eVTOL, The Case for a Bigger Bird

The Beginnings

When Orville Wright was ready to launch, looking down the stretch of sand at Kill Devil Hills that was going to be the location for that historic first flight, striking a bird or any other flying object was probably very low down on his list of concerns.

It was only almost two years later, in September 1905, when the first recorded bird strike occurred, mostly self-inflicted as Wilbur Wright was chasing a flock of birds over Huffman Prairie. It was then another seven years for the first fatality to be attributed to a bird strike, when Calbraith Perry Rodgers’ Wright Model B struck a flock of gulls near Long Beach, California, on April 3, 1912.

In the CS-25 / CFR 14 Part 25 (Large Aeroplanes) world, we nowadays tend to associate bird strikes mostly with disruptions following strikes to the engines or propellers which may lead to engine shutdowns and ensuing diversions. There have been a handful of cases over the years where multiple engines were affected, with Captain Sullenberger’s and his crews’ epic feat of airmanship at New York being the most recent and vivid in our memories. This is encroaching into the CS-E / CFR 14 Part 33 and CS-P / CFR 14 Part 35 realms that I will not go into in this article. At the moment it is unclear if engines and rotors will fall under an all-encompassing eVTOL regulation or if they will require a separate certification regimen.

Airframe strikes although significantly more numerous, rarely result in an immediate effect on a flight in large aircraft, even if some panel or composite repair work may be needed down the line. There are of course some cases which get more media coverage, where birds penetrated the flight deck, radome, etc. and even caused the loss of an aircraft and crew, but thankfully the latter are rare exceptions.

As the overly quoted maxim goes, safety is no accident, and this good performance with respect to the airframe and windshield bird strike resistance in large aircraft is the result of certification standards that have evolved over the century since Wilbur learnt that chasing birds was not a great idea.

Current Bird Strike Certification

Both CS-25 and Part 25 have a performance requirement that the airframe design must assure capability of continued safe flight and landing after the impact of a 4 lb (1.814 kg) bird at Vc at sea-level or 0·85 Vc at 2,438 m (8,000 ft), whichever is the more critical. This speed, Vc, is the design cruise speed. It is not a speed normally documented in the Aircraft Flight Manual as it is used mainly in certification, but for the purpose of this article we can assume it is the maximum speed that the aircraft is designed to fly in normal operation.

The term (Large Aeroplanes) may be a bit misleading as CS-25 / Part 25 apply to any aircraft heavier than 19,000 lb (8,618 kg) or with more than 19 seats. Most aircraft you will see at a major airport, from a commuter jet with more than 19 seat to the A380 will be under this certification specification.

Part 25 has an additional requirement that the empennage must resist an impact of a 8 lb bird at the same speed at sea level. This was introduced after a single accident in 1962 where a Vickers Viscount struck one or more Whistling Swans (average of 14 to 16 lb) causing damage to the tail and subsequent crash near Baltimore which resulted in 17 fatalities.

CS-23 / CFR 14 Part 23 (Normal-Category Aeroplanes), around which eVTOL certification standards are being moulded, are quite sparse when it comes to dealing with bird strike resistance. The performance requirement is only for the windshield and its supporting structure directly in front of the pilot to withstand, without penetration, the impact equivalent to a 2 lb bird when the speed of the aeroplane is equal to its maximum approach flap speed. This is only for Level 4 aircraft in this category, i.e. those with a maximum seating configuration of 10 to 19 passengers.

As we shall see later, speed is the main factor, so the performance requirement for Normal-Category aeroplanes is effectively much less than that for CS-25 and Part 25 aircraft. This also means that most general aviation (GA) aircraft that are not Level 4 have no performance requirements to demonstrate bird strike resistance. Unsurprisingly, a number of GA aircraft losses and fatalities have been attributed to bird strikes and while CS-25 / Part 25 aircraft report the higher rate of strikes, the rate of strikes causing damage in the latter is then much lower.

