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HAND REACH TO CONTROLS 

PURPOSE

The purpose of these measurements is to determine the minimum Equivalent Thumbtip Reach necessary to reach and actuate selected hand controls under Zones 1, 2, and 3 restraint as described in Mil-Std-1333B, Aircrew Station Geometry for Military Aircraft.  

DISCUSSION

Thumbtip Reach, frequently referred to as "Functional Reach," is a familiar body dimension.  Although difficult to obtain good repeatability, it is the most commonly used dimension when attempting to understand reach capability with the hand.

The manner in which hand controls in cockpits are operated can be classified into four general types:

            - those that are operated typically with the tip of the forefinger (push buttons and toggle switches).*  These will be designated as "F" type controls.

            - those that are operated with the thumb and forefinger (locked toggle switches, knobs, and most circuit breakers): "T" type controls.

            - those gripped with the whole hand (control stick grip or wheel):  "G" type controls. **

            - those operated by using the fingers as a hook (T-handles): "H" type controls.  

* Obviously, in an ultimate stretch, if the reach cannot be made with the forefinger, the third (medial) finger can be attempted. For this reason, we measure to the latter. 

** A case can be made for the elimination of the "G" measurement for some controls and substitute the "H" measurement. It can be argued that, in reaching for some "Grip" type controls, it would be possible to pull the hand around the controller if the pilot can initially "crawl" around it using a "hooking" action. If, however, as in the case of the stick grip of the primary controller, the control is undesirably displaced in the process, the true "G" type measurement must be used.

To completely understand accommodation it is not sufficient to merely make a can/can't appraisal of a series of subjects' abilities to reach and actuate hand operated controls.  Such determinations do not yield information on the actual reach capability necessary to access controls.  We only know that that particular subject could or could not reach.  We also may not know how much farther the subject might have been able to reach or that he/she was performing an excessively extended reach.  Also, if the subject cannot reach a given control, we will not know how much longer his/her arm must be so as to reach it.

It is essential, therefore, to determine the minimum arm length necessary to reach controls.  Initially, we attempted to measure reach miss-distances between the control and the appropriate interface of the hand, with the hand in the operating attitude.  It was very quickly found that such measurements are both difficult and unreliable.  A much more convenient procedure was developed and is recommended for use here.  It consists of marking a short line on the thumb side of the lower forearm perpendicular to the axis of the forearm.  The distances from this line to the interface points on the hand are measured in advance of the examination session.  These special dimensions are referred to as "X-to" dimensions and are described and illustrated in the section "Anthropometric Dimensions" in Contents. 

When gathering reach data, rather than attempt to measure from the interface point  on the hand to a control, we measured from the line on the lower forearm (the"X" point) to the control.  This made it possible to express reach capability, regardless of the method of control actuation, in terms of the familiar "Thumbtip Reach."  We referred to this as "Equivalent Thumbtip Reach," since it included not only reaches to "T" type controls, but to "G," "F," and "H" controls as well.  The logic of the Equivalent Thumbtip Reach calculation is discussed in the Analysis and Results section.  

For any given seat position it is obvious that, all other factors being equal, controls that can be reached and actuated by subjects with short arms can also be reached and actuated by those with longer arms.  In the vertically adjusting ejection seat, this relationship is somewhat complicated, since seat position is strongly influenced by the requirement to gain adequate vision out of the cockpit.  Pilots with lower Sitting Eye Heights need to adjust the seat toward the upper end of its range and invariably farther away from hand controls to gain adequate vision over the nose of the aircraft. The worst case, insofar as reach to controls below shoulder level is concerned, is the pilot with short Thumbtip Reach and Sitting Eye Height, and relatively high shoulders.  Such a pilot may have to adjust the seat full-up, moving the shoulders the greatest possible distance away from all such controls.  In high performance aircraft, this constitutes most controls. 

The Figure below illustrates a Seat Position Selection Chart which can be used to ensure that reach data are obtained on subjects representing an appropriate range of shoulder levels within the cockpit.  The logic of its construction and use is based on the following discussion.  

SEAT POSITION SELECTION CHART FOR EXAMINATION OF REACH CAPABILITY

AIRCRAFT I.D._____________________________COCKPIT  _____________

                                                     S I T T I N G   S H O U L D E R   H E I G H T S


SEAT POSITION SELECTION CHART. This chart is used as a guide to assure that reach data are obtained at an appropriate range of shoulder levels within the cockpit. Ideally, all combinations of Sitting Shoulder Height and Seat Position should be used. Short of that, those for the full-up seat should be emphasized. If it is impossible to obtain subjects for every Sitting Shoulder Height, those of up to 1 inch larger or smaller can be used with seat positions adjusted to simulate the desired shoulder height. 

