Our analysis characterized the injury distribution for fatal helicopter crash victims and demonstrated variations in injury frequency and proportion of minor, moderate, and serious or worse injuries within each AIS body region. The face and upper extremities had a high number of total injuries, but the majority of those injuries were minor. The thorax and lower extremity body regions had the highest frequency of injuries of at least moderate severity. Our analysis also provided a more detailed injury classification of moderate or worse (AIS 2+) injuries in order to help characterize crash biomechanics and identify potential areas of enhanced safety design. Because the greater number of body regions resulted in fewer injuries per region, the detailed injury classification system could not be organized into minor, moderate, and serious or worse categories. Rather, all moderate or worse injuries were categorized into one of detailed body region categories. Moderate injuries (e.g. organ lacerations, fractures) were included with serious or worse injuries in this classification system because they contribute to injury morbidity, ability to exit a sinking helicopter, and mortality from multiple coexisting injuries. Furthermore, describing injuries in this manner can more readily identify potential engineering and safety design solutions that can improve their prevention. Among these detailed injuries, organ injuries within the thorax accounted for the greatest number of moderate or worse injuries, followed by thoracic bone, and abdominal organ injuries. Among the lower extremity injuries, lower leg injuries were most common, followed by ankle/foot, pelvis, and thigh injuries. Moderate and severe brain injuries also accounted for a relatively high proportion of total moderate or worse injuries.
Similar to our study, Taneja and Wiegmann (2003) reported that thorax/abdominal organ and lower extremity injuries were common among pilots killed in helicopter crashes, but brain, rib, and skull injuries were also very common. Research in U.S. Army and Navy helicopter crashes during 1985–2005 found that decedents had the highest frequency of injuries to the head and chest, often followed by the spine and lower extremities (Kent 2010; Mapes et al. 2008). Another study of U.S. Army helicopter accidents found differing injury distributions between decedents and survivors, with head, upper torso, lower torso, and lower extremity injuries occurring most frequently among decedents, and lower extremity, head, face, and upper extremity injuries occurring most frequently among survivors (Barth n.d.). Because data on survivors’ injuries were not available, we were unable to compare injury distributions between decedents and survivors. Nevertheless, we have demonstrated high frequency and severity of thorax, lower extremity, and head injuries among helicopter crash fatalities, consistent with other studies.
The injury distributions observed in our study contrast with the distributions often seen in airplane and motor vehicle crashes. An analysis of fatal injuries among pilots in general aviation airplane accidents found that the most common injuries sustained were fractures of the ribs, skull, facial bones, tibia, and pelvis (Weigmann & Taneja, 2003). Upper and lower extremities were the most commonly injured body regions among MVC victims with at least one AIS 2 + injury (Forman et al., 2019; Ye et al., 2015; Poplin et al., 2015). However, Mallory et al. (2017) demonstrated injury distributions similar to our analysis, with higher injury rates in the thorax than other AIS body regions for most types of MVC impacts (i.e. rear, rollover, side, frontal oblique, or frontal impacts). Additionally, a majority of fatalities were attributed to head or thorax injuries (Mallory et al., 2017). Injury distributions between similar studies can be difficult to compare because of various analysis methods, with some studies reporting the proportion of persons with specific injuries, other studies reporting the proportion of specific injuries among all injuries, and other studies reporting all injuries or injuries specifically of survivors or decedents. Different versions of AIS coding also affect comparisons between studies. Despite differences in analysis methods, thoracic organ injuries appear to be a more prominent finding in our study compared with those found in fixed wing aircraft and motor vehicle crashes.
Knowledge of injury patterns allows investigators to evaluate strategies to reduce morbidity and improve survivability from helicopter crashes. Types of injuries sustained in helicopter crashes depend on a number of factors, most notably the crash mode, speed, position, and orientation of the occupant, along with the safety features of the helicopter. However, occupants of helicopters still die in crashes that are considered survivable based on acceleration forces that are within limits of human tolerance (Dodd, 1994; Shanahan, 2004). To survive a crash, occupants must survive impact forces, remain capable of egress, and continue survival until rescue. In our analysis, not all decedents had injuries classified as critical or maximal severity, but they still died, most likely as a result of concomitant injuries or drowning. With better protective equipment, some of these decedents might have survived. Restraint technology, use of personal protective equipment, training in helicopter extrication, and standardized record keeping and documentation of circumstantial information and injury details are all areas that should be improved to decrease morbidity and mortality from helicopter accidents. The more detailed body region classification system presented in this analysis aims to identify opportunities for interventions to improve crash survivability. Specifically, restraint systems to decrease torso and lower extremity injuries, helmets to decrease head injuries, and safety equipment to prevent drownings should be evaluated for their impact on injuries and fatalities.
