Discussion Panel Topic: Military Flight Helmets - Is Head Protection Still Important?

Program Chairs: R. Munson, Brooks AFB, TX; A. Eke, Farnborough, UK

Sponsor: International Association of Medical Flight Surgeon Pilots

Place: Aerospace Medical Association (AsMA) Annual Scientific Meeting - Detroit, MI - Tuesday, May 18, 1999 - 1400-1530 and 1600-1730 (double sessions).


ABSTRACTS:

HELMET TRENDS: UPSETTING THE BALANCE OF CAPABILITY AND PROTECTION

R. A. Munson. Human Systems Program Office, HSW/YASA, Brooks AFB, TX 78235-5218.

Introduction: The combat aviator’s helmet has been designed with often competing functions: mount equipment, enhance combat flying, and provide protection. The current helmet design intentionally reflects a balance of tradeoffs between those competing requirements. Developments in helmet mounted devices (HMDs) may upset this balance; increasing combat capability while degrading safety and comfort.

Methods: Review accident reports, product quality deficiency reports, helmet tests and anecdotes from the field on the USAF HGU-55/P to provide historical information on failures. Review requirements to test and qualify for flight new equipment on helmets and projected changes in the combatant population suggest problems to come.

Results: Most pilots like the HGU-55/P for it’s light weight, comfort, and performance in maneuvering flight. Accidents show that the helmet is often lost during ejections and that major head injury is not uncommon. Visors are also lost because there is no capability to lock them down. Impact protection is adequate, though less than other helmets as a tradeoff for size and weight. Stability is a problem with helmet mounted devices, and accidents suggest there is increased risk to pilots with ejection. The light thin shell cuts weight and seems to improve noise protection, but increases risk to head impacts. Increased HMD weights increase risk due to inertial loading with drogue and parachute deployment. Currently all air forces lose appreciable man-days due to soft tissue neck injuries with aircrews flying high performance jets; anecdotes from test pilots flying HMDs suggest these rates will jump. Increasing helmet size and inattention to shape makes helmets more susceptible to adverse effects of windblast. Failing to recognize that the ejection seat acts in concert with the helmeted head during ejection can lead to unexpected mechanisms of injury with HMDs. Smaller lighter occupants may result in higher head/neck injury rates with the current helmet.

Conclusion: Increasing the weight and size of helmet mounted equipment upsets the current engineering tradeoffs; future helmets with HMDs will be less safe and degrade combat flying in a tradeoff for more system capability and lethality.



WHY THE LIGHTS GO OUT: THE CURRENT UNDERSTANDING OF CONCUSSION

SM Archer. Impact Medical Technologies (IMT), Carmel, CA.

The last ten years has seen a revolution in our understanding of how the human brain is injured. Thanks to work by Thomas Genarelli and his successors at both the University of Pennsylvania and Glasgow University, we now know that concussion , the most common and potentially severe brain injury, is caused by the stretching of the nerve output fibers called axons. Thus the new term: Diffuse Axonal Injury. When axons are stretched, calcium ion transport is disrupted and nerve function ceases. Depending upon where this occurs in the brain, the result may be unconsciousness; and the duration of unconsciousness is directly related to the time it takes for the nerve axon to repair itself and re-establish calcium ion transport. Genarelli’s and other work has established that the brain is most susceptible to rapid rotational movements rather than simple linear acceleration (‘a whack on the head’) as had been thought for centuries. Further work by Margulies, etal, established that the most dangerous motion is rapid rotation followed by counter-rotation such as occurs when the head rebounds off an object. This new knowledge has profound significance for all areas of head protection: helmet design, vehicle interiors, occupant restraint systems, and so on. Understanding the mechanism of Diffuse Axonal Injury, the disruption of the nerve’s ability to generate a ‘depolarization wave’ which is the basis for it’s function, and the mechanical properties of the human brain that make it susceptible to rotational forces provide the basis for designing improved head protection. The design and testing of IMT’s Small Business Innovative Research project – the Low Lift Aviator Helmet – was aimed at preventing such injuries as well as correcting properties of the current HGU-55/P helmet that generate lift in the ejection environment and limit impact protection.



