By Elan Head

An award-winning journalist, Elan is also a commercial helicopter pilot and an FAA Gold Seal flight instructor with helicopter and instrument ratings. Follow her on Twitter @elanhead


Experts highlight importance of crashworthiness in eVTOL design

Aircraft crashworthiness will be one fundamental element of assuring the overall safety of urban air mobility operations, and must be designed into eVTOLs from an early stage.

Those were two key takeaways from an eVTOL crashworthiness panel held Jan. 22 at the Vertical Flight Society’s (VFS’s) Transformative Vertical Flight 2020 conference in San Jose, California. Speakers from NASA, the Federal Aviation Administration (FAA), Uber, Bell, and Jaunt Air Mobility discussed the role that crash safety will play in urban air mobility, and some of the technical and regulatory challenges associated with protecting the occupants of future eVTOL aircraft.

NASA deployable energy absorber for eVTOL crashworthiness
Deployable energy absorbers, shown here on an MD 500 helicopter during NASA testing in 2009, could enhance the crashworthiness of certain eVTOL designs. NASA/Sean Smith Photo

“The success of eVTOL is going to very much be linked to how well we protect our public image,” pointed out Michael Smith, a long-serving engineer at Bell who is now part of the company’s Innovation team. By mitigating the consequences of otherwise catastrophic accidents, he said, a focus on crashworthiness not only “protects the occupant . . . it protects our industry.”

Ryan Naru, vehicle standards lead for Uber Elevate, described crashworthiness as part of Uber’s “layered” approach to eVTOL safety, which takes into account operational factors as well as aircraft design.

“Crashworthiness is one piece of mitigating the total fatal accidents and reducing fatal accident rates,” he said. “You’re going to have to accept the fact that accidents will occur, [so] what can we do to mitigate those accidents away from fatalities and towards a more survivable incident?”

With more than 250 models currently listed in VFS’s World eVTOL Aircraft Directory, many eVTOL developers may be more focused on simply getting their aircraft aloft than preparing for worst-case scenarios. But the panelists emphasized that early design choices will have significant downstream effects on crash safety.

For example, batteries located in the sub-floor could potentially negate some occupant protection systems, said Justin Littell, an engineer with NASA Langley Research Center’s Structural Dynamics Branch. “If you have a Kevlar honeycomb in your sub-floor and now you put a rigid battery in there, the rigid battery is going to take up the load and any kind of energy-absorbing additions you have are not going to work,” he noted.

Emphasizing the role that crashworthy structures play in protecting aircraft occupants, FAA chief scientific and technical advisor for crash dynamics Joseph Pellettiere urged eVTOL designers to give primacy to structural design, rather than counting on the safety benefits of add-on features.

“Yes, you can come up with other techniques like a deployable absorber, an airbag, a parachute, or something, but all of that is going to add complexity, weight, other aerodynamic issues,” he said. “The more space that you can show for crushable space and attenuating energy is going to allow you to size those systems differently, it’s always going to be reliable, and it’s going to be a much easier way to show that you have some protection for the occupant.”

The importance of crushable space notwithstanding, Jaunt Air Mobility chief technology officer Martin Peryea suggested that external airbags are an add-on feature that “could buy its way onto” some aircraft designs. While Jaunt’s Reduced rotor Operating Speed Aircraft (ROSA) concept has a large overhead rotor and, like helicopters, is capable of autorotation, airbags could be an especially promising solution for other eVTOL designs that can’t perform the same type of emergency landing.

“It is a challenge with these types of vehicles in terms of performance, but the fact that they don’t have large rotor systems to do autorotations or big wing systems to do a run-on landing, we have to visit some of these technologies — and external airbags is probably the answer for a lot of these aircraft,” he said.

Such external technologies are not new. Littell was part of the NASA team that tested a so-called “deployable energy absorber” on an MD 500 helicopter in December 2009. The composite honeycomb concept — which has a flexible hinge design that allows the entire structure to lie flat until deployed — successfully protected the airframe from much structural damage, and the four crash test dummies within from significant injuries.

“I’m a very big proponent for external airbags. They work well on every surface,” said Smith, although he granted that they do add about 1.5% of an aircraft’s gross weight. Like some other panelists, he was less enthusiastic about ballistic parachutes, which don’t necessarily work reliably at the low speeds and altitudes associated with VTOL operations.