Appropriate restraints for the upper torso immobilize occupants and are essential for survival in a helicopter crash (Bolukbasi et al., 2011; Shanahan, 2004). A study of civilian (i.e. non-military) helicopter crashes by Coltman et al. (1985) found that only vertical impact forces exceeded thresholds that would be predicted to cause severe injuries for well-restrained occupants. These forces may be mitigated through energy attenuation to reduce impact loads through crashworthy or stroking seats, which incorporate controlled deformation in a vertical direction, cables and links in restraint systems, crushable subfloor structures, or a combination thereof (Bolukbasi et al., 2011; Coltman et al., 1985).
Injuries can also occur from direct impact with the helicopter structure or objects within the helicopter, and these contact injuries may be more important than those caused by deceleration forces in survivable crashes (Taneja and Wiegmann 2003). Taneja and Wiegmann (2003) posit that use of shoulder restraints and head protection may significantly influence the pattern of injuries in potentially survivable helicopter accidents. However, availability of these safety features is limited. Labun (2014) found that lap belts, shoulder harnesses, restraints using inertia reels, and seats frequently used by pilots were largely effective, although the same protective equipment is less available and less effective when used by cabin occupants. Modern restraint system technologies include 4- and 5-point restraint systems with locking inertia reels, shoulder harnesses, airbag restraints, and lap belts.
Airbag restraints are of particular interest given the high frequency of thoracic organ injuries seen in our analysis. Studies using crash test dummies have found that airbag restraints provide greater protection than standard restraints (Wright and Albery 2013). Airbags decrease pilot forward motion (Vadlamudi et al. 2011) and can prevent impact with objects or the helicopter structure, reduce slack in restraints, and reduce head movement (Bolukbasi et al. 2011; DOD 2009; Ford 1995). Cockpit airbag systems and belt retractors that position occupants appropriately prior to impact can reduce flailing (Bolukbasi et al. 2011). Decreased severity of head strikes was demonstrated when airbags supplemented restraint systems in U.S. Army helicopters (Alem et al. 1991). Despite these findings, airbags are not readily available in civilian (i.e. non-military) helicopters, and effective restraint systems for all occupants should become a standard in helicopter design (Bolukbasi et al., 2011; Mapes et al., 2008). Airbag restraints did not appear to be available on any aircraft in this study, based on information detailed on the accident/incident report forms, which were updated in 2013 to reflect the potential installation and deployment of inflatable restraints. Collaboration between manufacturers and industry should be fostered to accelerate the development and installation of airbags as standard equipment for helicopters. Further study of the impact of airbags on preventing thorax injuries in helicopter crashes should be considered.
Helmets can supplement restraints and improve injury and fatality rates in helicopter accidents. Crowley (1991) demonstrated that helmets significantly protect occupants from serious head injuries in survivable U.S. Army helicopter accidents. Head injury is the leading cause of injuries and fatalities on U.S. military helicopters (Bolukbasi et al. 2011) and is associated with the greatest risk of death (Kent 2010). Given the frequency of head injuries, head protection is recommended for all occupants of helicopters operated by the U.S. military (Bolukbasi 2011; DOD 2009; Mapes et al. 2008). The same level of protection should be considered for civilian operations as well. In our analysis, head injuries occurred frequently, and further research is needed to confirm our finding of a higher proportion of moderate brain injuries among drowning victims to evaluate whether this is a contributing factor in drownings. Water-based crash victims with brain injuries might be less able to extricate themselves from the helicopter and thus, more likely to drown, especially with concomitant injuries.
If restraint technology and helmets allow occupants to survive the initial impact, further survival of water-based crashes depends on the ability to evacuate the aircraft quickly. Previous research demonstrated that accidents occurring in water resulted in a high percentage of drowning-related fatalities (Bolukbasi et al. 2011; Brooks et al. 2008; CAA 2003). We analyzed injuries by drowning status to evaluate disparities in injuries between drowning and non-drowning victims. Drowning victims had fewer documented injuries compared with non-drowning victims. However, medical examiners/coroners conceivably could have provided less detailed examinations for drowning victims after the initial cause of death determination. The higher proportion of moderate brain injuries among drowning victims could be explained in part by drowning-associated brain edema, but moderate brain injuries were still more common among drowning victims after excluding brain edema. Regardless of other injuries, drowning occurred in approximately 37% of decedents in our analysis, and strategies to prevent drowning should be considered. Impact with water could allow for lower impact forces and potentially fewer injuries of less severity, but helicopter inversion and sinking contribute to the increased frequency of drowning in survivable incidents (Bolukbasi et al. 2011; Taber and McCabe 2007). Therefore, research should be prioritized on improving egress from sinking helicopters and on the impact of helmet use on preventing drownings.
