Most aviators feel pretty secure when ensconced in that expensive, fleece-covered, form-fitted cockpit seat, their bodies held firmly in place by a five-point harness. However, research has shown just how vulnerable the human body is to the severe trauma found in many aircraft crashes. And that susceptibility is leading toward some remarkable new restraint technology for helicopter pilots and, perhaps later, airplane pilots.
The math is ghoulishly actuarial. According to a U.S. Army study published in 1989, some 80 percent of all the Army’s helicopter crashes are survivable. Thirty percent of all fatalities occurring in those crashes were preventable. Fifty percent of those crashes involved fatal injuries to the head and neck. Therefore, a full 15 percent of all Army helo fatalities could be prevented if a suitable system for avoiding head and neck trauma could be developed.
In tests conducted at Wayne State University in Detroit, human cadavers were strapped into crashworthy stroking aircraft seats via standard five-point harnesses and subjected to severe, yet “survivable” crashes. Contrary to the popular belief that the shoulder strap component of the five-point harness would hold the subject in place, the bodies flailed about dramatically, with severe head and upper torso injuries prevailing.
In a living subject, such flailing would have resulted in serious injury or death due to secondary strikes on cockpit structure, flight controls, instrument panel or glareshield. These also well demonstrated the human torso’s propensity for dramatic amounts of compression when subjected to severe high-g impacts. Further injuries resulted from harness webbing stretch, failure of harness inertia reels and what researchers term the human body’s “jelly-like” extrusion through harness restraints.
Tasked by the Department of Defense (DOD) with research into ways to alleviate helicopter crash trauma, Simula Safety Systems researchers correlated eight years’ worth of U.S. Army UH-60 Black Hawk and AH-64 Apache helicopter crash injury data with the Wayne State University findings. They found the real-world injuries closely mirrored the university’s experimental data. Of the 144 injuries resulting from those accidents, 93 were due to excessive motion allowed by the restraint system, the seat or the aircraft structure itself.
Simula and DOD researchers concluded the best answer to preventing and reducing these injuries was the same solution required of the automotive industry years ago–the airbag. The challenge presented to the Simula designers was the DOD’s demanding design considerations, much more demanding (although in some ways similar) to those faced by automakers.
Dubbed the cockpit airbag system (Cabs), the device had to:
• Provide protection for aviators ranging in size from what’s known as “fifth percentile female” to “95th percentile male.”
• Present no risk of physical injury in their deployment.
• Have a low risk of inadvertent deployment.
• Present no interference with cockpit controls, either when stowed or during and following a deployment.
• Provide a high tolerance for pilots out of position in their seats.
• Maintain a high level of reliability, even in severe military, thermal, vibration and electromagnetic environments.
• Have an extensive self-test capability.
• Have a low maintenance and life-cycle cost.
• Be deployable without dependency on aircraft power and without batteries.
• Present no obstacle to aircraft egress on land or in water.
• Have a high-speed crash data recording capability.
Presented with those daunting demands, Simula came up with a Cabs system the DOD liked so much it won a contract for production of 490 Cabs systems for installation in U.S. Army UH-60A/L Sikorsky Black Hawk helicopters. The 20-month production deal is worth $11.1 million and is seen as an opportunity for groundbreaking operational validation of the concept.
Cabs consists of forward and lateral airbags for each pilot controlled by an electronic crash sensor unit (ECSU) capable of detecting a crash in three axes. Unlike automotive airbags, which inflate then deflate almost immediately, Cabs stays inflated for three seconds to provide protection throughout an extended crash sequence.
The ECSU is another important difference between Cabs and the system found in automobiles. Car airbag sensor units are set to react in just a single axis. The ECSU is a three-axis, solid-state, microprocessor-controlled, fully programmable sensor. It continually monitors aircraft accelerations and velocity changes and compares them with preset deployment parameters. The unit also works like a kind of specialized flight data recorder, storing acceleration/time data at repeated 40-sec intervals before a crash and for 20 sec after impact. Simula said the ECSU is suitable for use in both rotary- and fixed-wing aircraft and can be programmed for specific models and types.