REVIEW OF HEAD IMPACT INJURY CRITERIA

L. A. Obergefell, C. E. Perry. Air Force Research Laboratory (AFRL/HEPA), Wright-Patterson Air Force Base OH 45433-7947

Head impact injury has a long history of research. A great deal of this work has been conducted for the automotive arena, because of the high incidences of head injury before the application of seat belts and air bags. The helmet industry, including sports and motorcycle helmets, has also done considerable work in understanding how to protect the head during blunt and sharp impacts. Injury criteria have been developed based on head impact forces, linear accelerations, and angular accelerations. This presentation will cover the available criteria, including the Wayne State Tolerance Curve, the Head Injury Criteria (HIC), and Maximum Strain Criteria (MSC), and review their application to the aircraft ejection environment.



A COMPARISON OF U.S. AND BRITISH HELMET TESTING METHODOLOGIES BY TESTING HELMETS AGAINST THE ALTERNATIVE STANDARD

Johnson P. A., Crowley J. S., McEntire B. J. United States Army Aeromedical Research Laboratory, Ft. Rucker Alabama.

A U.S. military helmet was tested in a British helmet test facility using the British standards for military helicopter helmets. The current British military helicopter helmet was tested in a U.S. facility using the current testing standard for U.S. military helicopter helmets. The results of these tests were interpreted and compared to show the specific relationship between the performance of a helmet and the standards it has been designed to meet. The implications of this relationship and the relevance to future standards are discussed and proposals made for setting standards which would allow wider comparison of helmets and a closer relationship of performance to purpose.



UK APPROACH TO STANDARDISATION OF AVIATION HELMET IMPACT REQUIREMENTS

A. Foreman and N. J. Eastaugh. Defence Evaluation and Research Agency.

Rationale: Surveys have shown that 40% of injuries sustained during aircraft accidents are craniofacial with some 14-20% of fatalities attributed to serious head injury. Although it has long been recognised that helmets prevent or minimise head injury, military aircrew helmets must also fulfil other functions. Recent documented cases of aircrew neck pain and discomfort have suggested that a trade-off may be necessary between impact protection and the other functions of the helmet in order to optimise the balance between protection and operational effectiveness. This has precipitated a re-examination of the protective requirements for military aircrew helmets in the UK.

Methods: At present, helmets worn by UK military aviators must pass impact tests which are similar to British Standards for Road Vehicles users’ helmets. Over the next few years, the UK aims to develop and refine military aviation helmet impact standard(s), using existing knowledge, and the results of new research which aims to reveal the underlying mechanisms of head injury and the impact hazard in the aviation environment.

Results: A consensus statement, which will provide the basis for the standard(s), has been issued by UK experts in helmet impact test methodology, standardisation, accident investigation and head injury. Deficiencies in current knowledge have also been highlighted. In addition, aircrew helmets worn in accidents are being analysed to reveal the extent and cause of the damage, and a computer model formulated to investigate head injury mechanisms.

Conclusions: Head injury is complex, and is still poorly understood after many years of research. Further work is needed to define the relationship between mechanical insult and head injury outcome. This integrated work programme aims to address this deficiency, and provide the evidence necessary to develop impact standards for future aircrew helmets.



HEAD INJURIES CAUSED BY THE HEADBOX OF THE ACES II SEAT

Maj GC Peters*, Lt Col R Munson*. USAF School of Aerospace Medicine; Human Systems Program Office, HSW/YASA, Brooks AFB, TX 78235.

Purpose: There have been several head injuries documented from ejections of the ACES II ejection seat. This project intended to document the head injuries, and if there was a tendency to cause injury, to propose a solution to the problem.

