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MAXIMUM SITTING HEIGHT ACCOMMODATION

 
PURPOSE

The purpose of these measurements is to determine the threshold value for Sitting Height at which the bare head contacts the underside of the canopy or other overhead.

DISCUSSION

At least 4 subjects in the upper ranges of Sitting Height should be used to determine the maximum Sitting Height that can be accommodated under the canopy or other overhead.  The ranges of values for Sitting Height and other body dimensions can be found in the table, Anthropometric Multivariate Models, at TABLE. 

Measurements should be made parallel to the angle of seat adjustment.  Accuracy can be difficult if that part of the canopy or overhead immediately above the head varies much from the horizontal.  For this reason, measurements are usually made with the seat at or near full-up.  

PROCEDURE

              1.  Subjects representative of the largest Sitting Height expected to be accommodated should be used as subjects.  The subject is installed into the aircraft seat.  The harnesses are cinched up and the lab belt is buckled.  The seat should initially be full-down.

              2.  The canopy is then closed.  The subject raises the seat to the top of its range, or until the head contacts the underside of the canopy.  The subject nods his head forward and back, and side to side to be sure there is clearance.  If the overhead interferes with head motion, the seat is lowered until there is no interference.  The distance, if any, above the head is measured.  In the latter case, the seat is considered to be the highest it should be raised for this subject - bare headed.

              3.  The space between the head and the underside of the canopy can be measured either with a carpenter’s retractable tape or with soft, non-abrasive, measuring pads, depending upon the distance.  If measured with a tape, it must be oriented parallel to the seat adjustment trajectory. An assistant can provide hand signals to the subject from in front of the aircraft to assure that the head is held vertically.  The measurer can do the same from the side to be sure that the head is oriented in the Frankfurt Plane.  In side-by-side cockpits and flight decks clearance measurements can usually be made by the investigator from the other cockpit.  

Click on FIGURE for a data form* for recording maximum Sitting Height accommodation with vertically adjusting seats.  

*This and all other data forms are available by hyper-link. They are not included in the text.

              4.  For non-ejection seats adjusting in both vertical and fore-aft directions, examine at the full-down/full-forward, full-down/mid-forward, and full-down/full-aft seat positions.  Seats adjusting along an up-and-forward ramp should be examined at full-up and forward, midpoint, and full-down and aft positions.

ANALYSIS AND RESULTS

Because helmets are subject to periodic design changes, accommodation for Sitting Height is examined with the subject bare headed.  When applying the results, the increase in functional Sitting Height due to the helmet is subtracted from the bare head accommodation values.  For the HGU-55/P this increase is 1.5 inches.  This value, of course, may be different for other helmets.  An additional amount, ranging from the diameter of the fist to the thickness of the hand, depending upon Service policy, is also subtracted to provide adequate clearance for head and torso motion. This latter policy adds to the indecisiveness of Maximum Sitting Height accommodation.

If the subject is able to raise the seat to the full-up position without interfering with head motion, we know that the cockpit will accommodate this given Sitting Height, plus the distance the seat has been raised, plus the space above the head below that at which head motion is compromised.  The distance between the top of the head and the top of the helmet is subtracted, as is the distance from the top of the head and the canopy for the subject who encounters motion interference in the first example under Procedures.

The maximum Sitting Height accommodated in both cockpits of the T-37B is 40.0 inches.  This value allows 1.5 inches for the HGU-55/P helmet and one inch of free space clearance above the helmet.

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