Évaluation

The three point belted seat model was strengthened to prevent any structural failure during the simulation. It is possible from this analysis to calculate loads in the seat structure and output belt forces. This would allow an assessment o f the seat strength requirements to permit the fitting o f seat back mounted shoulder belts.

The installation o f three point belt systems in wide bodied passenger aircraft may not be a feasible proposition. However, it should be stated that this is not an impossibility. The installation o f such systems would involve structural modifications with resultant cost implications to aircraft manufacturers.

Introducing this restraint method into smaller commuter aircraft is more appropriate.

neck hyperextension occurs as the seat back is not designed with a suitable head restraint.

The restraint o f the lower legs imposes a high loading in the ankle area. This is undesirable but could be easily remedied by fitting a heel plate to the luggage restraint bar. The detailed design o f the seat back and seat base would also need to be modified to prevent injury being caused to the occupant by high localised contact loads with the seat structure. An example o f this would be the transverse spar in the seat back which would result in localised loading on the spinal column. In addition, the seat back should be shaped to provide lateral restraint to the upper torso. The armrest design should be modified to prevent any risk o f abdominal injuries and to provide lateral restraint to the occupants lower torso. The latter requirement is applicable to all three configurations.

The occupant kinematics have been predicted for an occupant initially seated in an upright position. However, if an occupant was leaning forward on impact, this would cause higher loadings on the seat back and increase the forces and accelerations imposed upon the occupant. It would cause problems in restraining the head where wraparound seats and head rests are not fitted.

If significant lateral motion occurred, the head trajectory could be such that it did not contact the head restraint. The aisle side occupant could potentially end up falling towards the aisle. The fitment o f a shoulder belt which initially located the occupant in an upright position would prevent this and would also be beneficial in any accident which resulted in the occupant being accelerated towards the rear o f the aircraft, for example, on rebound. If it was necessary to only fit a lap belt, it would be desirable for breakover seat backs to be retained.

A major advantage of rearward facing seats is that seat back video screens could be more safely incorporated into the seat design. In the forward facing configurations, even with shoulder belts fitted, they would present difficulties in meeting accepted injury criteria. There is a much lower chance o f head impact with the seat back in the rearward facing configuration. Also it is highly probable that the impact velocity o f the head would be lower if contact occurred. These two factors could make the seat back video screen an seat leg design would have to be modified to suit the new direction o f loading.

As only one occupant was seated in the rearward facing simulation, the seat

attachment point loads cannot be directly compared with the other two simulated configurations.

The introduction o f rear facing seats in civil aircraft has, so far, not been widely adopted. Principally, the objections have been passenger preference and comfort, but the issue continues to be controversial. Rear facing seats have been used more widely in military aircraft. The crew of G-OBME who were seated in rear facing seats sustained only minor injuries.

Notwithstanding the controversy surrounding rear facing seats, definitive observations have been made in this study. The analysis o f the occupant kinematics shows penetration into the seatback, with neck hyperextension and tibia strike against the seat front bar. Extending the head rest and 'padding' the seat front bar would greatly reduce these phenomena.

S ig n ifican t im provem ents in injury levels are obtained with rear fa c in g seats, pa rticu la rly in H IC, fe m u r a n d belt load. Whilst reduced belt loading may be an obvious consequence o f a rear facing installation, the HIC is also favourable having been reduced significantly. It is likely that much greater improvement may be obtained in a purpose designed rear facing seat.

Increased loading on the lower leg may be readily addressed in a well designed rear facing seat.

It has previously been argued that cabin debris would be more injurious to a rear facing occupant, since this will be moving forward through the cabin.

There is substantial evidence that debris was responsible for death and injury in G-OBME, despite the use o f forward facing seats [128], Regardless of seating configuration, debris is a major hazard worthy o f improvement.

