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ANTHROPOMETRIC ACCOMMODATION IN THE T-38

Gregory F. Zehner
Human Engineering Division
Armstrong Laboratory
Wright-Patterson AFB, Ohio

Kenneth W. Kennedy, Ph.D.
Engineering Anthropometry Consultant
Yellow Springs, Ohio

Jeffrey A. Hudson
Sytronics Inc.
Dayton, Ohio

ABSTRACT: "The  USAF [United States Air Force] may expand the body size entrance requirements for Undergraduate Pilot Training (UPT). We are now conducting a research project to quantify the smallest and largest people that can safely and efficiently operate all types of USAF aircraft prior to changing these requirements. Our accommodation analysis of the T-38 was based on AETC's [Air Education and Training Command] list of operational requirements (tasks that a pilot must be able to carry out to safely fly the aircraft).

Our results indicate that the T-38 accommodates large pilots quite well, except in the rear cockpit, where pilots with Sitting Heights higher than 39" may have their helmets pressed against the canopy during negative-G flight. Accommodation for small pilots is much worse in both cockpits. Seventy-three percent of the JPATS [Joint Primary Aircraft Training System]-eligible female population and 13% of the JPATS-eligible male population cannot perform one or more of AETC's operational requirements. These pilots would have to stretch to see over the nose, and they either would not be able to reach full rudders or they would have to slide forward in the seat to reach full rudders. With locked inertial reels, they would not be able to reach to retard a full throttle, so they would be forced to unlock the reels or unload restrictive G-forces."

Note: For more information regarding the JPATS program, see http://www.wpafb.af.mil/ascpa/factshts/programs/jpats98.htm

EXTRACTS: 

INTRODUCTION: "With the procurement of the Joint Primary Aircraft Training System (JPATS) and its eventual introduction into the USAF and USN inventories, it will be possible to train pilots whose body sizes are considerably smaller than ever before (Zehner, 1996).  The USAF is now considering expanding the body size entrance requirements for Undergraduate Pilot Training (UPT) (AFI 48-123) to take advantage of the increased accommodation offered by the JPATS aircraft and to provide equal access to flight training for both male and female pilot candidates."

"Potential USAF pilots must first fly the T-3 Firefly (the initial "Flight Screener") and then, after JPATS . . . must continue training in either the T-1 Tanker/Transport trainer or the T-38 Fighter/Bomber trainer. If small pilots cannot safely fly existing trainers, it may be pointless to allow them to enter UPT, and unless these [training?] aircraft are modified, there may be no point in designing follow-on aircraft to JPATS-level accommodation limits."

BACKGROUND: "For JPATS, the size range . . . is defined as a series of body size test cases (1-7). Each case represents a separate individual with either extreme size or body proportions. It will be possible for pilots as small as 58" in Stature and 31" in Sitting Height, or as large as 77" in Stature and 40" in Sitting Height, to operate the JPATS aircraft."

". . . the T-38 was intended to accommodate 5th through 95th percentile male anthropometric dimensions based on the 1950 USAF anthropometric survey of pilots (Hertzberg, Daniels, & Churchill, 1954). Because of the improper use of percentiles in the design specifications for the T-38 (Zehner, Meindl, & Hudson, 1992), and the potential for the actual designs to exceed requirements in some areas of accommodation, we had to complete our investigation to determine  the real accommodation levels of the aircraft." 

"Previous investigations of accommodation in training aircraft have focused on the existing pilot population body size, and the results from these studies indicate that, in some areas, the extreme ends of the size distribution of current pilots are at or very near the limits of accommodation. There is clearly a potential for accommodation problems if even larger and smaller pilots are allowed to fly these aircraft." 

COCKPIT ACCOMMODATION: "We examine seven aspects of anthropometric accommodation:
     1. Overhead clearance.
     2. Rudder pedal operation.
     3. Internal and external visual field.
     4. Static ejection clearances of the knee, leg, and torso with cockpit structure.
     5. Operational leg clearances with the main instrument panel.
     6. Operational leg clearance with control stick motion envelope (the pilot's ability to attain the full
         range  of stick travel).
     7. Hand reach to controls."

"We test subjects of various sizes with  the seat adjusted to numerous positions in the cockpit. This allows us to examine the subject in progress, and . . . to extrapolate measurements for subjects of neighboring sizes and varying proportions."

ANTHROPOMETRY: "We have assembled a pool of over 50 test subjects for these studies.  . . . Our sample was not selected to exactly represent the body size distribution of the pilot population. Small subjects were selected to over-represent the extremes of the general USAF population  while retaining a reasonably normal distribution for each measure. A small number of large subjects were selected to represent the largest potential pilots in the USAF population."  

