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STATIC KNEE, LEG AND TORSO EJECTION CLEARANCES 

PURPOSE

The purpose of these measurements is to determine the largest values of Buttock-Knee Length and body breadth, usually at the shoulders, to clear cockpit structures during ejection.  

DISCUSSION

Interference between the legs and torso and cockpit structures such as the main instrument panel, controls and structures extending aft of the instrument panel, the canopy bow, and cockpit side-sills and centerline canopy braces during ejection, is associated almost exclusively with the upper ends of the ranges for Buttock-Knee Length and Shoulder Breadth.  There can be an association between seat location and leg clearances, since seat adjustment can occur along an angle other than that of the ejection rails.  In such ejection systems, the pilot can drift fore and aft to positions of greater and lesser possible interference depending on seat adjustment.  Contact by the feet, shins and elbows is related less to torso size and more to leg and arm placement at the onset of the ejection process.

Static dimensions tell us little about leg and foot trajectories and submarining during ejection and only very crudely approximate the violence of the ejection process.  The seat and its occupant are sometimes pulled up the ejection rails to simulate the effects of knee depression and, to a minor extent, submarining.  However, this also has not been shown to be a completely adequate technique for simulating the ejection sequence, and will yield little additional information beyond that obtained with the seat in the cockpit.

Ejection clearance subjects should be near the top of the range for Buttock-Knee Length, Sitting Knee Height, and Bideltoid (Shoulder) Breadth.  

PROCEDURE

Knee Clearance

Our procedure for examining ejection knee clearance assumes that the canopy has been blown off the aircraft. It is the first choice procedure and the easier procedure to measure since it can be done with the canopy open. A through-the-canopy ejection, however, represents the worst-case.  We found that the most accurate measurements can be made directly to the forward transverse part of the canopy frame.  This does not account for shards of canopy material remaining in the canopy frame in a through-the-canopy ejection.  

            1. The subject, dressed in full flight gear, including boots, is installed in the seat.  Using an inclinometer, the thighs should be set at right angles to the ejection rails.  To do this, the feet may have to be withdrawn aft and the seat adjusted. An inclinometer can be used to set thigh angle. If the seat adjustment and ejection angles differ, the seat should be adjusted to either full-up or full-down, whichever causes the pilot to drift farthest forward.  The knees are set 12" apart - centerline to centerline.

            2.  A rigid straightedge (usually two sections of an anthropometer), equipped with an inclinometer, is held against the forward surface of the left knee (Typ.) in single cockpits (or the outboard knee in side-by-side cockpits) and held in the vertical (X-Z) fore-aft plane.  By viewing an attached inclinometer, the top end of the straightedge is adjusted forward or aft until the angle of the straight edge is equal to that of the ejection rails.

            3. Using a carpenter's retractable tape, the distance is measured perpendicular from the aft surface of the straight edge to the nearest structure or other threatening surface or edge forward of the knee, usually the canopy bow, windscreen bow, or glare shield.  The procedure is illustrated below. 

Click on FIGURE for a proposed form for recording ejection clearance data.  

            1a. Single Crew Station.  Measure the distance across the crew station between the sills at the shoulder-elbow station line forward of the seat back.

            1b. Side-by-side Crew Stations.  Measure the distance between the outboard sills across both cockpits and to the centerline of each cockpit at the shoulder-elbow station line.  Where there is a centerline longitudinal canopy brace, measure the width of the brace.  By calculation, the extent to which the centerline brace may encroach into the ejection envelopes can be determined.  Data forms to record these measurements are not included in this report.

            2. The subject, dressed in appropriate flight gear, is installed in the seat and instructed to simulate hand and arm positions appropriate for ejection.  In the case of a D-ring, the subject should grasp the D-ring with three fingers of each hand or by one whole hand with the second hand grasping the first's wrist.  In the case of side seat-mounted ejection handles, the subject should simulate a full hand grasp with the handles hinged upward.

            3. Measure the distance from the most lateral body part, the shoulder, elbow or hand to the inside of each side-sill. Clearance with a centerline canopy brace between side-by-side cockpits can be calculated.

ANALYSIS AND RESULTS

There is a direct relationship between increases in Buttock-Knee Length and reduced knee clearance with the glare shield and canopy bow during ejection.  It must be remembered, however, that this is a static measurement. While it cannot be relied upon to pinpoint true clearances associated with ejection, during which the total body is under severely dynamic loading, it is the only technique available for general use in the field.

Since Buttock-Knee Length is the body dimension most closely associated with ejection clearance, data analysis consists of adding the clearance between the knee and canopy or windscreen bow and glare shield to the subject's Buttock-Knee Length to obtain the threshold contact values.  Averages of such values were calculated.  It is understood that, ideally, there should be adequate free space between the knees, shins and feet and the windscreen bow, canopy frame and glare shield.  For most aircraft, there is insufficient information regarding the amount of space forward of the knees to account for the effects of submarining.

Ejection Clearances

Differentiation should be made between canopy-open and canopy-closed ejections.  For the former we need be concerned with the potential interference offered by the windscreen bow.  Since through-the-canopy ejection can be either the first or second alternative, clearance with the canopy frame (and canopy shards remaining in the frame) must also be considered - thus the measurement of the additional potential interference offered by the canopy bow and frame.  The maximum static Buttock-Knee Length accommodated for ejection clearance with the canopy bow in the T-37B (both cockpits) was found to be 26.4 inches.

Lateral clearance in the T-38A appears to be ample.  The T-37B has limited side-to-side clearance.  On ejection, the pilot may experience interference between the inboard hand and the centerline canopy brace.

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