While further research and development could conceivably improve the outlook for ballistic parachutes in urban air mobility, Naru said, “a significant amount of research is going to need to be done if we expect the next generation of eVTOL vehicles to include a parachute as an above-and-beyond step to mitigate these potential fatal accidents.”

Meanwhile, Uber sees next-generation energy-absorbing seats as a particularly high-priority research area for eVTOL crashworthiness, according to Naru.

“We’re looking at what would potentially be a mass-market product,” he said. “That means that there will be children flying with us; there will be people who are above the traditional 150 pounds of the anthropomorphic test dummy, and both categories of individuals will not be well protected by a stroking seat. So we need to think about . . . what is the next generation for a seat that can be adapted specifically to the occupant that is in it?”

Several panelists also called attention to the post-crash fire risks associated with the lithium batteries in many eVTOL designs.

Said Peryea, “Obviously we have to design systems that will survive a crash and not catch fire. You do have a lot of stored-up energy in the battery system, and the impact loads could ignite that battery system. And you’ve got to give the occupants enough time to leave the aircraft. . . . This is going to be a challenge.”

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  1. For just over 2 years now we have been collecting data on eVTOL development to try and anticipate what the various designs will look like, what the common features will be, crumple zones, construction techniques, what materials will be used etc in an attempt to start preparing first responders now already.

    Many valid points have been raised here and it’s encouraging to see that safety is of the utmost importance but the emergency services are not equipped yet, let alone trained, to handle the types of incidents that they could be confronted with where eVTOL craft are involved.

    The standards that ICAO, EASA, FAA and various Civil Aviation organisations require for airport and heliport emergency services or ARFF (Aerodrome Rescue & Fire Fighting) are some of the highest and these very same standards will have to be implemented in rural, municipal and metropolitan emergency services too when eVTOL craft start getting flown just about everywhere.

    The reality is that new training materials, techniques, equipment etc will have to be developed and emergency medical services will also have to rethink the way they handle patient management, extrication and identifying the different mechanisms of injury they can anticipate, very similarly to the way ARFF first responders are trained.

    No longer will it be a case of simply “putting the wet stuff on the red stuff” for Fire & Rescue services as now high voltage systems and battery isolation techniques will come in to play, new types of cutting tools that can deal with composite fibers and materials and lets not forget the ever present danger of electrocution meaning that Firefighters will have to include a high level of electrical knowledge to the already wide scope of skills they practice daily.

    We cannot afford to wait till the first few incidents involving eVTOL craft happen and statistically they will which is why we’ve been looking at solutions to some the points I’ve raised here and many many more so that injuries are minimized and risks are mitigated when having to deal with emergencies involving eVTOL craft.

    I welcome any guidance, collaboration, knowledge sharing and even constructive criticism.

