Objectifs

In assuming the brace position, some occupants maintained a brace position with their lower legs "tucked" backwards slightly under their seat, whilst

others placed their legs forward o f the vertical. Two positions were consequently investigated one with the lower legs 11 degrees rearward o f the vertical and the other 11 degrees forward o f the vertical. The former is depicted by the baseline model. The luggage bar forms a natural stop for the lower legs against which they were positioned. This formed an angle o f 11 degrees rearward o f the vertical. In the two dimensional model the luggage bar was excluded, as no contact was shown to occur with it anyway.

8.6.9 Knee and Lower Leg (to Front Seat) Contact Stiffness

In automotive impacts, the knee bolster, which forms part o f the instrument panel, plays an important role in reducing the forward travel o f the occupant and thus the change in velocity. This parametric study was conducted on the upright occupant, where considerable knee and tibial contact took place. The contact stiffness o f the knee panel was doubled and halved to study this effect.

8.6.10 Knee Panel Position

Investigation o f the knee panel position was made to assess the differences in injury levels on the upright occupant. The knee panel was moved 50mm forward and rearward o f the mean position.

8.7 RESULTS

The results obtained from the analysis are displayed in the tables and figures.

A summary o f maximum values is presented in Tables 15 to 17. Also presented is the Head Injury Criteria (HIC) which defines acceleration time history in a single digit. Regulations require this figure to be below 1000, which is considered to be a survivability threshold and therefore injury may occur at lower levels. This study considers femur loading to be at the limit of injury threshold at 4000N for bending and 10000N axially.

8.7.1 Brace Position Sensitivity Study

The Baseline model consisted o f belts of 13% strain at a load of lOkN, seat stiffness as shown in Appendix 3 and the front seat of type 444 construction (with a nonbreakover seatback). The model was verified by correlation with the recorded injuries o f the occupant o f 15F and with the structural damage to seats 15F and 14F. Figures 4 to 27 show the occupant kinematics and the sensitivity study associated with the analysis. Table 15 presents the peak injury levels sustained by the rear occupant.

Belt Stiffness

Figures 7 and 9 show the results o f the belt stiffness analysis. Decreasing the belt stiffness from 8% strain at lOkN to 16% strain at lOkN increases the Head Injury Criterion (HIC), Figure 7. It is also observed that both the head and thorax accelerations have increased.

Figure 8 shows the load versus percentage elongation. As the percentage elongation is increased the axial femur load is decreased, however, the vertical femur loads are increased. The pelvis load is also reduced.

Folding Seatback Breakover Stiffness

Figures 10 and 12 show the results o f the seat breakover stiffness. Increasing the stiffness causes a slight reduction in the head acceleration and consequently the HIC value. The thorax accelerations remained relatively constant, Figure 10.

Figure 11 shows the load versus seatback breakover stiffness. The belt, femur vertical and femur axial loads are largely independent o f the breakover stiffness, but pelvis loads are increased with increasing stiffness o f breakover.

Seat Pitch

Figures 13 to 15 show the results o f the seat pitch analysis. Increasing the seat pitch for the brace position reduces the HIC value, Figure 13. This also reduces the peak head acceleration. The thorax acceleration remains relatively constant.

Figure 14 shows the load versus seat pitch. It is observed that the axial and vertical femur loads are reduced together with the pelvis load. An increase in belt load is also apparent in Figure 15.

Fixed Seatback Failure Stiffness

Figures 16 to 18 show the nonbreakover seatback stiffness. Figure 16 shows the HIC and acceleration results associated with increasing the stiffness o f the seats (Type 444). It is apparent the HIC and head and thorax accelerations are reduced.

Figure 17 shows the load versus seatback nonbreakover stiffness. It is apparent that both axial and vertical loads decrease as the nonbreakover seatback stiffness is increased. This decreasing trend is also apparent with the belt loads, Figure 18.

Seat Front Cushion Angle

Figures 19 and 21 show the results o f the seat front cushion analysis. Figure 19 shows the HIC and head acceleration. The thorax acceleration decreases as the angle o f the seat base front cushion is increased.

Figure 20 shows that the axial femur load decreases as the cushion angle is increased. The vertical femur load and belt load, Figure 21, increase with an increase in the front cushion angle. The pelvis load increases as the angle is increased from 7 to 11 degrees, however, this decreases as the angle is increased from 11 to 15 degrees.

Seat Base Stiffness

Figures 22 and 24 show the results o f the seat base stiffness sensitivity study.

It is observed in Figure 22 that the HIC and head acceleration increase as the stiffness o f the seat base is increased. There is an increasing tendency in thorax acceleration as the seat base stiffness is decreased by a factor o f 2.

Figure 23 shows that the axial femur load decreases with an increase in the seat base stiffness, however, the vertical femur load decreases as the stiffness is increased. Both pelvis and belt loads increase with an increase in seat base stiffness, Figures 23 and 24 respectively.

Lower Lee Position

Figures 25 to 27 show the kinematic results o f the lower leg positioned 11 degrees forward o f the vertical. It is apparent the lower leg rotates forward and the tibia strikes the seat in front. A significant increase in injury level is obtained above the established baseline value, Table 17.

8.7.2 U pright Position Sensitivity Study

The Baseline model consists o f belts o f 13% strain at a load o f lOkN, seat stiffness as shown in Appendix 3 and front seat o f type 444 construction (with a nonbreakover seatback). Figures 28 to 62 show the sequential kinematics and the sensitivity results o f the upright position baseline analysis. A greater degree o f knee and tibia contact occurred, compared with the brace position.

Higher rotation o f the knee and ankle joints also took place as the feet struck the aircraft floor.

