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181 APPENDIX E Human Injury Study A previous study (31) investigating the effectiveness of restrained (i.e., the body does not come in contact with any arresting systems for civil aircraft suggested a deceleration objects inside the cabin), the risk of injuries has been attrib- limit of 1 g for this type of analysis but provided no reference uted to the rapid displacement of body fluids. The rate of for this design requirement. To establish a design require- displacement, and therefore the human tolerance to the ment for the airplane with a medical basis, literature from the deceleration forces, is primarily determined by the rate of development of ejection seats, high-g maneuvers, and air- application of force acting on the fluids. The rate of applica- plane crash reconstructions were reviewed. From this review, tion of force is the third derivative of motion, called "Jerk," human injuries were related to the deceleration of the fixture with the units of g-per-second (g/sec). The literature docu- volunteers were riding. This appendix is a summary of the lit- ments that, in this category, the point of reversible incapaci- erature review. It provides insight into the human tolerance tation is 38 g when the rate of application is 1350 g/sec and to deceleration for a wide range of durations and suggests increases to 50 g when the rate of application is reduced to deceleration limits for the evaluation of arrestor systems. 500 g/sec or less (46). Reversible incapacitation is defined as momentary lack of awareness of an individual that can limit their ability to extricate themselves from the airplane if E.1. Range of Human Tolerance necessary. These symptoms can range from disorientation to to Deceleration blackout and last for just a few seconds to several days. The To obtain an evaluation criterion based on human toler- lower bound for these effects begins within the human body ance levels, reports from ejection seat tests, airplane deceler- at 10 g (45). ation tests, and aeronautical accident reconstructions were In addition to the displacement of body fluids, injuries can reviewed. The range of decelerations reviewed was from 2 g be also induced by the interaction of the body with the to 220 g, with durations from 0.002 seconds up to 115 sec- restraining device or the motion of unrestrained body regions onds. This encompasses the range of decelerations within such as the head and limbs. These injuries are similar to those which aircraft arrestor systems will most likely stop an air- observed in automotive crashes. These injuries include bruis- plane. From the literature reviewed, this deceleration range ing (46, 47), muscle strains, muscle tears, concussions (mild has been broken down, based on the duration of the deceler- traumatic brain injuries) (45), and fractures when the body ation pulse, into three broad categories as shown in Table E-1: impacts the seatbelt with force (48) or strikes an object inside (1) less than 0.2 seconds, (2) 0.2 to 3 seconds and (3) greater the cabin. In an exercise examining the injury threshold for than 3 seconds (45). Each category is also graphically repre- the occurrence of traumatic brain injuries (TBI) based on sented in Figure E-1 to illustrate the trends in the data. The idealized deceleration curves, the results suggested that following sections describe in more detail the types of injuries injuries to the brain can be induced by deceleration starting observed within each category. at 10 g for impact durations of 0.2 sec. These findings are based on the response of the average male occupant. The response can vary slightly based on the gender and age of the E.1.1. Deceleration Durations occupant. This difference is illustrated by the upper and lower Less Than 0.2 Seconds bounds in addition to age and gender. Simple items such as For decelerations less than 0.2 seconds (200 ms) in dura- foam pads on the back of the seat can also significantly change tion (Figure E-1 Region 1) and when the body is sufficiently the peak deceleration and the impact duration (49); there-
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182 Table E-1. Summary of the human response to deceleration from the literature reviewed. Decel (g) Ave t (sec) Reported Injury Cited 220 0.060 Lethal in Anesthetized Hogs (45) 125 0.060 Reversible Injury in Anesthetized Hogs (45) 70 0.130 Lung Injury in Chimp Similar to Blast Lung (45) 50 0.200 Reversible Loss of Consciousness (500 g/sec) (45) 45 0.050 Concussion (45) 38 0.200 Reversible Loss of Consciousness (1350 g/sec) (45),(55),(56) 26 0.002 Back Muscle Soreness (47) 18 0.100 Upper Limit Head and Limb Motion Can be Braked by (57) Musculature 17 0.300 Sudden Pressure to the Epigastrium and Rib Margin (55) 16 0.090 FAA Std for Peak Floor Deceleration (52) 15 -- Suggested Human Tolerance Limit (57) 14 >0.200 Lumbar Fx if Max Torso Flexion is Attained (45) 13 >0.200 Abdominal Strain (47) 12 >0.200 Where Seat Belt Rides Up (46) 10 0.002 Bruising (46), (47) 10 0.200 Hydrostatic Pressure Effects Begin (45),(56) 9 -- FAA Std for Seat Decal Limit (52) 8 0.310 Neck Muscle Soreness (46) 8 60.0 150% Loss in Respiratory Capacity (57) 6 3.0 Pilot Can Still Maneuver Plane (58) 6 60.0 15% Loss in Respiratory Capacity (57) 4 2.0 Eye and Sinus Hemorrhages (46) 4 80.0 Dimming of Vision (59) 2 115.0 Flatting of T-Wave (Serious Arrhythmia) (59) fore, great care should be exercised when using an aircraft R egion 1 R egion 2 R egion 3 deceleration of 10 g as an injury criterion by itself. In a review of aircraft accidents, Moseley (50) observed that although the aircraft's deceleration was within human tolerance limits (<10 g), injuries were still sustained. In the Moseley report, many of the injuries sustained were due to unrestrained body regions striking objects in front of them (50). One possible reason for the higher rate of injury, as reported by the Committee for Aeronautics, is that the accel- erations measured in the instrumented Anthropomorphic Test Device's (ATD) chest were 1.2 to 1.5 times the peak acceleration measured at the floor (48). Therefore, a sled deceleration of 40 g approaches the injury threshold for the chest as specified by the 3 ms clip criterion. The 3 ms clip cri- terion is an automotive injury criterion that specifies that an Figure E-1. Decelerations with documented human acceleration of the chest greater than 60 g can cause serious response. injuries (51).
