick or wheel motion envelope is driven by seat position, Thigh Circumference, Buttock-Knee Length, and sometimes Abdominal Depth.  The upper seat positions and Thigh Circumference seem to be the most critical.  With regard to the control stick, we can readily visualize this when we appreciate that the motion of the upper end of the control grip is around the base of an inverted cone.  As the seat is raised, the greater the possibility of interfering with its motion - especially if the pilot has large thighs.  For the same reason, the potential of interfering with control wheel motion is also increased.  If the pilot can retain adequate vision, it might be possible to move the seat downward to relieve interference.  Since the large pilot will typically use the full down seat position, the control stick grip/wheel is usually above the thighs and interference may not occur.  Also the legs are often sufficiently long as to cause the knees to rise high enough to clear the seat side fence and side consoles, permitting greater space between them for control stick movement.  Occasionally full aft motion of the control stick or wheel is interfered with by the pilot's belly.  Again, if adequate vision over the nose can be maintained, this can sometimes be relieved by lowering the seat.

The ability to reach and actuate rudder pedals is also effected by seat position.  The pilot who is small in Sitting Eye Height may have to raise the seat to achieve adequate vision.  If the legs are not disproportionately long, the pedal carriage may have to be adjusted aft to have access to the full range of pedal motion and to be able to actuate the brakes.  If the pilot has disproportionately short legs, he or she may not be able to actuate full rudder and brakes, even though the carriage is adjusted full aft.  If the seat can be lowered and minimally acceptable vision out of the aircraft maintained, access to rudder pedals can be improved - along with reach to hand controls below shoulder level.  Under no circumstances, however, should the pilot sacrifice vision.

Reach with the arm and hand is not only influenced by the dimension Thumbtip Reach, or, as some have called it, "Functional Reach," but also by Sitting Eye Height, Sitting Shoulder Height and the length of the legs.  Sitting Eye Height plays a decisive role in seat adjustment, since the pilot must seek at least minimally adequate vision not only over the canopy, but also to the instrument panel.  The seat may have to be moved to still a different position to obtain full control of the rudder pedals.  The level of the shoulders in the cockpit, which directly influences hand reach, is heavily influenced by attempts to meet vision and rudder pedal requirements.  Finally, any factor that effects mobility at the shoulder and elbow, such as design, fit, and adjustment of harnesses and personal protective and survival gear, body strength, and motivation as well, come into play in the act of reaching.

It is typical for pilots to change seat positions to achieve optimum accommodation to a variety of needs.  It follows that several subjects with the same arm length will achieve different levels of reach accommodation, depending on his/her other body dimensions.  If only one subject is used in the evaluation of operational leg clearance, access to rudder pedals, and  hand reach to controls and other aspect of accommodation, the results will be relevant only to that individual.

Examinations of overhead, operational and ejection clearances were usually performed using subjects at the upper ends of the ranges for relevant body size dimensions such as Sitting Height, Buttock Knee Length, Sitting Knee Height, Shoulder Breadth, and Thigh Circumference.

Examinations of internal and external vision were performed on subjects throughout ranges for Sitting Eye Height and Sitting Height.

Measurements of rudder pedal operation and hand reach to controls are most effectively examined using subjects at the smaller ends of the required ranges for dimensions such as Buttock-Knee Length, Sitting Knee Height, Thumbtip Reach and a range of Sitting Shoulder Heights.

The procedures described here concentrate on high performance aircraft with single, side by side, and tandem cockpits with transparent canopies.  The procedures will necessarily vary for use on flight decks without transparent overheads.

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