ANNOTATOR'S ADDENDUM: The following are anthropometric multivariate Cases 1, 5 and 7 values that are critical for the examination of JPATS cockpits. They were derived specifically for the JPATS program, in which potential pilots as small as 58" in Standing Height are specified for accommodation. 

JPATS ANTHROPOMETRIC MULTIVARIATE CASES 1, 5 AND 7

                                              Case 1          Case 5      Case 7*
                                         Generalized     Longest    Objective
                                          Small Pilot       Limbs      Small Pilot
                                           (Female)          (Male)      (Female)

Sitting Height                     32.8"              38.0"           31.0" 
Sitting Eye Height             28.0                32.9            26.8
Sitting Shoulder Height   20.6                25.0            19.5
Sitting Knee Height          18.7                24.8            18.1
Buttock-Knee Length       21.3                27.9            20.8
Thumbtip Reach**             27.0                36.0            26.1

* Case 7 was proposed by the USN to target smaller female pilots than the USAF Case 1. 
** Equivalent to Functional Reach.

RESULTS FOR SMALL PILOTS

VISION OVER THE NOSE: "The JPATS cases range from 26.8" to 35.0" for Sitting Eye Height, while the range for the current pilot population is 28.9" to 35.4". We measured over-the-nose vision (ONV) with the subject's head held level (in the Frankfurt Plane), and again with the subject's head stretched up for maximum possible downward vision. AETC instructor pilots have insisted that trainees should fly with their heads level, and that the additional degree of vision attained by stretching should be held in reserve as a safety margin." 

"The [regression plot] . . . below predicts head-level ONV angles for flyers based on their Sitting Eye Heights, with the seat in the full-up position. The graph shows that people of very small Eye Height Sitting may only be able to see a few degrees over the nose when the aircraft is in level flight. Depending on the aircraft angle of attack during landing, these pilots may not be able to see the runway over the nose of the aircraft."

"Fifty-eight percent of the JPATS female population and 10% of the JPATS male population falls below the size necessary for acceptable ONV. For the current USAF pilots, 14% of females and 6% of males will  have an ONV angle worse than -11 degrees."

"For the JPATS smallest sizes (cases 1 and 7, eye heights = 28.0" and 26.8", respectively), external visual field is so restricted that the pilots cannot see the runway during a no-flap approach.  . . . Subjects near case 7 size typically have eye positions below the aft edge of the glare shield in the T-38. When the HUD is added, small JPATS cases will see very little of the display."

RUDDER THROW: "The measurement which best identifies the minimum leg length required to reach full rudder throw is a combined leg length. We add Buttock-Knee Lengthy and Sitting Knee Height to arrive at a new measure that we call "Comboleg." . . . Using Coboleg would not be appropriate . . . in aircraft where the pilot cannot fully extend his or her knee.  . . . The range of Comboleg measures for current pilots is 40.7" to 52.4". The JPATS range is 38.9" to 52.7". "

"We defined rudder accommodation limit as full rudder input and full brake with the knee fully extended. The subjects were tightly restrained and not allowed to slide forward in the seat. We then measured subject miss or excess reach to rudders for regression analysis. The [regression plot] . . .  below shows miss/excess distance (negative numbers for miss distance, and positive numbers for excess distance) to full rudder and brake for a variety of leg lengths. With the seat in the full-up position, a Comboleg length of 43" is required to attain full rudder and full brake simultaneously. This applies to both the front and rear cockpits in the T-38." [Miss distances is the additional leg reach that is needed by a given individual to gain full rudder and brake actuation. Excess distance is the leg reach beyond that which is minimal for full rudder and brake actuation.]


Figure 2. [Regression plot for predicting] Leg reach to rudders (seat full up).

"We used a two-step process to determine the percentage of the various populations accommodated on rudder pedals. Two steps are required because, if a pilot's legs are too short to reach the rudder pedals, he or she may be able to lower the seat to get closer. This is acceptable if the pilot still has equal to or better than -11 degrees over-the-nose vision in the lower seat position. Therefore, we adjust the seat so that each subject in the following calculations sees -11 degrees ONV. From that seat position, we determine if the subject can reach full rudder input and full brake." 

"Fifty-four percent of the JPATS female population and 5% of the JPATS male population are too small to both reach the pedals and see -11 degrees out of the cockpit. JPATS Case 7 represents a Comboleg of 38.9 inches, and is the smallest multivariate body size to be accommodated in JPATS. Case 7 would miss full rudder by 4.1" with the seat full-up. This is a misleading figure, however, because Case 7 would need to raise the seat an additional 3.2" beyond full-up to see the minimum -11 degrees ONV. If it were possible to raise Case 7 that much, miss distance to the rudders would be 5.7"."

