711 HPW engineers design improved restraint system for mobile aircrew

  • Published
  • By John Schutte
  • Human Effectiveness Directorate
WRIGHT-PATTERSON AIR FORCE BASE, Ohio - --  A new restraint system based on the same technology that brings speeding roller coasters to a smooth stop may soon help save warfighter lives and reduce military mobile aircrew injuries. 

Engineers at the Air Force Research Laboratory's 711th Human Performance Wing, Human Effectiveness Directorate (RH) teamed with Wolf Technical Services, Inc., to develop the Universal Mobile Aircrew Restraint System, or UMARS, under a Small Business Innovative Research agreement, after Air Force statistics showed the need for an improved helicopter safety system for loadmasters, flight engineers, medics, pararescuers and gunners--all of whom must remain mobile while working in the cabin of military aircraft under unpredictable conditions. 

That Air Force Special Operations Command data pointed to slack in existing tether systems as the culprit leading to five fatalities in incidents defined as "Class A survivable crashes." 

The UMARS is a smart system that automatically self-adjusts tension level based on the severity of flight maneuvers or anticipated impacts, according to John Plaga, senior aerospace engineer with 711 HPW/RH. 

With the new restraint system, a crewmember can adjust tether length incrementally up to eight feet for moving around the cabin freely during normal flight. During a violent maneuver or crash, tension increases fluidly to soften the impact of being restrained.
 
"It's a damper, which means that the faster you try to pull the webbing out, the more force you get," explained Stuart Nightenhelser of Wolf Technical Services. "It will give you a little bit of restraint force in a benign situation but a lot of restraint force in a crash."
 
Existing military aircraft restraints consist of either a long, fixed leash or a locking inertia-based system, both with inherent shortcomings, according to Plaga. For example, the inertia-based system locks up if the reel rotates too quickly; the retraction force increases with tether length, so at full extension it may be difficult for crewmembers to stay standing; and, when unhooked from a safety harness, the webbing can retract violently into the reel like an oversized, lethal tape measure. 

Those systems were not designed for the unpredictable environment inside a military transport aircraft, Plaga said. The current leash-style tether must be constantly manually adjusted to keep it at a safe length yet allow a crewmember to do his job; if the tether is set at a long length during a mishap, the crewmember may be ejected from the aircraft. 

Either way, the force transmitted to the crewmember is great when he or she reaches the end of the line. 

"If you transmit all the load to the person without attenuating it, it's kind of like using the steering column in a car to stop you rather than an airbag," Plaga explained. "Obviously we want to get away from that."
 
In contrast, UMARS is designed specifically to meet the rigorous demands of military flight and to attenuate energy but not necessarily stop motion entirely, Plaga said. It has an adjustable maximum working length and can be used at multiple locations in an aircraft. 

"Another advantage is that when UMARS engages, you can release it with just a small pull," said Nathan Wright, associate aerospace engineer with 711 HPW/RH, similar to releasing a roll-up window blind. "With existing systems you have to retract the full tether length, which limits crewmembers' ability to quickly do their jobs." 

Personnel from the 311th Human Systems Wing at Brooks City-Base, Texas, and AFSOC at Hurlburt Field, Florida, specified the new system based on mishaps involving large helicopters such as the MH-53 used for quick troop insertions and search-and-rescue missions. They sought the human biodynamics engineering expertise at AFRL's 711 HPW/RH to select a contractor and oversee design and testing. 

Other applications for UMARS included fixed-wing aircraft such as C-130s and commercial medical transport aircraft, tall-building escape systems and fall-arrest systems for construction workers and aircraft maintenance personnel who need safety restraints while working high above the ground. 

"The 711 HPW engineers get a lot of credit for starting this project off right," Nightenhelser said. "They studied the issue, surveyed advantages and disadvantages of existing systems, and came up with an optimum set of design criteria." 

Wright said flight certification test plans are ready, including tests for effects of vibration and salt fog. Tests so far have been for fit and function and to receive usability feedback from aircrew. 

The 445th Airlift Wing at Wright-Patterson, which flies the large C-5 transport aircraft, has shown interest in conducting in-flight tests once the system obtains interim safe-to-fly approval, Wright said.