Issue 35: November/December 1986

Analysis of 39 typical accidents of the period revealed that survival could be expected in crashes in which longitudinal acceleration loads were in the 30 to 35G range with a velocity change of 60 to 70 feet per second. In the vertical direction, the acceleration loads were in the 25 to 30G range with a velocity change of 50 to 60fps.

There are significant differences in how the longitudinal and vertical loads affect the occupant. An occupant for instance can withstand a 30 to 35G longitudinal load and sustain only minor injuries as long as the severity of the secondary impact with the instrument panel or other objects is limited. Where plots wore their shoulder helmets injuries were slight, where they did not many died or were seriously injured in accidents that would have been entirely survivable had a shoulder harness been worn.

The figures tell their own story. Of the 39 typical accidents (each accident included a fatality and at least one survivor), in some 70% of the cases those involved would have survived it they had been wearing a shoulder harness. Again, 82% of those seriously injured would have benefited from wearing a shoulder harness. In America, where wearing seatbelts in cars is not compulsory, the respect held for these life-saving devices is much less than in countries, such as Australia, where seat belt wearing has been compulsory for many years. In Australia, though, it is generally felt that too many GA plots still fly without using the shoulder portion of their harness, a mistake that could cost them dearly when least they expect.

Seats were the other primary culprit. Many light aircraft simply do not have property-anchored seats. Years of wear and tear and constant resetting, far more than in a car, often lead to seats running bose in their tracks while the tracks themselves are often not designed strongly enough to accommodate otherwise survivable Gloadings that may occur in what would have been a survivable crash.

During the investigation of the accidents, many seat and restraint problems were documented. These included problems with seat strength, lack of energy absorption capability, seat feet design, inadequately designed seat pans, lack of shoulder harness installation and equipment added to the seat which inevitably weakened its structure. The NTSB felt it noteworthy that, although the number of aircraft in this study was small but representative, in almost every accident the aircraft had design features that were less than adequately crashworthy. This suggests that these problems are widespread in the GÁ fleet.

In survivable accidents, the most commonly seen injuries of a severe nature were to the head and torso. The report established that 20% of the fatally injured and 88% of the seriously wounded occupants had serious but preventable head and upper body injuries and that only 16% of the occupants were wearing shoulder harnesses. Almost all the case studies in this report showed examples of head and upper-body injuries that could have been prevented or minimized with the use of upper-body restraints. Shoulder harness use not only prevents or minimizes head and upper body injury, but the harness acts to redistribute a large portion of the crash loads to the upper body so that lap belt and pelvic loads are reduced. High lap belt loads can cause pelvic and internal injuries.

Many of the seats in the GA fleet broke or came off their tracks during survivable crashes. The report indicated that 44% of the occupied seats became detached from the aircraft structure, usually because the seat feet or legs broke or separated. In one case, a seat came loose which allowed the occupant, who had survivable injuries, to fall out of the aircraft and drown in shallow water. Had the seat remained in place, the occupant would have been held in place by the lapbell and survived.

Many of the seats separated because of small side loads to the seat coupled with the warping of the floor and seat tracks. The NTSB felt that the present side load requirements of 1. 5Gs are totally inadequate. With a misalignment of only 10 degrees, a 9G for-ward acceleration can create a side load of 1. 5G to the seat feet. Small side loads can thus result in seat separation because of seat feet spread. To combat this problem, several manufacturers have added extra structure to the seat foot area.

For warping can also cause seats to fail. Many such floor distortions were caused by the nosewheel gear deforming into the cabin floor and warping the floor pan, thus exposing the seat tracks to additional loads while the crash deceleration was in progress. As the floor warps, extra stresses can be placed on the seat feet as the seat resists changing its original shape to accommodate the floor warp.

Seatpans can also tear in a catastrophic manner causing the occupants to literally bottom out on the floor. The NTSB is now pressing for more stringent design requirements on both seat tracks, seal feet and floorplan design.

In conclusion, the NTSB report advocates the constant use of a shoulderless, the life-saving potential of this simple device is incomparable when related to the low cost involved. The report is also urging a stricter set of design requirements on the floor and seat structure in an effort to ensure that secondary failure does not turn a survivable accident into a fatality. Closer to home, do YOU wear your shoulder harness?