"For current USAF pilots, 19% of females and 3% of males cannot apply full rudder and brake while maintaining -11 degrees ONV. All current pilots are within one inch of reaching full rudder and brake while seeing at least -11 degrees ONV.  All current pilots are within one inch of reach full rudder and brake while seeing at least -11 degrees ONV." [Annotator's italics]

ARM REACH TO CONTROLS: "The most difficult area in which to establish pass/fail criteria is reach to [hand] controls.  . . . reach to a particular control is a function of arm reach, shoulder height, shoulder width, and seat position.  . . . even though two pilots might have the same arm length, their other body measurements will almost certainly be different. Pilots typically select a seat position to optimize external [ONV] vision, and then, if necessary, adjust the seat to improve reach to rudders and [hand] controls. [Typically, in ejection type seats, the large torso pilot, because of minimal clearance above the helmet, must lower the seat.] [Therefore, t]he aft-angled ejection seat moves a large-torso pilot lower and more forward in the cockpit (closer to controls and rudders) and short-torso pilots higher and more aft in the cockpit (further from controls and rudders) [, the opposite that good human factors design]."

"To eliminate the need for three-measurement regression, we substituted the variable Span for Thumb-tip Reach and Biacromial Breadth, and created a two-step regression using Span and Sitting Shoulder Height. The multiple correlation between Shoulder Height, Span, and miss distance to the throttle is .95 with a standard error of .5 inches." 

"We based reach to control measurements on the Zone 2 harness configurations as defined in Mil. Std. 1333, with inertial reels locked but shoulders reaching out toward the control." 

"While USAF pilots usually do not lock their reels, safety concerns dictate looking at "worst-case" scenarios. Locking the inertial reels is meant to simulate the restricted mobility a pilot experiences during adverse-G conditions, and it also tests whether the pilot can control the aircraft when there is an inadvertent restraint lock." 

"AETC determined that, with locked inertial reels, pilots must be able to operate the inertial reel lock, the ejection handles, and retard the throttles. In the T-38, the throttles are the most difficult of this group to reach, so they are the only control[s] discussed here. [The inertia reel lock and the ejection handles are mounted on the seat and, therefore, reach distances to them are not affected by seat position.]"

"Since this is multiple regression, a number of combinations of Shoulder Height and Span [can] equate to zero miss distance.  . . .  Miss distance rises as span decreases and as shoulder height increases. A two varible [sic] example graph is shown below. The required Span length for reaching throttles is approximately 65 inches." [Without further explanation, announcing that a 65-inch "required," that is, minimum Span for reaching the throttles is confusing.]

"As with rudders, we used a two-step process to determine the percentage of the various populations accommodated on reach to throttles. Two steps are required because, if a pilot's arms are too short to reach the controls, it may be possible to lower the seat to get closer. This is acceptable if the subject still has adequate (-11 degrees) ONV in the lower seat position. Therefore, we adjust the seat so that each subject in the following calculations sees -11 degrees over the nose. From that seat position, we determine if the subject can reach the throttles well enough to retract them when they are full forward." 


Figure 3. Arm reach to throttles (seat full up).

"Sixty percent of the JPATS female population and 2% of the JPATS male population are too small to reach the throttles and still see -11degrees over the nose. For current USAF pilots, 23% of females and 1% of males will not be able to reach and retract full throttle.  All current pilots are within one inch of reaching and retracting full throttle while maintaining -11 degrees ONV.  [Annotator's italics] 

SUMMARY FOR SMALL PILOT RESULTS: "The T-38 is not a very accommodating aircraft for small pilots. . . . percentages of pilot populations failing to meet the operational requirements . . . are 73% of the JPATS female population, 13% of the JPATS male population, 36% of the current female pilots, and 8% of current male pilots. It is not surprising that such large percentages of the JPATS population fall outside the T-38 accommodation limits, but it is surprising that over a third of women [currently] eligible to enter UPT do not meet the AETC requirements. These pilots have to stretch to see over the nose, and they either cannot reach full rudders or have to loosen their lap belts and slide forward to reach full rudders. With locked inertial reels, they cannot reach to retract a full throttle, so they must unlock the reels or unload the G forces that are limiting movement."

LARGE PILOT ACCOMMODATION: "The front cockpit of the T-38 can accommodate pilots much larger than the range of Sitting Heights found in the military population . . ."

"In the T-38's rear cockpit, however, the current 40" maximum Sitting Height is only minimally acceptable. [Their helmets] will touch the canopy . . . and will press hard against the canopy during negative-G flight."

"In the T-38, there is a great deal of room in front of the pilot's knees, and the canopy bow and glareshield are well outside the ejection envelope."

"Both small and large subjects found it difficult to pull the control stick full-aft and then roll it to the stops on the left and right sides. This action was most difficult with the seat in the full-up position. The correlation between stick interference and the body measurements we expected to be related to it (thigh circumference, thigh clearance, and buttock-knee length) were around zero."


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