Methods: The Air Force Safety Center files on ejections were reviewed and the ejections with head injuries were documented. A project was undertaken to evaluate replacing the current headbox with a foam-padded plate. Two representative foam samples were tested against the current ACES II seat headbox to demonstrate the protective value of the adding energy absorbent materials to the headbox. A head form shape in a HGU-55P was dropped onto the materials and the resultant G forces were measured. Biostatistical analytical methods were used to measure statistical significance of all findings. Results. Head injuries were documented to have occurred in all platforms except the F-15E. The injuries varied from mild concussions to subdural hemotomas. During laboratory evaluations, it was documented that energy absorbent materials did decrease the G forces transferred to the headform.

Conclusions:The current configuration of the ACES II seat headbox tends to cause head injuries, particularly during high speed ejections. A minor modification using energy absorbing materials would potentially decrease the number and severity of future head injuries during ejections.



INTEGRATING AIRCREW PROTECTION AND MAN MOUNTED DISPLAY TECHNOLOGY – THE EUROFIGHTER SOLUTION

R. J. Croft. Royal Air Force, RAF Abbey Wood.

In order to gain any advantage, however small, over the air to air adversary, the operators of high performance agile combat aircraft are enhancing the sensory and weapons acquisition capabilities of their particular systems. The EF2000 Eurofighter head equipment requirement calls for a fully integrated, helmet mounted symbology system that is functional throughout the whole day and night mission envelope. The Eurofighter weapon system performance specification also, however, requires the helmet system to provide levels of protection for the wearer to an agreed standard. These specifications pit the very demanding physiological protection requirements expected in an item of aircrew equipment, against what were traditionally aircraft mounted avionic display technologies that will now be put into a single man-mounted and head-supported system. The integration of both display and protection capabilities into a single assembly has proven to be anything but complementary, and what may be considered as routine engineering practice to “bolt a black box” into an aircraft presents some specific and very unique man/machine integration challenges within the context of a high performance assisted escape system.



DEVELOPMENT OF A VIRTUAL HEAD-MOUNTED EQUIPMENT ANALYSIS AND DEVELOPMENT SYSTEM

C. D. White*. Biodynamics and Protection Division, Human Effectiveness Directorate, Air Force Research Laboratory (AFMC), Brooks AFB, TX.

Purpose: The ability to assess the influence of these new helmet mounted systems on head center of gravity, mass moments of inertia, line of sight, and equipment integration would greatly enhance our capabilities to evaluate and develop head mounted technologies. Three-dimensional solid models of equipment components and of the head would allow the evaluation to be performed in a virtual CAD/CAM environment before costly EMD prototypes are created. A cost-effective design, development and evaluation tool of this nature is not currently available.

Methods: To develop a Virtual Head-mounted Equipment Analysis and Development System (VHEADS), three-dimensional solid models of current equipment must be created. This will be accomplished using the Brooks AFB CAD/CAM Laboratory’s reverse engineering capabilities, a 3-D solids based computer aided design and manufacturing (CAD/CAM) package and rapid prototyping. The current HGU-55/P and the new Lightweight HGU-55/P will be reverse engineered so that changes in the two helmets can be examined. The second step in developing the VHEADS is to establish an algorithm to place the helmet on a head form in a virtual environment. Methodologies for “placing” reverse engineered solid model of the HGU-55/P helmet onto the scan of an unencumbered subject will be examined. Approaches will include a force equilibrium system of equations, a pressure minimization routine, and a displacement minimization routine. Developing a helmet fit routine on individual subjects is useful, but does not provide a long-term solution for head-mounted equipment integration issues. The feasibility of developing population virtual head form will be examined. This will allow new equipment to be examined for accommodation and helmet integration issues in a virtual environment.

Conclusion: VHEADS will provide the Air Force with the ability to design, prototype, and evaluate enhancements to existing helmet/mask systems. Additionally, it will enable the design, development, prototyping, and field evaluation of new helmet-mounted systems, helmets, and masks.

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