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183 For emergency crash landings for which the deceleration E.2. Injury-Based Criteria for duration is typically very short, the FAA has set standards for Evaluating Arrestor Systems airplane safety. The FAA standard stipulates that the peak floor deceleration of the plane must not exceed 16 g, or have To define the human tolerance to deceleration and suggest a rate of application (jerk) greater than 178 g/sec (52). The a human tolerance limit for use in the analysis of existing and structures within the aircraft must be able to withstand the future arresting systems, literature was reviewed and an exer- forces generated by this deceleration pulse for at least 3 sec- cise examining the injury threshold for the occurrence of TBI was undertaken. The results of the literature review indicate onds. From this deceleration pulse, the standard also requires that a single injury criterion is not applicable for the full range that the peak deceleration experienced by the occupant must of deceleration durations anticipated in the evaluation of the not exceed 9 g (52). arresting systems. Therefore, a three-tiered deceleration limit for the evaluation is proposed (Figure E-1). The first is for E.1.2. Deceleration Durations decelerations with durations under one second, the second for from 0.2 to 3 Seconds decelerations with durations between one and three seconds, and the third for deceleration durations over three seconds. If the duration of the deceleration pulse is lengthened to For deceleration durations less than 1 second, the data between 0.2 and 3 seconds (Figure E-1 Region 2), a greater reviewed indicates that 9 g is a reasonable limit. For this tier, the transfer of energy to the fluid systems of the body can be risk of incapacitation or injury due to rapid displacement of expected (46). In this range, injuries reported for rapid dis- body fluids is minimized. In addition to a lower risk of incapac- placement of body fluids at 10 g now begin at 4 g due to the itation, there is a reduced risk of bodily injury. For these short longer deceleration duration (45). In this interval, the heart decelerations below 9 g, the airplane cabin will remain intact and circulatory system has time to react, and is working at and is within the design limit for the seat anchors. By remain- maximum output to counteract the effects of the decelera- ing within the design limits of the airplane structure, the poten- tion. This increased cardiac output, in some individuals tial of injury from the body striking objects inside the cabin is tested, led to an irregular, rapid, and inefficient heartbeat reduced. If the head comes into contact with the structures (tachycardia) (53). If not corrected, extreme tachycardia inside the cabin, the potential for injury is estimated to be low. cases can be life threatening (54). In addition to circulatory For the analysis of airplane deceleration between one and problems, the deceleration forces can force fluids to pool in three seconds, the peak acceptable deceleration must be body regions, exceeding the elasticity of tissues and causing reduced to 6 g. By limiting the peak deceleration to 6 g, the lit- blood vessel rupturing (45). These injuries can be observed as erature suggests that the forces placed on the body by the longer ruptures of the blood vessels in the eyes and skin. In the eval- deceleration will not affect the passengers' cognitive ability to uation of pilots' responses it was determined that pilots still extricate themselves from the cabin. had the cognitive facilities to control the plane if exposed to When the duration of the deceleration exposure is 3 seconds accelerations of 6 g for up to 3 seconds. We hypothesize that or greater, it is proposed that the criterion be further reduced passengers of an aircraft could extricate themselves from the to 4 g. The reduction should minimize the occurrence of cabin after a similar experience. reported symptoms such as dimming of vision, loss of breath, dizziness, and ultimately, for long duration deceleration, loss E.1.3. Deceleration Durations of consciousness. All these symptoms could contribute to the Greater Than 3 Seconds passenger's ability to exit the airplane after an incident. The results of this effort indicate that a tiered approach is When the duration of the deceleration exposure is 3 sec- required for the analysis of airplane arresting systems. For onds or greater (Figure E-1 Region 3), the observed injuries deceleration durations less than 1 second, the proposed injury are primarily determined by the hypoxic effects (45). The threshold is 9 g to reduce the risk of incapacitation or injury hypoxia is caused by a combination of poor blood circulation due to rapid displacement of body fluids and head impacts to and poor respiratory output. Since the deceleration occurs interior structures within the cabin, respectively. For the analy- over a period of time up to 120 seconds, the onset of symp- sis of airplane deceleration between one and three seconds, the toms will be more gradual. The initially reported symptom is peak acceptable deceleration proposed is 6 g. When the dura- dimming of vision (53), then loss of breath, dizziness and tion of the deceleration exposure is 3 seconds or greater, it is ultimately, for long duration deceleration, loss of conscious- proposed the criterion be further reduced to 4 g. By maintain- ness (55). The injury threshold for these long duration decel- ing the deceleration to these limits, passengers should have the eration pulses is between 2 and 8 g. physical and cognitive ability to safely exit the plane.