In the cockpit, the seat can usually be adjusted upward or downward to produce different elevations of the shoulder in the cockpit.  For example, the shoulders of a pilot with a 22.0" Sitting Shoulder Height and in the full-up seat adjustment will be at essentially the same level as another pilot with a 23.0 inch Sitting Shoulder Height in the seat adjusted 1 inch down from full-up, and a third pilot with a 24.0" shoulder in the seat at -2.0".  It follows, then, that the Equivalent Thumbtip Reach required to access a given control will be essentially equal for such shoulder heights and seat adjustment combinations.  In selecting subjects to be representative of those who will potentially experience difficulty in reaching controls, it is necessary to target the uppermost seat position.  That is, to examine subjects in the full-up seat or simulated for the full-up seat.  Because of the above relationships, then, a subject with a Sitting Shoulder Height of 22 inches in the seat adjusted to 2 inches down from full-up can simulate the subject with a 20 inch Sitting Shoulder Height in the full-up seat.  Evidence for the validity of this assumption will be discussed further on.  This eases the persistent problem of finding subjects who are of the exact sizes needed for the examination of reach.  Simulation probably should not be attempted to Sitting Shoulder Heights more than two inches less than that of the subject.  We reached this conclusion only after a large number of simulations were attempted.  It is rare that a larger shoulder height has to be simulated. 

When it is necessary to determine if controls have been located appropriately to accommodate to specific values for Sitting Shoulder Height and Thumbtip Reach, using subjects to simulate a target value for Sitting Shoulder Height can be quite useful.  If, for example, a cockpit must accommodate to a minimum Sitting Shoulder Height of 22.7 inches and a Thumbtip Reach of 27.6 inches (see TABLE), we can simulate the small shoulder height with subjects as large as 24.7 inches.  Since the worst case seat position for pilots meeting the target value of 22.7 inches would be full-up in order to gain adequate over-the-nose vision, we can simulate this shoulder height by lowering the seat by an amount equal to the difference between the subject's Sitting Shoulder Height and 22.7 inches.  Again, we recommend that subjects not be more than two inches larger than the target value.  Using the reach measuring procedures described in the previous paragraphs, the minimum necessary Equivalent Thumbtip Reach can be determined and compared to the 27.6-inch minimum target value.  

Three reach zones specifically for use in aircraft cockpits have been defined in Mil-Std-1333B, Aircrew Station Geometry for Military Aircraft.  Although this Mil-Std is no longer recognized by the military services, the guidance it offers as regards reach zones is still followed by the majority of aircraft companies and many agencies of the Department of Defense.

Reach Zone 1 requires that the operator's shoulders be relaxed, but "fully restrained and equipped without stretch of arm or shoulder muscles."  The harnesses are snugged and the pilot is held back against the seat back with the inertia reel locked.  Forward and side-to-side motions of the torso and shoulders are not permitted, and should not be required.  Zone 1 controls are defined as "critical and emergency controls," further, that all controls specifically related to "takeoff, landing, low altitude high speed flight, weapons delivery, and escape should be located within Zone 1."   Many crew station engineers and pilots feel that Zone 1 should include, at the most, only the control stick or wheel at neutral, seat ejection grips and handles, and ignition and fuel controls.  These categories of controls are not universally agreed to and reach requirements vary with aircraft type.

Zone 2 reaches are defined as those requiring the restraint system to remain as described for Zone 1, but the operator is free to move his/her shoulders and torso forward and to the sides to the maximum limit permitted by the total restraint system.  Mil-Std-1333B calls for "essential" controls to be placed within Zone 2.  

Reach Zone 3 specifies that the inertia reel be unlocked and the shoulders and torso permitted to move forward and to the sides as necessary for a maximum reaches.  Mil-Std-1333B specifies these controls as "non-critical" or "non-essential."

A convenient way to contain shoulder and torso motion for Zone 1 measurements is to attach a cord to the seat or cockpit structure aft of each shoulder, stretching it forward, and with the hand, holding it firmly against the bony prominence at the tip of the shoulder (acromial process).  If motion of the shoulder occurs during the reach measurement, it can be readily detected through disturbance of the shoulder/cord contact. This is illustrated below. 