  2. Delighted to see at least some people recognising the criticality of the UAM safety issue and looking at the crash-worthiness issues. We all know that accidents, or at least incidents that result in un-controlled landings of eVTOL aircraft will occur and mitigating the results by adding systems that reduce G-loadings on impact is obviously desirable.
    But why not go further, and focus on preventing the crash/un-controlled landing happening, and save the aircraft as well as the occupants? And if this can be done at about the same weight and cost as all the different crashworthiness measures, then why not? So here’s the logic.
    The obvious crash-worthiness measures of stroking seats, crushable structure and airbags are all sensible measures but they do add weight, cost and volume and probably frontal area/drag as well.
    But what scenarios do you design these measures for? Currently the US and European regulatory authorities are defining the eVTOL crashworthiness certification standards, which are being based on current helicopter standards such as a 30fps Ground Impact Velocity (GIV). That’s logical but only up to a point. This GIV is based on what is deemed to be a ‘survivable’ helicopter accident based on previous accident statistics, but it does not take into account the reality of those accidents which result in a higher GIV in which it is assumed that mostly everyone dies.
    The question should be asked as to whether this is an appropriate basis for the certification of eVTOL’s for the UAM application. Let’s consider the scenarios that might exist.
    Firstly, VTOL aircraft are especially vulnerable to any power or control issue during the VTOL phase – gravity takes over very quickly. So what height equates to a 30fps GIV? That’s about 5metres/16ft if there is complete power/lift loss, ignoring aerodynamic drag effects, not very high.
    So what is the potential worst case scenario for GIV? Let’s consider that the aircraft has a ballistic parachute, and the system uses multiple rapid-opening parachutes to reduce the current effective height from 300ft to optimistically about 30m/100ft at which the potential GIV is about 24m/s or 80fps. So currently there is a 300ft ‘Safety Gap’ possibly to be reduced to about 100ft with parachute developments, but let’s be clear, no practical crash-worthiness measures are going to prevent serious injuries and deaths at a GIV of 80fps, which clearly can, and eventually will occur with people on board unless something is done to prevent it.
    So what is the solution? The only possible solution that physics provides is the use of retro-rockets, which Elon Musk has clearly demonstrated works pretty well in returning his launch boosters back to the launch site. The situation we have with eVTOL’s is relatively simple compared to this.
    So the best potential solution is quite obvious, and is nothing novel as Soyuz capsules, moon and planetary landers etc. all use the same principles, which are well proven. The Russians have been dropping tanks and APC’s this way for many years (sometimes manned!) and the parachute retro-rocket combination has also been used for dropping heavy logistics loads by both Russia and the USA.
    So for eVTOL’s the combined use of a ballistic parachute and retro-rockets can provide a controlled landing at 1-2m/s under practically all emergency circumstances, probably at a weight and cost similar to the full suite of stroking seats, crushable structure, and airbags, so why not adopt it?
    There seems to be an inherent resistance to putting an energetic system (rocket motors and their solid propellant) on an aircraft, with the vision of long motor efflux flames reaching downwards being a distinctly negative image, especially in such as California where fires are such a big concern.
    But is this fully logical? With tons of Li ion batteries and high voltage electrics the likelihood of fire after a crash is very high anyway. The rocket launched Ballistic Recovery Systems used on Cirrus and other aircraft for nearly 20 years have never, as far as I know fired un-commanded or caused a fire hazard on the ground or after an accident. But they have saved hundreds of lives, so what is the fear and where has logic gone when it comes to eVTOL’s?
    If the retro-rockets are located between the aircraft and the parachute with a sufficiently long cables to the aircraft the efflux flames will never reach the ground, as the burn time is only about 1s and the motors burn out very shortly (much less than 1s) after the aircraft lands. At this point I have to declare an interest as MD of Active VTOL Crash Prevention Ltd. (AVCP) based in the UK, where we have been working on this concept for the last four years. The sophisticated Exploding Foil Initiator (EFI) system we use is not powered up until a few milliseconds after the emergency is declared, so cannot go off accidentally, even under lightening strikes. The system is used on air-launched missiles and torpedoes so is designed to be ultra-safe and can only initiate when it receives the correct command signal.
    Turning to the current status of the adoption of safety systems for the current eVTOL designs, the great majority of projects are currently proposing just to use system redundancy for primary safety, with no secondary safety system to prevent a crash if things go wrong. Hence the focus on crash-worthiness measures to mitigate the results of the crash. My view is that this is very short-sighted, but such as EASA seem to be prepared to go along with the risk that is inherent in this approach, and I assume that the FAA have a similar position currently. Is this sensible?
    UBER has suggested that certification credits might be given if eVTOL’s fit secondary safety systems, i.e. pure ballistic parachutes, or parachutes combined with retrorockets, but EASA has rejected this proposal, and I’m not sure how the FAA are reacting to it. To me it seems strange that organisations responsible for ensuring the safety of air vehicles are not positively encouraging the adoption of secondary safety systems which can prevent a crash, rather than staying with the conventional and limited basis of ‘survivable’ helicopter crashes.
    For me the issue is that relying on system redundancy is potentially a mirage, as several factors can easily overwhelm any level of redundancy. Multiple bird strikes, blade loss from one rotor destroying another adjacent to it, power failure, lightning strike, battery fires, and the most common causes of helicopter accidents, pilot error and poor maintenance can all make system redundancy irrelevant.
    Hence it will be interesting to see how the industry responds to these issues, and whether the combined parachute/rocket systems being developed by AVCP in the UK and ASR in the USA are adopted by the more cautious (sensible?) players.
    Roger Sloman

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