In addition to difficulties from egress related to injuries, high fatality rates in water-related incidents are also related to difficulties with egress from the sudden rush of water, disorientation, difficulty in visualizing the environment, entanglement with debris, and challenges in releasing restraints and opening doors (Bolukbasi et al. 2011; Brooks et al. 2008). Helicopter underwater escape training (HUET) reduces disorientation and allows occupants to practice aircraft escape from an inverted position in the dark (Bolukbasi et al. 2011). Training is recommended for pilots and personnel who regularly fly offshore for work duties (HSAC, 2004). Some studies have reported higher survival of water-based helicopter crashes among persons receiving escape training compared with untrained persons (Cunningham 1978; Hytten 1989). However, research findings are not in agreement regarding the effectiveness of HUET, due in part to lack of training standards, the degree to which simulations reflect the real-world physical and cognitive elements of egress, skill retention, and training recency (Taber 2014; Taber and McGarr 2013). Providing information specific to previous HUET experience in accident reports would provide safety researchers with data to direct efforts. In addition to HUET training, supplemental breathing devices can provide several minutes of air while occupants exit a submerged helicopter. These devices, also called helicopter emergency egress devices, have been used successfully by the U.S. military to increase survivability for occupants in accidents where helicopters are submerged (DOD 2009; Mapes et al. 2008). Given the high number of drownings in our analysis, further research is needed on these and other strategies to prevent drowning.
In addition to consideration of more advanced restraint technologies, use of helmets, and strategies to prevent drowning, standardized record keeping could facilitate future evaluations of the impact of these safety features on injury severity and mortality from helicopter accidents. Data should be collected systematically on both survivors and decedents of helicopter crashes to evaluate patterns of injuries. Autopsies are typically performed when accidents result in pilot or worker fatalities, but documentation of nonfatal injuries sustained could be helpful in identifying patterns and intervention strategies for these injuries. During survivor interviews as part of each accident investigation, nonfatal injury results identified by occupant seating location could be obtained and included in the NTSB database. NTSB accident reports typically include recommendations to prevent future accidents, such as recommendations for requirements for avionics, enhancement of infrastructure, or increased oversight of operations. NTSB investigators have access to information from company records, including training records, interviews with witnesses and survivors, and evidence from investigations and inspections. Including this information on NTSB form 6120.1 (Pilot/Operator Aircraft Accident/Incident Report) could be beneficial in assessing the role of additional training and equipment (NTSB, 2013). Specifically, history of water survival training and use of equipment such as restraints, personal flotation devices, life rafts, supplemental breathing devices, and helmets, should be recorded. Additionally, standardized reporting of injuries is needed for autopsies, because no federal regulations exist for autopsy reporting, although practice guidelines for autopsies have been developed by national organizations (NAME 2005; Hutchins et al. 1999).
The oil and gas industry can promote the implementation of additional safety features, support research investigating the impact of these safety features on injuries and fatalities, and promote improved data collection among helicopter accidents involving oil and gas personnel. Additionally, the industry can investigate the impact of including these safety features on payload, range, cost of operations, and occupant comfort. Operations supporting the oil and gas industry in the Gulf of Mexico were selected for this study based on this highly regulated industry’s focus on safety, comprehensive surveillance, and opportunity for development of meaningful interventions. Industry best practices and guidelines for reducing risk in aviation operations have already been developed and call for higher safety standards than those required by regulation. With the adoption of satellite tracking, location and investigation of accidents occurs quickly and thoroughly. Results of this study can be used by the oil and gas industry to focus attention on aspects of accident investigations and reporting that can improve research and practice in this area. Improvements in aviation operations in the Gulf of Mexico can also be implemented by international offshore helicopter operations and helicopter operations in other industries, and thus the opportunity exists for wide dissemination of research findings and recommendations.
Further research is needed to confirm the findings and generalizability of our study, which was limited by several factors. Information was not available for all decedents on demographics, seat location of deceased passengers within the helicopter, and use of specific safety features such as seat belts or personal flotation devices. Furthermore, flight data recorders were not available or required, and thus information regarding velocity vectors and deceleration rates was unknown. Autopsy reports varied in type (i.e., internal exam vs. external view), quality, and amount of detail, thus limiting AIS coding of each injury. Nevertheless, the majority of examinations were internal (i.e., dissection of the body); only four (11.4%) examinations were external (i.e., inspection of the outside of the body). Information on survivors was not available, and thus evaluating differences in injury distributions between decedents and survivors was not possible. Sample size was small for detailed body region classifications, limiting comparisons. Finally, our results cannot be generalized to injuries in land-based helicopter crashes.