Proper programming of the ECSU is its own special challenge. If the direction of the crash is primarily vertical, the shock-absorbing characteristics of, in the Black Hawk’s case, hydraulic helicopter landing gear must be accounted for if the system is to understand the difference between a hard landing and a crash. As Simula test and certification engineer Brett Burgeles put it, “Cabs should not deploy during the stroking of the gear, because that situation could be just a hard landing, rather than a crash. Therefore the Cabs deployment is delayed, meaning that the five-point seat restraint system is fully loaded before Cabs can deploy. So the system must react intelligently and quickly.”
Cabs installation on the Black Hawk requires minor modifications to the helicopter. The ECSU is riveted to a small aluminum tray mounted in the copilot’s (in the case of most helicopters, the left seat) seat well. Pilot and copilot lateral airbag modules are strap-bolted onto an existing piece of armor that’s already part of the doorframe assembly. The forward airbag requires the most installation time since it requires removal of the glareshield. That structure must be removed from the aircraft and cut to accommodate the airbag modules. Fiberglass reinforcements are then riveted around the cutout areas and additional aluminum angles are installed to support the glareshield.
The last step is installation of the control wiring and its routing between a new circuit breaker, aircraft power, the ECSU and each airbag. Simula estimates total installation time to be 5.7 hr.
As evidence of the military propensity for designing and testing equipment in environments in which human beings themselves could probably not operate, Cabs was designed to be fully functional between the temperatures of -25 deg F to 160 deg F and capable of operation after exposure to between -65 deg F and 230 deg F. The system passed qualification tests that exposed it to salt, fog, solar radiation, flight vibration, weapons fire vibration, sand and dust, fungus and “the combined effects of temperature, altitude, humidity and explosive atmosphere.”
Inadvertent Deployment ‘No Big Deal’
Cabs becomes passive when a failure in its systems occurs, thereby rendering the possibility of inadvertent deployment to what Simula estimates to be less than one in 27 million flight hours.
Army aviators were at first concerned about being startled when, in the midst of what should already be an under way or pending emergency, two large airbags pop out of the wall next to their heads from the glareshield in front of them. To test this surprise factor, a pair of prototype Cabs installations, one on a Black Hawk, the other on a Bell OH-58D Kiowa Warrior, were deployed in flight without warning, once during a 150-ft hover, and again at 1,500 ft. Pilots flying both aircraft reported the deployments as “no big deal” and “a non-event.” The deflated but deployed airbag can be folded back out of the way.
While the funding was not available for the full-scale test crash of an instrumented Cabs-equipped helicopter, Simula did validate it in a steel-cage mockup complete with crew seats, cyclic and collective controls, instrument panel and armor. Anthropomorphic test devices (ATDs, a laboratory term for “crash-test dummies”) were dressed in the Army’s standard aviator uniform, including survival vest, handgun, survival knife, helmet, night-vision goggles, nuclear/biological/chemical gear and even chest armor plate. Thus accoutered, the ATDs in the mockup cockpit were dropped vertically in a 30-deg nose-down pitch at a rate of 50 ft/sec with an impact designed to provide a maximum deceleration of 50gs. Other tests involved a horizontal acceleration at 50 ft/sec with a 30g deceleration. DOD specifications called on Cabs to provide head and chest protection under those conditions, and it did.
Results of the tests were evaluated by engineers from the Army’s Aeromedical Research Lab, U.S. Air Force Lab, the U.S. Navy, the FAA and Simula. When they were asked afterward if they would want this system in their aircraft, 115 voted yes, five voted maybe and only two voted no.
“One Army study estimated a 15-percent reduction in fatalities for Black Hawks equipped with Cabs,” said Burgeles. “The number is higher for less crashworthy aircraft.”
The technology is easily transferred to civil aircraft, with the main obstacle securing enough money to pay for the test and certification of such a system. “We’ve been in conversations with the leading manufacturers,” said Burgeles, “and they’ve all expressed interest. The market is there; it’s just a question of cost.
“The challenge of selling OEMs on aircraft airbag systems is similar to that in the 1980s when the issue of automotive airbags was being debated. While car companies had developed much of the technology, they later balked at having it required by law, saying the costs were too high and the consumer wouldn’t want to pay for it. But Congress acted and now the figures show that some 4,000 lives are saved by them every year.”