CONTROLLING SHOULDER AND TORSO MOVEMENT. 

There is no set formula for selecting controls to which measurements will be made.  Since there can be a very large number, however, only a sampling of them is attempted.  The examiner should always be mindful of primary, safety-of-flight, emergency controls and contractually binding controls from the RFP or SOW, etc., and include them among those examined.  Additional controls spaced at regular intervals over the surfaces and at the boundaries of the main instrument panel, sub-panels, pedestals, side consoles, overhead, and bulkhead panels should also be selected.  Where there are groups of related controls within a relatively small area, an attempt should be made to select those that represent the variety of types and spatial distribution within the group.  If a large area is found to contain only displays, or is vacant, landmarks on the surface of the panel, such as screw heads, can be selected to represent the area.  For consistency, we assumed all such latter landmarks to be thumbtip interfaces.  If, at a later date, such panel spaces are considered for the location of controls, information regarding reach will be available.

Examiners must designate which of the selected controls will be operated by the right and left hands, and which can conveniently be operated by either hand.  In the latter case, reach measurements are made on each arm.  Since primary control grips are designed to be grasped in the right hand and usually located between the knees, it should be assumed that, in such aircraft, controls on the main instrument panel to the left of the centerline of the cockpit will generally be operated exclusively with the left hand.  Since the right hand is assumed to be preoccupied with the control stick, controls on the main instrument panel to the right of the crew station centerline and all those on a center pedestal may be operated by either hand.  Controls on left and right sub and side panels will be operated only with the corresponding hand.  Occasionally, a control will be found on the bulkhead just to the side of the shoulder.  Such controls are sometimes more conveniently reached with the opposite hand.  In wheel controlled aircraft, the pilot should never be expected to operate a control on one side of the yoke with the opposite hand.

Go to C-5A, C-141B, F-15A,BF,C&DF and F-16A/B,C/D,CG/D&CJ/D for complete lists of emergency controls for each model of each aircraft as well as a proposed list of hand controls for these aircraft to which reach measurements should be made.

PROCEDURE                                                                                           

To determine the shortest arm length necessary to reach a control, it is necessary that the test subject's elbow be fully extended and locked during the reach.  Some controls are close enough that they do not require the elbow to be fully extended, indeed some are so close that the elbow cannot be extended.  To obtain a measurement for such controls, subjects can often compensate and obtain a fully extended elbow by hyper-flexing the wrist.

Because measurements can be made only when the subject's elbow is fully extended and we want to obtain reach values for as many controls as possible, subjects with Thumbtip Reaches as close as possible to the minimum value should be used.  A control too close to obtain a valid measurement, and therefore easily reached by such subjects, virtually assures that it can be reached by the full range of subjects.

The examination proceeds as follows.  

            1.  Record Sitting Shoulder Height and Thumbtip Reaches on a Reach Data Forms Cover Page such as that illustrated in FIGURE. 

            2.  Wearing typical flight gear less helmet, the subject is installed into the seat.  In high performance ejection cockpits, the seat is adjusted to a position based upon the subject's Sitting Shoulder Height.  The Seat Position Selection Chart above can be used to assure that a range of actual and simulated values for this dimension are represented.  If a specific minimum accommodation value for Sitting Shoulder Height has been designated, the seat is adjusted down by an amount equal to the difference between his/her shoulder height and the target value to a maximum of 2 inches.  In cockpits and flight decks with seats with both fore and aft and vertical adjustability and those that adjust along a ramp, the seat should be full-forward and full-up.  Record seat position on the Reach Data Forms Cover Page.  

            3.  All torso and shoulder harnesses are buckled and appropriately snugged up.  The subject should lean forward, lock the inertia reel, and then settle comfortably against the seat back, allowing the reel to take up the slack in the restraint system.  The inertia reel should remain locked for all Reach Zones 1 and 2 measurements.  It is important to determine that both shoulders are equally restrained.  This can be checked by selecting a point or control located in the centerline of the main instrument panel and taking right and left hand Zones 1 and 2 reach measurements to it.  Equivalent Thumbtip Reaches that agree in correspondence with the differences in the subject's right and left Thumbtip Reaches, if any, will indicate equivalent restraint for both shoulders.  Zone 1 measurements can be simulated in aircraft not equipped with manual locking inertia reels, but Zone 2 measurements cannot.

