write:While aft facing seating will be mentioned briefly side facing seating will not be discussed. When restraining humans it is important that the restraint system design does not con

uneven sharing of loads when multiple restraints are used
the propensity of the restraint system to injure the occupant
and the ease of release of the restrain system in a post-crash environ-ment. It wasn’t too long ago that airline seats were designed to survive a static load and not a very large one at that. This made little sense as most crash loads are dynamic and can be quite high. Now
airline seats must be tested to a fairly high dynamic load and be able to withstand some side loads
too. Progress!When discussing human restraint systems the normal assumption will be that the occupant is being restrained while in the forward facing position. While aft facing seating will be mentioned briefly
side facing seating will not be discussed.When restraining humans it is important that the restraint system design does not contribute to the injuries while preventing unwanted movement of the occupant. The geometry of the restraint is critical. The lap belt in a system preventing forward movement needs to transmit loads to the portion of the body which is best able to withstand theses loads; the pelvic joint. Belts which provide restraint above the joint put excessive loads on stomach and other internal organs. Belts which provide restraint below the pelvic joint are likely to allow submarining especially during impacts with a significant vertical com-ponent. During the submarining process the occupant slides under the belt
suffering additional injuries due to both the lack of restraint and the process of being squeezed through the gap between the belt and the seat.Shoulder harnesses are important because they limit the upper body torso’s ability to rotate forward and down during decelerations. This motion should be avoided because it causes dynamic loads to the upper body which can exceed the load at the body’s center of gravity. Also
the occupant’s flailing envelope is enlarged and it increases the likelihood of head injuries which
even if not fatal or even serious in the long term
could degrade the occupant’s ability escape unassisted from the aircraft.In order to minimize back injuries
shoulder harness restraint should be applied at an angle between horizontal and approximately 25° upward. Shoulder harnesses which extend over the shoulder and then downward behind the seat may increase the likelihood of back injuries and submarining. When designed or installed in this manner
crash related tension loads on the shoulder harness place additional loads on the spine
and any negative G (upward) loads are carried through the shoulders to the spine instead of the though the pelvic joint. In addition
the ten-sion load in the harness created by a forward deceleration also creates a downward force on the torso which can increase the potential for submarining.Before leaving the lap belt and shoulder harness
two more areas need to be touched on; crotch straps and web width. Although use of a lap belt tie-down (crotch strap) is not normally seen on other than aerobatic aircraft
they can provide additional protection on any aircraft (or fast moving vehicle). The primary purpose of the this device is not to restrain the body at a point between the legs
but to resist the upward movement of the lap-belt caused by the dual action of the up-ward pulling shoulder harness and the dynamic reaction of the body as it responds to forward/vertical decelerations by attempting to submarine under the lap belt.Use of a restraint webbing with a large width is desirable from a safety perspective because it spreads the restraining load over a wider area. Nar-rower belts and harnesses are more comfortable to wear and their associated hardware is lighter. The minimum recommended widths for forward facing seats are 2.5 inches for lap belts with a desirable width of 4 inches in the center abdominal area and 2 inches for shoulder straps.When discussing restraint systems
a brief review of the coordinate axis system used by structural engineers and its relationship to crashworthiness re-quirements is in order. Crash loads generated in response to crash generated dec-larations are three dimensional vectors. Structural requirements to withstand these loads are expressed in terms of longitudinal (X or parallel to the fuselage)