CS-27 / CFR 14 Part 27 (Small Rotorcraft) have no bird strike resistance performance requirements whatsoever. CS-29 / CFR 14 Part 29 (Large Rotorcraft) fare slightly better by having a requirement to resist a 2.2 lb (1 kg) impact at the lesser of Vh or Vne to 8,000 ft, and continue safe flight and landing for Category A, and a safe landing for Category B.

To give some perspective, a 2 lb bird is equivalent to a red-tailed hawk. On January 4, 2009, a Sikorsky S-76C++ crashed near Morgan City, Louisiana. 7 minutes after take-off, when cruising at 135 kt at 850 ft a loud bang was heard on the cockpit voice recorder. Remains of a female red-tailed hawk were found on the windshield in the wreckage. Six of the seven occupants died in the crash. 4 lb would be the equivalent to a small snow goose. A Bell 407 helicopter flying at 116 kt at 1,200 ft crashed near Stuttgart, Arkansas on November 19, 2017. Remains of snow geese were identified in the cockpit area to the first bulkhead. The pilot and two medical crew members did not survive.

The Proposal for eVTOL

EASA’s Proposed Special Condition for small-category VTOL aircraft states that for Category Enhanced, occupants must be sufficiently protected from likely bird impact. In particular, the flight crew must be able to perform duties and the passengers must be protected from serious injury. This appears to be putting more emphasis on the windshield structure than the general airframe but one could argue, in a roundabout way, that if a bird strike does not allow continued safe flight, wherever it hits, then it would not be letting ‘the flight crew perform their duties’.

However, there is no specified bird mass in this proposed Special Condition. Considering that eVTOL will be spending most of their hours below 3,000 ft AGL, where the vast majority (over 90%) of bird strikes occur, a ‘likely bird impact’ would probably be one involving birds with the same mass as those affecting CS-23 / Part 23 and CS-25 /Part 25 aircraft.

An EASA-commissioned study states that kinetic energy (KE) is a better indicator of damage probability than bird mass alone. This is what certification achieves. In practice, it fixes a maximum certificated KE value by incorporating the bird mass and the maximum speed (Vc) with the simple equation, KE = ?mv2

As we are squaring the speed, the maximum speed will have a much larger impact on the ‘certified’ maximum KE value than the mass of the bird. Impact dynamics would also depend on bird shape, density, rigidity, angle of impact, impact-surface shape and impact-surface rigidity but KE remains a good indicator.

We can see the relationship of the KE value vs. speed in Chart 1 for two bird masses, i.e. 2 lb (0.907kg) and 4 lb (1.814 kg) which are used in the current certification standards. Using the recently launched Jaunt eVTOL as an example, assuming the airframers are quoting the maximum speed, 150 mph (130 kt), this would result in a performance requirement for the design to withstand an impact energy of 2,029 Joules (J) if using the 2 lb bird mass certification value and 4,058 J for a 4 lb bird mass certification value.

We can now look on what we can do with certified maximum KE value but before returning to the eVTOL example, let us see why CS-25 / Part 25 aircraft fare so well. The maximum speed used in certification is normally much higher than for currently proposed eVTOLs. Let us take an example with a maximum speed of 340 kt. The maximum KE value of an impact with a 4 lb (1.814 kg) bird at this speed would be 27,754 J.

From Chart 2 we can easily visualize that any speed and mass combination in the green area of the chart would represent a KE less than the maximum certificated value hence, potentially decreasing the chance of damage to the structure.

Large aircraft operations are somewhat protected since in most cases when flying below 3,000 ft where most bird strikes occur, they are normally at a much lower speed mostly in the range of 140 to 220 kt. During departure, widespread use of Noise Abatement Departure Procedures (NADPs) helps to keep the aircraft relatively slow in this zone as aircraft only accelerate after passing 3,000 ft AGL. Similarly, on approach and landing ATC terminal procedures usually require speeds below 220 kt in the latter part of Standard Terminal Arrivals (STARs), and final approach speeds are in a range between 140 kt and 160 kt. The widespread use of Continuous Descent Approaches (CDAs) and decelerated approaches help to avoid flying level segments below 3,000 ft AGL.