10.6 CONCLUSIONS

An aircraft impact is generally a three dimensional event, although lateral and yaw effects may be comparatively small. Two dimensional modelling often adequately represents the impact. The ability to model in three dimensions is clearly beneficial. This is o f most use when there is a significant lateral component which may change the strike zones such that a less friendly surface is contacted.

The extension from 2D into 3D shows an increased capability to model occupant interaction with an aircraft interior during an impact. The limitations imposed by utilising only 2D analysis techniques are overcome. The 3D geometry allows the occupant's motion to be fully determined. Examples o f the effects which were modelled in the analysis include, asymmetric deformation o f the seats, oblique occupant impacts with complex structures and lateral acceleration and motion of the aircraft.

The comparison o f three dimensional with two dimensional simulations is not pure, since the lateral component is, o f necessity, neglected in the two dimensional analysis. This limitation not withstanding, the levels o f correlation are generally satisfactory in terms o f injury criteria. The increased sophistication provided by three dimensional analysis gives rise to improved modelling o f the legs in particular, and their contacts with the chest and seat structure. In addition, 3D modelling is required to represent the requirements o f the 16G dynamic test.

It has proved possible to simulate the asymmetric deformation o f the seat structure and its interaction with the occupant. It is anticipated that this capability will be an increasing requirement in the future. It may be that improvements in processing power o f computers will favourably influence the availability o f finite element based analysis, rather than mechanisms based analysis. This would enhance the ability to model occupant kinematics and non-linear structural deformation simultaneously.

The analyses showed that forward facing lap belted occupants are more vulnerable to injury compared to rear facing lap belted occupants.

However, in the rear facing analysis, the lateral component combined with the low height o f the head rest cause the head and neck to impinge on an area where in reality there may be internal framework o f the seat structure. High and full width headrest would improve this situation.

Forward facing occupants restrained by lap and diagonal belts experienced similar upper body forces consistent with similar injury levels to the rear facing occupant. Although the head and neck contacts o f the rear facing seat are avoided, lap and diagonal belts for the forward facing occupant result in spinal loads close to the maximum permitted.

Additionally, considerable lower leg flailing occurs which require measures to combat injury under the seat in front.

For the NACA pulse, both rear facing seats and forward facing lap and diagonal belted seats, impose higher floor loadings than the forward facing lap belted seats. On average the percentage increase in loads are 52% for the rear facing seat and 33% for the lap and diagonal belted seat. However, it must be stated that with the exception o f the lap belted seats, the other seating in question are not purpose designed. In the analysis, both rear facing and lap and diagonal belted seats are rigid.

10.7 RECOMMENDATIONS

Analysis work should be undertaken to establish crashworthiness requirements for aircraft seat rail and floor support structures. A programme o f research should be undertaken to assess the most efficient design for impact, while optimising the mass of the floor structure.

Simulation techniques should be encouraged as a method o f identifying worst case seating configurations for subsequent certification tests.

Further research is required to establish the potential for injury using the regulatory 16G pulse.

Securing the seats to the fuselage side at the outer edge o f the seat base (a single pedestal installation) would represent a cost and mass effective method o f improving seat strength. A further study is recommended of the feasibility o f such designs.

Three point belts offer major improvements in the reduction o f head, femoral and pelvic injuries, due to improved kinematics and load distribution. Such installations should be considered for small commuter type aircraft.

R ear fa c in g seats w o u ld reduce the risk o f injury significantly. The sea t b ack m u st be strengthened a n d in creased in height, with sh apin g o f th e h ea d rest to p reven t lateral displacem ent o f th e head. A lso, a h eel p la te to p reven t rearw ard displacem ent o f the f e e t sh o u ld be incorporated. The use o f a lap belt is still required in th e event o f occu pan t reb o u n d

With the introduction o f seat back video screens the potential risk o f injury to forward facing passengers is significantly increased. The use o f rear facing seats when seat back video screens are installed should be strongly encouraged.

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11. BRACE POSITION CORRELATION WITH IMPACT