            4.  With the back resting comfortably against the seat-back and the forearms and hands resting in his/her lap, the shoulder cord for the left shoulder is brought forward over the bony prominence at the tip of the shoulder and held against the shoulder to encourage the maintenance of contact with the seat back while measuring Zone 1 reaches.

            5.  Ask the subject to extend the hand toward each of the controls to be examined, in turn, usually beginning with the extreme left and proceeding around to the right.  All Zone 1 reaches for a given hand are usually completed before beginning Zone 2 measurements.

            6.  Measure and record the distances measured from the "X" mark on the forearm to the interface points on the controls.  An example of such a measurement is illustrated below.

MEASURING REACH CAPABILITY IN THE COCKPIT. Measurement is made between the mark on the thumb-side of the lower forearm to the interface of the control.

In the case of the control grip, toggle switch, or pushbutton, for instance, the interface point is the near surface of the control.  In the case of a T-handle, ejection D-ring or handle, the interface point is the back side of the "T", strap, or handle.  Most measurements will have positive values.  That is, the mark on the forearm will be on the near side of the control.  Occasionally the arm will be extended far enough that the mark on the forearm will be found on the far side of the interface point.  If it is possible to make such measurements, the values are recorded and analyzed as negative values.  Record results on the data forms shown in the below hyper-links.

GO TO FIGURE to data form for left handed reaches, left and right handed reaches and for right handed reaches. 

GO TO FIGURE for left and right handed reaches. 

GO TO FIGURE for right handed reaches. 

            7.  Steps 5 and 6 are repeated for left hand Zone 2 reach - that is, without containing the shoulder and allowing the subject to lunge forward against the restraint system.  The subject should not be expected to obtain absolute maximum reach.

            8.  After all Zone 2 measurements have been made, those potentially reachable only under Zone 3 restraint are measured.  For Zone 3 measurements the harness is unlocked.  The subject, in attempting to reach the control, is permitted to lunge or lean the torso in the direction of the control to a comfortable maximum permitted by the unlocked restraint system.  

ANALYSIS AND RESULTS

Analysis of reach data requires the use of both left and right Thumbtip  Reaches, the "X" to hand interface measurements for both hands, "X" to control interface  measurements, Sitting Shoulder Height, and seat position.  Examples of left and right Thumbtip Reaches and "X" to hand interface measurements can be found in the first table below: examples of "X" to control "F" type interface in the second. 

SAMPLE VALUES FOR THUMBTIP REACH AND "X-TO-INTERFACE" POINTS ON THE HAND.

THUMBTIP REACH (R) ___29.0"___ , (L) ___29.0"___ 

LEFT X TO G _3_5/8"_ , X TO F _8_3/8"_ , X TO H ___7"___ , X TO T ___6"___

RIGHT X TO G __4"___ , X TO F _8_7/8"_ , X TO H _7_1/2"_ , X TO T _6_1/4"_

- - - - - - - - - -

SAMPLE DATA FOR ONE SUBJECT REACHING WITH EACH HAND TO AN "F" TYPE CONTROL. 

CONTROL ____________________

 

 

EQUIVALENT

(LOCUS)
      (HAND)  
             (MODE)

MEASUREMENTS

THUMBTIP REACH

 

 

 

 

 

ZONE 1 ____33.5"____

 

(MIP)( L )( F)       

 

Z-1 __12_7/8"__, Z-2 __8_5/8"___

 

ZONE 2 ____29.3"____

 

                               

 

Z-3 ____7_____

 

ZONE 3 ____27.6_____

 

 

                                                - - - - - - - - - -

 

 

 

 

ZONE 1 ____31.6"____

 

(MIP)( R )( F ) 

 

Z-1 __11_1/2"__, Z-2 __7_1/2"___

 

ZONE 2 ____27.6"____

 

                    

 

Z-3 ____5.5_____

 

ZONE 3 ____25.6_____

Analysis proceeds as follows.  The distance from "X" to the control minus the distance from "X" to the interface point on the hand is  equal to the distance between the interface point on the hand to the control.  If the "X"-to-control distance is greater than the "X" to hand interface distance, the subject cannot be expected to reach the control under the specified reach  conditions.  If less, the subject should be able to reach to and likely beyond the control.

