For those of us operating the 25-30,000-strong civilian transport fleet, with quiet and comfortable flight decks, the idea of wearing a helmet for extended periods would be regarded as a form of torture - although, as my modern, company-supplied, ill-fitting and rather heavy ANR (Active Noise Reduction) headset slips off my head as I change radio frequencies, there are times when I wonder! To that total, if we add the huge number of commercial and private twin- and single-engine FW aircraft, we can see it’s a relatively small proportion of the fixed-wing world that actually uses helmets.
By contrast, helicopter pilots on both sides of the military/civil divide can wear helmets (both options generally exist), although, again, within large/civilian air transport operations, most don’t. There is, however, a distinct difference when we look at the significant worldwide fleet of aircraft conducting specialised operations, such as Helicopter Emergency Medical Service (HEMS), Search and Rescue (SAR) and Police Aviation, often including Night Vision Imaging System (NVIS) operations, where helmets are mandated. Some civilian agencies, including the US Department of the Interior and United States Fore Service (DOI/USFS) have produced their own aviation helmet standards, producing a database of compliant equipment.
Perhaps unsurprisingly, where we have seen considerable activity in the development of helicopter helmets, both civilian and military, notably in support of growing demand for NVIS operations, with multiple vendors offering lightweight multi-function helmets, there has been comparatively little on the fixed-wing side of the house. Commercially, this is arguably as a result of a smaller market, but also undoubtedly due to the driving requirements of fast jet operations, where head protection during ejection represents a mighty (and expensive...) hurdle for any new design. The ubiquitous HGU-55 aircrew helmet has been in service since the 1980s with many air forces, in a number of variants, so there's definitely scope for a new design, exploiting modern, lightweight materials and with due consideration of associated head-borne technologies 'baked in'. The conflicting demands of head protection and comfort, combined with basic helmets pressed into applications they were not designed for (e.g., HGU-55/P and NVIS), have resulted in pain and increased workload for both fixed- and rotary-wing crews, with neck injuries common. On that point and acknowledging the core differences in crew workstation requirements for fixed wing and helicopters, and not least for the sake of brevity, I will concentrate on the fixed wing side for now.
As a user/sufferer of various helmets over the years, including the venerable UK Mk4, Mk10 and US HGU-55/P, including Night Vision Goggles (NVG) and attached Helmet Mounted Displays (HMD), I am genuinely excited that the fast jet world has a truly new design of basic helmet – announced last year, LIFT Airborne Technologies’ AV2.2.
It is fitting (sic!) therefore to discuss the issues with the introduction of any new helmet which seeks to meet this difficult set of requirements, including ejection and the incorporation of helmet-attached equipment, and employ the AV2.2 to illustrate how this new design addresses the issues:
The Basic Flight Helmet – long overdue evolution
Particularly for fighter aircrew, the approach to the design of the basic flight helmet has always been one of compromise and trade-off. Given the basic protection requirements for the helmet, including:
- Physical protection (impact, ejection)
- Eye protection (glare/blast visor)
- Aural protection (environmental noise)
Add the operational requirements:
- Stable and comfortable (eyeline/LoS, high G, long missions)
- Operational protections (laser, CBRN)
- Life Support (oxygen mask)
- Communications (microphone, earphones)
- Support (repair, adjustment/range, including female aircrew)
there are inevitable conflicts:
To illustrate the many tensions between safety and (operational) utility, all of the above ‘protections’ add mass, and affect the combined head/helmet Centre of Mass (CM); our neck muscles are not particularly good at dealing with significantly increased mass an/or offset loading. So, if we are going to load the head abnormally, a thorough understanding of the effects of the resultant modified total mass/balance is absolutely vital. Physiologically, it will come as no surprise that the muscles in our neck, allowing for extension, flexion and rotation, have developed to cope with no more than our own head mass at 1G – if we mess with this too much, like snap/pulling lots of G while wearing a heavy helmet and twisting to look over the shoulder (who would do that?), the result would be a potentially serious neck injury. Targeted strength training and posture guidance is a critical mitigation in this respect - just ask a Formula 1 driver about lateral G load and a fighter pilot about vertical G load; both spend a lot of time strengthening neck muscles not naturally designed to keep the head stable under such conditions. In terms of how far we can push head borne mass and CM, various studies indicate approximately 2.5 kg total mass, within a prescribed envelope of vertical/longitudinal/lateral CM, is tolerable, although I would question if the derivation of this figure is more applicable to the helicopter crash case than fast jest manoeuvre/ejection. Given this maximum helmet mass, which should be emphasised covers everything head borne (I do see things like the oxygen mask missed out occasionally) we should also be careful of CM position – generally, moving it forwards engages our stronger neck muscles, so we are better able to cope than, say, moving the centre too far aft where we are naturally weaker.