Using the same aircraft example, i.e. with the maximum certificated impact KE of 27,754 J, if it is flying at 220 kt, this would be approximately equivalent to striking a 4.3 kg (9.5 lb) bird which the structure should survive albeit with some damage. At this point it is important to reiterate that this is only relating to the structure, not engines or rotors. Another advantage that large aircraft have is that critical systems can be segregated around different parts of the aircraft thus a strike would unlikely disable multiple critical systems. However, eVTOL, although presenting a smaller target, would probably have quite a few systems concentrated in small areas due to space limitations and these locations housing flight critical systems would need to be protected.

Another area that would need to be protected are sensors. These have been a sort of Achilles’ heel in highly automated aircraft in recent times. It is expected that eVTOLs will make extensive use of optical sensors, LiDAR etc. as they potentially move towards pilot-less operation. There must be some provision to protect the sensors e.g. by redundancy and segregation as otherwise a strike on a sensor's transparency could 'blind' the aircraft.

Now, going back to the eVTOL example with a maximum speed of 130 kt, let us assume the 4 lb standard is adopted. Seeing the excellent performance of CS-25 / Part 25 aircraft in this respect, this would not seem unreasonable.

Major airports tend to have wildlife management policies in place to control bird and other wildlife concentrations. Implementing the same in cities would somewhat dent the green credentials of eVTOL.

As per their nature, cities normally have a number of attractants which include bodies of water, parks and green areas as well as a constant food supply. Many studies show that bird populations, especially some of the larger species are showing an upward trend, so it appears that eVTOL exposure to this threat is all but inevitable.

With Chart 3 in hand we are in position to apply operational mitigation on the basis of a known certification standard.

Uber has announced that Melbourne will be the first city outside the U.S. for Uber Air operations. Anyone who has been to Australia knows that the wildlife there tends to be big. Grey-headed flying foxes are a good example even if they are not a bird. While the aim would always be to avoid hitting any wildlife by using routes to avoid their habitat etc., if one is struck it should be well within the structure’s capability up to the maximum speed as their mass is around 680 g for an adult.

However, let us say that there are reports of greater than average concentration of Australian White Ibis, where a male adult could be up to 2.5 kg. Again, while avoidance would be the primary objective, there is also an additional possible mitigation by capping the speed to 110 kt. Should the eVTOL strike such a bird, it would then still be within its certificated envelope. The lower the speed, the greater the margin. Naturally, this is a simplification as a detailed risk analysis would be needed, but it is an important tool which will not be available if no certification performance requirement is in place as a starting point.

Interestingly, the Rotorcraft Bird Strike Working Group in the U.S. noted no bird strikes were recorded with aircraft speeds less than 55 kt. This could be because these speeds are closer to speeds that birds are used to in nature such as other birds in flight, and thus they are able to detect and avoid the aircraft. Other mitigating strategies could include using lights and aircraft colour schemes that increase the aircraft’s visibility.

The effect of an impact on eVTOL motors and rotors would require a separate analysis outside the scope of this article. While multicopter configurations with ducted horizontal rotors appear to have an inherent protection with the duct doubling up as a guard, some configurations with multiple small exposed rotors appear more vulnerable. Multiple rotors should provide a level of redundancy, but you would have to think of a scenario where a large bird could affect a number of these closely spaced, bare rotors.

Alternative Compliance - Possibilities & Challenges

The most obvious and robust way to bird strike resistance certification is to strengthen the structures. This may come at a weight penalty that manufacturers may find penalizing, but there may be other options to explore.

Aircraft could have on-board LiDAR to detect large birds or also other flying objects such as UAVs. Avoiding UAVs should not be an insurmountable task provided all aircraft are using a system similar to today’s ACAS. However, an active detection system such as LiDAR could protect against rogue, or otherwise non-transponding targets in addition to wildlife. LiDAR is already in use to conduct bird population surveys, but it is yet to be seen if the system can be used for avoidance including in specific light and visibility conditions that may be encountered operationally.