Using the sample subject whose data are shown in these first two tables above, the values for the minimum Equivalent Thumbtip Reaches necessary to access this  control under Zones 1 and 2 restraint are calculated in a straightforward manner as follows.  The subject's Thumbtip Reaches, both left and right, were measured at 29" each.  The distance from "X" to fingertip was 8 3/8 inches for the left arm.  The distance from "X" to the control interface on the main instrument panel (MIP) for the left arm was 12 7/8 inches.  Therefore, this control would require a Zone 1 left hand Equivalent Thumbtip Reach equal to 29 + (12 7/8 - 8 3/8) or 33.5 inches. Equivalent Thumbtip Reach, Zone 2, equals this value (33.5) minus the  difference between Zone 1 and Zone 2, or 33.5 - (12 7/8 - 8 5/8) or 29 1/4  (29.3) inches.*  The minimum Zone 1 and 2 Equivalent Thumbtip Reaches for the right hand (given this subject's Sitting Shoulder Height and seat position) to this control were found to be 31.6 and 27.6 inches, respectively.  

* All data were rounded off to the nearest 0.1 inch. 

Reach data can be examined to correct or remove questionable data. This is a straightforward process of arranging the data by control and individual Equivalent Thumbtip Reaches. In the below table, reach data for the left hand for subjects "A" through "E" in the right cockpit of the T-37B are reported. Subject "A" had the lowest shoulder level in the cockpit and subject "E" the highest. Since higher shoulders are farther away from controls below shoulder level, we would expect an increasing progression of values for Equivalent Thumbtip Reach for those subjects when reaching to such controls. As expected, subject "A" consistently needed the least Equivalent Thumbtip Reach and subject "E" the greatest. The order of values for subjects "B" through "D," also generally reflects this pattern. Outliers and values that significantly depart from the established relationships are questioned. Decisions to eliminate values are made only after consideration of the pattern of values for controls near the one in question. Only minor departures from the expected order of values are accepted. Those that depart by an amount that is obviously misrepresentative can be altered to agree more closely with those located nearby. For controls above shoulder level, a decreasing progression of values would be expected, although a severe aft angle of seat travel can complicate this relationship.

REACH DATA SMOOTHING GRAPH FOR THE LEFT HAND, ZONE 1, RIGHT COCKPIT OF THE T-37B. Subject "A" had the lowest shoulder level in the cockpit and subject "E" the highest. For controls below shoulder level we would expect an increasing progression of values of Equivalent Thumb-Tip Reach, subject "A" reporting the smallest, subject "E" the largest.

Reach data can be analyzed in at least two ways, depending upon the purpose of the examination. If the purpose is to provide an extensive mapping of reach capability, or if values are to be used in the pilot candidate selection process, regression plots can be developed. The variables in the regression plots are Sitting Shoulder Height, Equivalent Thumbtip Reach and seat position. If regression plots are to be prepared, as many subjects as possible should be used, not so few as are presented here for the purpose of illustration. Sitting Shoulder Height/seat position combinations should be selected so as to obtain a distribution of shoulder levels within the cockpit that is representative of the smaller pilots. Therefore, if subjects at the lower end of the range for Sitting Shoulder Height are not available, they may sometimes be simulated by lowering the seat. Lowering the seat by 1 inch, therefore, would simulate a pilot with 1 inch lesser Sitting Shoulder Height than the subject in the seat. Otherwise, measurements of reach are typically made with the seat full up. Regression plots can be seen at: F-16A, T-1A, T-37, T-38, C-141A, and T-37(a).

"To eliminate the need for a regression requiring three predictive variables, we substituted the variable Span for Thumb-Tip Reach and Biacromial Breadth, and created a two variable regression using Span and Sitting Shoulder Height. For some controls, particularly those overhead or on the aft portion of the side consoles, Shoulder Height is a significant variable in the regression equations. However, most of the controls . . . are forward of the shoulder, and the height of the shoulder was not significant in the resulting equation. Therefore, most of the time, only arm span is necessary to predict reach capability." (From Zehner, G.F. and J.A. Hudson, Body Size Accommodation in USAF Aircraft, AFRL-HE-WP-TR-2002-0118, United States Air Force Research Laboratory, Human Effectiveness Directorate, Wright-Patterson AFB, OH.)

If the purpose of the evaluation is to decide compliance with a requirement for body size accommodation, it is usually sufficient to calculate average minimum Equivalent Thumbtip Reaches for each control and compare these with the anthropometric accommodation requirements detailed in the specification documents.
 

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