So far so good, but aren’t we already equipped with helmets far lighter than 2.5 kg? Yes, most basic helmets are around 1.8 kg, and the CM is very close to the ‘head only’ position, but that unfortunately is not the whole story, and this is where I get back to NVGs.
Why include NVG integration as part of the basic helmet? Given their universal employment in nearly all fixed wing military aircraft around the world, and their considerable (adverse) effect on some of the core helmet design aspects, particularly total mass and longitudinal/vertical CM (the employment of NVGs is a major source of aircrew neck injury and pain), we continue to procure Image Intensifying Tube (IIT)-based systems. Binocular aircrew systems, where the optical invariant conspires to deny making any significant packaging modifications to mitigate adverse mass/CM, are unlikely to evolve radically in the foreseeable future. Indeed, this ongoing love affair with analogue night vision (with some entrenched ‘opinion’ on the operational side clearly driving) is unlikely to end anytime soon despite the maturity of much lighter and better packaged digital technologies, so any new head gear should therefore mitigate the associated negative mass/balance effects if operator acceptance is expected.
So how bad is the problem for fighter pilots wearing legacy helmets? The binocular section of a F4949 (AN/AVS-9(V)) weighs around 535 g; if we add the ubiquitous banana clip mount for the current HGU-55/P helmet, including two half AA batteries, then the total is 775 g of additional mass – with a basic helmet weight of, say, 1.8 kg, we’re now up to the aforementioned 2.5 kg, with a significantly forward longitudinal CM. Adding other equipment, like display modules (Scorpion etc.) etc just makes the total weight/ forward CM problem worse.
So, if the analogue NVG is not going to develop radically; it is up to the helmet designer to mitigate their impact. In essence, we need to reduce helmet mass and move the CM of the basic helmet backwards – both will mitigate the adverse effect of adding NVGs. If we consider an operational aircrew ‘wish list’, this would top the bill IMHO, and, funnily enough, this is a very noticeable aspect of the NGFWH design:
NGFWH – LIFT AV2.2
During July 2022, the US Air Force (USAF) selected LIFT Airborne Technologies‘ AV2.2 helmet as the Next Generation Fixed Wing Helmet (NGFWH), designed to replace the 1980s vintage HGU-55/P in current US service. The new lightweight AV2.2 NGFWH addresses specific issues with the HGU-55/P, including comfort, cooling and stability, and features an integrated single visor and Night Vision Imaging System (NVIS) mount.
In late 2018, AFWERX, a Technology Directorate (TD) of the Air Force Research Laboratory (AFRL) and the innovation arm of the US Air Force, instigated a $20 million Challenge for the NGFWH prototype effort, with the entire helmet project estimated as a $400 million Air Force Life Cycle Management Centre programme requirement.
Air Combat Command (ACC) wanted “a next-generation helmet to address issues with long-term neck and back injuries, optimise aircraft technology, improve pilot longevity, and provide better fitment to diverse aircrews,” AFMC said in a June 27 2022 statement. Dissecting this statement into specifics, the current HGU-55/P helmet, particularly when installed with display devices including NVGs, is uncomfortable, difficult to stabilise, hot to wear and difficult to support, especially when paired with the aforementioned devices, including the Joint Helmet Mounted Cueing System (JHMCS) day/night modules. The new helmet is destined for all US bomber and fighter aircraft pilots, apart from F-35 pilots who will continue to fly with the F-35 Gen III Helmet Mounted Display System (HMDS).
During March/April 2023, engineers with the 46th Test Squadron and the 28th Test and Evaluation Squadron, based at Eglin AFB Florida, conducted tests to validate the performance of the NGFWH. Just down the road from Eglin, F-22A Raptor pilots from the 301st Fighter Squadron from Tyndall AFB undertook developmental flight testing. Commenting on the testing, a 301st Fighter Squadron representative was reported as stating the helmet is a “massive improvement” over the legacy head gear.
Since then, the ground test team has visited Patrick AFB and Joint Base Langley-Eustis, enabling more pilots to participate in the development effort, the HC-130J, F-15, F-16 and B-1B Lancer crews experiencing the new head gear. Some exposure to NVIS has been accomplished, and attached displays, such as the Joint Helmet Mounted Cueing System (JHMCS) and the Hybrid Optically-based Inertial Tracker (HObIT)/Scorpion HMD will be following in due course. LIFT has stated that, assuming this development effort goes according to plan, it plans to begin producing helmets in 2024, and a phased approach will be taken in the delivery of NGFWH, beginning with the F-15E Strike Eagle.