Another option is to have a network of ground based avian radars that covers the eVTOLs’ area of operation. This type of radar is in use today to conduct bird tracking and migration surveys. This type of system could initially provide information on areas to avoid or when to apply operational mitigation due to high bird concentrations. It may also be possible to develop a system where the bird primary target data is processed and assigned a form of virtual transponder which could then interface with whatever anti-collision and ATM system is in use where the bird would be seen as just another transponding target. There are of course a few limitations with this and we cannot expect to track every single sparrow (24 - 40 g) in a city, but ever-increasing computational power and the data free flow bonanza promised by 5G could make it possible to think about tracking the larger birds that pose the bigger hazard. The performance of the radar itself has to be evaluated as it may be adversely affected by building echoes etc. in a close-in urban setting.

The applicants would need to demonstrate that these systems meet their assigned Function Development Assurance Level (FDAL), on par with the effectiveness of structure strengthening to achieve bird strike resistance and it is unclear if there would be such a certification pathway.

Preparing the future, today

If eVTOLs will become as ubiquitous and flying the number of hours as some projections would lead us to believe, then we should be aiming for a robust certification standard from now. Even a single accident or incident, especially in the early days of a nascent mode of transport, happening over a major city could be disastrous for the industry. Some theorize that the crash in 1977 of a helicopter on a Manhattan tower heliport was a big factor of why rooftop helicopter services have all but disappeared in the U.S.

Having a repeat with eVTOL would be unforgivable not just from the human aspect, but would also have the potential to morph into a financial calamity for all stakeholders.

Certification provides a level playing field that would ensure a common standard. If a manufacturer is not bound by certification and goes for a design with low bird strike resistance, if its aircraft are involved in any such event, the negative publicity, reputational damage and financial impact will be shared across the board, even by those airframers who invested in more robust designs.

Airworthiness standards are always a balance of acceptability and practicability.

However, there is a lot of data that points to the necessity of protecting eVTOLs from bird strikes and the acceptability must be from the point of view of the user and general public, through the regulator.

While manufacturers may initially decide to not certify aircraft for operations into icing conditions and other avoidable hazards or operating conditions, sharing the sky with birds is not optional.

Report conclusions about aircraft qualified under the existing standards almost always state that it is extremely difficult to retrofit bird strike resistance. While it is understandable that the manufacturers would want to use the existing CS-23 / Part 23 rules, EASA’s stance of having a new separate standard for eVTOL would provide a perfect avenue to introduce these requirements. The time to do it is now, when eVTOLs are either still a CAD drawing or a prototype and changes are still manageable.

If regulators and the industry do not come up with adequate regulation and means of compliance to introduce bird strike resistance, if an eVTOL is lost due to a bird strike, it will not be a ‘black swan’ event, but an unpardonable disregard of over a century of aviation safety experience.

References and Suggested Reading

• NTSB - Bird-Strike Certification Standards and Damage Mitigation
• EASA - Bird Strike Damage & Windshield Bird Strike Final Report
• EASA - Proposed Special Condition for small-category VTOL aircraft
• Rotorcraft Bird Strike Working Group Recommendations to The Aviation Rulemaking Advisory Committee (ARAC)
• Australian Aviation Wildlife Strike Statistics
• FAA : Lessons Learned From Civil Aviation Accidents
• ICAO - 2008 - 2015 Wildlife Strike Analyses (IBIS)
• The Death and Life of Helicopter Commuting (Video)
• The Wright Stories - Bird Strikes

By the same author

• The B737 MAX, a Cautionary Tale for the eVTOL industry?
• eVTOL, Revolution or Evolution?

Note: The views expressed in this article are those of the author and do not necessarily reflect the policy or position of any other agency, organization, employer or company, past or present. The information contained in this article does not constitute investment advice.