Flailing o f the lower legs was observed. Flailing refers to the uncontrolled behaviour o f the lower limbs, following an impact. The lower legs are thrown forwards and upwards causing hyperextension o f the knee joints. Head and pelvis penetration into adjacent surfaces were more apparent with a HIC value o f 974, and pelvis loads in excess o f 7kN. These values are on the threshold o f serious debilitating injury.

B elt Stiffness

Figures 31 to 34 show the results o f the belt stiffness study. Increasing the percentage elongation from 8% strain at lOkN to 16% strain at lOkN increases the forward translation o f the occupant. This causes an increase in both HIC and head acceleration, Figure 31. There is a tendency for the thorax acceleration to increase at 7% strain with a subsequent decrease at 16% strain.

Figure 32 shows the load versus belt percentage elongation. The pelvis load increases rapidly with an increase in elongation. The pelvis contacts the seat front bar causing the occupant to rise, thus reducing the axial femur load. The belt load, Figure 34, increases with an increase in belt elongation. Figure 33 demonstrates that the tibia and foot loads increase as the elongation of the belts are increased.

Folding Seatback Local Stiffness

Figures 35 to 38 show the results o f increasing the seatback stiffness on which the rear occupant strikes. Figure 35 shows that increasing the stiffness causes the HIC and head acceleration to increase. A slight increase in thorax acceleration is also observed.

Figure 36 shows that a marked increase in pelvis load is observed.

Figure 37 shows that as the stiffness is increased a small increase in the tibia load occurs with a significant increase in foot load.

Seat Pitch

Figures 39 to 42 show the seat pitch sensitivity study. Increasing the seat pitch causes the HIC and head acceleration results to increase. The thorax acceleration reduces slightly.

Figure 40 shows that an increase in seat pitch slightly increases the vertical femur load. It is apparent that the axial femur loads increase at 32 inch pitch and reduce at 36 inch pitch. The belt loads, Figure 42, increase with an increase in seat pitch.

Figure 41 shows the results o f the foot and tibia loads. It is observed that a significant increase in foot load is obtained. The tibia contact loads decrease as the seat pitch is increased.

Fixed Seatback Local Stiffness

Figures 43 to 46 show the results of changing the local stiffness o f the locked seatback against which the rear occupant strikes. Figure 43 shows that the overall tendency is for an increase in HIC, head and thorax acceleration as the stiffness is increased.

Figure 44 shows that the femur vertical load remains relatively constant. An increase in the axial femur load is observed with subsequent reduction, as the pelvis load increases. The belt load, Figure 46, remains relatively constant.

Figure 45 shows the result o f the tibia and foot loads. It is apparent that as the tibia load increases, the foot load decreases.

Seat Front Cushion Angle

Figures 47 to 50 show the result o f changing the front seat cushion angle from 7 degrees to 15 degrees. Figure 47 shows that as the angle is increased, the HIC and head accelerations are increased. A small reduction in thorax acceleration is observed.

Figure 48 shows the effect o f load against change in angle. It is observed that the femur vertical load reduces slightly. A more marked reduction is obtained in the axial femur load. Both the pelvis and belt loads, Figure 50, also decrease as the angle is increased.

Figure 49 demonstrates that increasing the front seat cushion angle increases the tibia load. The increase in tibia load reduces the foot load.

Seat Base Stiffness

Figures 51 to 54 show the results o f changing the seat base stiffness.

Doubling the stiffness causes the body to ride higher in the seat, consequently the head contacts the front seatback above the region covered by the food tray.

Figure 51 shows that increasing the seat base stiffness increases the HIC, head and chest accelerations at the baseline factor o f 1, however, this decreases as it approaches a factor o f 2.

Figure 52 demonstrates that increasing the seatbase stiffness reduces the axial, vertical femur loads, however, this increases the pelvis load.

Figure 53 shows the result o f the tibia and foot loads, it is observed that as the tibia load increases from a factor o f 0.5 to 1.0, the foot load decreases. This is followed by an increase in foot load and a subsequent reduction in tibia load.

Lower Leg Position

No studies were made under this heading for the upright occupant. The principal reason for investigation o f the lower leg position was to improve the kinematics in the brace position.

Knee Bolster Contact Stiffness

Figures 55 to 58 demonstrate the results of the knee bolster leg contact stiffness sensitivity study. Figure 55 shows the HIC and thorax acceleration have a tendency to increase as the contact stiffness increases.

Figure 56 shows that the vertical femur load decreases with increasing contact stiffness. The axial femur load increases to a stiffness factor o f 1.0 followed by a decrease in the load. The pelvis load decreases to a factor o f 1.0 and increases to a factor o f 2.0. The tendency o f the belt load, Figure 58, is to increase as the knee bolster contact stiffness is increased.

Figure 57 shows the results o f the tibia and foot load. Increasing the knee bolster contact stiffness increases the tibia load. Consequently this reduces the foot load.

Knee Panel Position

Figures 59 to 62 show the effect o f moving the knee bolster panel position.

Figure 59 demonstrates that the head and thorax acceleration increases by positioning the knee bolster panel 50mm forward.

Figure 60 shows the change in load versus relative bolster panel position.

Moving the bolster panel forward increases the vertical and axial femur loads.

Increasing this parameter causes the pelvis and belt loads, Figure 62, to increase.

Figure 61 demonstrates the load on the foot increases with forward knee bolster panel movement. Consequently, the tibia load decreases, as the load is transferred onto the feet.

8.8 DISCUSSION

The analysis has yielded a large amount o f data, which is relevant to the consideration o f aircraft survivability. It is necessary, however, to consider the circumstances peculiar to this accident, and the nature o f the analysis performed.