The AV2.2 specification reveals significant effort has been made to improve the lot of aircrew, with some notable features:
Helmet mass/distribution and Field of View
As can readily be appreciated when viewed from the side, the AV2.2 moves helmet mass distribution rearward, bringing the head forwards within the shell. LIFT claim the ‘slick’ (basic) helmet is 21% lighter (than legacy designs). This is a significant weight saving, especially considering the efforts made to reduce weight in the HGU-55 over time. In this respect, building on the basic lightweight HGU-55/P shell, coming in at 540 gm (per MIL-DTL-871748), then adding liner, mask mounts, NVG mount, comms etc, then comparing like-for-like (including an integrated visor), a weight of around 1.2 kg for the slick NGFWH seems about right. More important however, the claimed 22% better CM will be felt keenly by aircrew – this is assumed a combination of vertical and longitudinal CM. The thinner EPS/Koroyd impact liner in the AV2.2 has facilitated bringing the helmet shell closer to the head, moving the attached NVG CM downwards. Also, the tighter shell brings the NVG mount closer to the brow line. Assuming a nominal 25 mm eye relief (distance between the eye surface and the goggle objective lens), this brings the effective longitudinal CM slightly rearwards; combining with the previously discussed basic helmet mass distribution, the result is a significant shift of the overall longitudinal CM of the helmet/goggle combination further rearwards, which would significantly alleviate neck loads attributed to NVG.
For those who have not suffered operating with NVG, this CM alleviation, combined with a more stable fit, is significant when we consider the physiology of the head. I will discuss this in more detail later, but by attaching NVGs to the brow of an aircrew helmet, we introduced arguably the most extreme head-borne mass/balance issue for aircrew. The key point here is that, by design, our head physiology trades stability for freedom of movement – the CM of the head lies slightly forward and above the C0/C1 juncture (the top of the cervical spine and bottom of the skull). So, finely balanced, low moments (the head weighs somewhere around 4.5 kg) with, relatively speaking, weak muscles stabilising, and providing for a wide range of flexion, extension, rotation and tilt. Protective helmets generally respect this arrangement with and even mass distribution around the skull and light weight, so minimising extra work for the muscles. Move away from this finely balanced situation at your peril….
An interesting corollary of the tighter helmet outer shell line and moving the head forward in the helmet is increased Field of View (FoV). LIFT claim an increase of 2.6 degrees – doesn’t sound like much, but even modest increases in peripheral vision make a significant difference, especially when wearing NVG, where the peripheral scan is important at low level.
Stability and Comfort
Generally, these requirements are in conflict, and the level of required stability depends on the detail of the application. As implied above, a reasonably fitted basic helmet (with CM approximately coincident with the head) is unlikely to move/rotate on the head too much, even under G; however, the situation changes markedly with attached devices, such as NVG and helmet sighting systems, where relatively significant mass is now attached to the helmet, changing the moments of inertia and promoting unbalancing moments, resulting in rotation and movement of the helmet. In addition, these offending attached devices are invariably there to provide an image to the eye, so precise eye relief and a relatively small exit pupil become factors – if the wearer wants to retain the projected image, the display surface needs to remain steady in front of the eye. The upshot? To achieve a stable fit, particularly when wearing heavy NVGs, legacy helmet fit means tight….very tight, especially if significant G is going to be introduced. Personally, the integrated chin/nape strap in my HGU-55 made a significant difference; combined with a snug mask fit, this combo made the difference for me, especially as an ‘older’ pilot, my linebacker neck muscles are long a thing of the past! Rotatable mask receivers (an optional feature of HGU-55/P) also mitigates helmet rotation on the head under load. The AV2.2 features nape support/adjustment and cradles the occipital bone (the lower curved part the back of the head) to ensure stability when tightening to avoid undesired helmet rotation. All of these ensure good stability, resisting the tendency of the helmet to rotate forward when wearing NVGs – rolling into the turn, pulling some G then losing the NVIS view as the goggle shifts downwards is, to say the least, frustrating!
If the helmet needs to be a tight fit for stability, (mission-long) comfort becomes an issue; a tight-fitting helmet also brings heat build-up, especially in a fast jet with a bubble canopy. In this respect, the HGU-55 also suffers; acknowledging the issue, Gentex’ X Liner® addresses the problem, achieving a claimed 50% better heat transfer than other liners. For the NGFWH, thermal management is achieved via intake/exhaust vents and a two-piece Airmesh comfort liner, promoting airflow inside the shell and providing a wicking layer to remove sweat.
Integrated systems and peripherals
I mentioned my lovely quiet, comfortable airliner flight deck previously; it’s a different situation in a fighter cockpit - it’s a tight fit, hot and bright at altitude, you’re exposed to direct fire (including lasers) and it’s noisy. If things go badly, you could be fired out of the cockpit into a high speed and freezing windblast; after the initial seat impulse, the next hazard is drogue deployment – if you’re tumbling laterally, you really don’t want a heavy helmet when the snap comes…
Dealing with the last resort first, the integrated MIL-STD-43511D approved visor, rated to 600 KEAS provides for ejection protection, matched to seats in US fighter aircraft; however, it is interesting to note the many reductions to this figure as more data is gathered on specific systems and pilot weights. For example, in the F/A-18E/F NATOPS (yes, the one Maverick threw in the bin…), the recommended maximum ejection speed for aircrew wearing JHMCS is 400 kts and the recommended max speed for lightweight pilots is 300 kts – makes the significant cost of qualifying a helmet to 600 kts seem a bit of stretch?
As for aural protection, one of the many personal mods to my own HGU-55/P is Communication Ear plugs (CEP) – ear plugs I place in my ears as I don the helmet, helping with comm clarity and noise attenuation. Nowadays there are Active Noise Reduction (ANR) kits available for most current helmets. The benefit of a quieter helmet, quite aside from aural protection/comfort, is that directional audio and split comm (monitoring multiple radios more easily) is facilitated. The ability to precisely control relative volume of inputs, including intercom vs. radio(s) is a major source of workload reduction, especially in very noisy cockpit environments – Harrier pilots will sympathise. For the NGFWH, the AV2.2 is available with standard Passive NR, ANR or CEP as required; this flexibility is important given the differing target environments – ANR designed for turbine aircraft does not necessarily work well in a jet!
In service support
Legacy fixed-wing helmet systems, including the HGU-55, were not designed for the attachment of NVIS or HMD, including the widely fielded JHMCS. The core modifications, and life-support servicing and maintenance effort associated with HMD/NVIS integration, particularly in the context of the comfort/stability requirements, is considerable.
Sum of the Parts
The AV2.2 clearly aims to address in-service/operational issues associated with legacy helmets, and represents a significantly upgraded capability over in-service equipment. Maintaining the core protection requirements, while making significant improvements in operational implementation vs. legacy systems, including support and maintenance, makes for a powerful upgrade for fighter pilots. Taking into consideration a much wider percentile of pilots, particularly more slightly built females, also increases safety and utility across the board. Considering where we are now, and particularly in the context of the variety of missions and associated attached devices, this really is a big deal – it will not just be a case of pilots preferring the AV2.2 in terms of comfort, the bottom line for the war fighter is reduced workload – this is quantifiable, and, considering the improvements, I would be confident that crews will not just feel the benefit in terms of increased comfort, but also greater capacity for the task, be it freer head movement in close combat to optimal operation of attached devices, including NVIS.
To date, testing is going well for the NGFWH; the system design elements indicate LIFT has addressed all of the issues with current/legacy head gear, while working within all of the extant requirements for fast jet helmets, even outliers like 600 kt ejection.
Fundamental integration of NVIS and HMD, like Scorpion and JHMCS, into flight helmets will make a big difference to crews during high workload situations, with a more stable and accurate presentation of image and (particularly conformal) data at the eye. These 'add-on' technologies are increasingly indispensable for the warfighter, so the ability of the basic head gear to accommodate enhanced vision and sighting equipment without introducing excessive workload and/or increased risk of injury, is critically important. Having described the benefits to fixed-wing crews, whilst acknowledging the different environment, it is clear some of the design aspects will flow down to rotary-wing/civilian applications which employ similar technologies. Personally, despite an available SPH-4 or HGU-56, I have often utilised my own (fixed wing) HGU-55/P when conducting flight test or instructing in a helicopter, using attached Low Profile Power Pack (LPPP) plus (modest) balance weights during NVIS operation.
Helmets are increasingly not just being worn for aural or impact protection, but to use as the basis to mount ever more complex vision and display systems. Evidenced by the aforementioned introduction of limited ejection parameters, it is important for crews, engineers, acquisition personnel and industry to understand the limitations and risks of employing legacy helmet designs with these ‘add ons’, and to balance those risks against the improved fit, comfort, and long-term health benefits offered by more modern designs, such as that being offered by LIFT.
The author would like to thank LIFT for permission to use their imagery to support this article.