Performance guarantees will be critical for eVTOL market adoption

By Alex Scerri | October 2, 2020

Estimated reading time 10 minutes, 52 seconds.

2020 is proving to be a fairly good year for eVTOL. COVID-19 does not seem to have stifled progress nor cooled investor sentiment as cash inflows continue notwithstanding the gloomy mood afflicting the wider aviation ecosystem.

EHang eVTOL deliveries
China’s EHang has already sold a number of its autonomous aerial vehicles to customers. EHang Image

Airframers at the more advanced design and production stages are already reporting some sales for their aircraft. Others are also fast approaching the moment when they will be signing sales and purchase agreements with their prospective customers.

This brings us to the intriguing subject of establishing warranties and performance guarantees. Anyone who has ever been involved in this process will vouch that this is one of the more demanding tasks that the technical and legal departments of a company will face as a team, whether on the operator or airframer side of the table.

The type of contracts signed will also depend on the operating model, e.g. if the aircraft will be purchased by a leasing company and then leased to operators, versus the latter buying the aircraft outright.

Certification: the cornerstone

One of the first clauses in such a contract would be that the aircraft is type certificated. The operator must also ensure that the manufacturer will obtain a type certificate from the regulatory authority in the jurisdiction where the aircraft will be operated, if different from that of manufacture.

With mainstream aircraft, this is fairly straightforward as the certification specification is already in place, but with eVTOL, the fact that the rules are still a work in progress will invariably add some complexity to the agreement. An example is who would bear the cost of design modifications if any regulatory changes come into effect between the signing of the agreement and delivery of the aircraft.

Although certification is indeed the cornerstone, aircraft under the same certification specification will have very different performance figures as they are designed for different missions. An essential part of the sales and purchase agreement is that the operator makes sure that the aircraft will accomplish the mission it is being purchased or leased for.

Performance: the devil is in the details

The contracts can go into various degrees of detail when it comes to performance guarantees. In some cases, there might be a guarantee that the aircraft will fly between two named and specified locations, while carrying x number of passengers. That is the easy part. Among other things, the parties will need to agree on what standard weight to use for passengers and any allowance for baggage. Even weight distribution will have an impact on performance.

Volocopter eVTOL Dubai performance
A Volocopter eVTOL prototype performs a demonstration in Dubai. The environmental conditions where an aircraft is operating will greatly impact its performance. Nikolay Kazakov/Volocopter Photo

Atmospheric conditions and elevation will have a big impact on performance, too. In aviation, many such figures are specified for a standard day, which at sea level corresponds to 15 C and an atmospheric pressure of 1013.25 hPa. A higher temperature and/or a lower atmospheric pressure will have a negative impact on rotor lift and aerodynamic performance in general. If you are planning to operate the aircraft in Dubai in summer, it will not be the same as flying it in Munich in winter.

Some contracts go into further minutiae such as specifying the temperature that must be maintained inside the cabin. This is regulated by the environmental control system, which will adversely impact the performance budget if it has to work harder to maintain a comfortable temperature in the cabin when the outside conditions are very different. If the aircraft is certified for flights in icing conditions, anti-icing features could be heavy power users and their use would also need to be accounted for where required.

When determining if the aircraft can operate between the two required points, an important consideration is of course the route. While a straight line would be the quickest path in most cases, airspace structure must also be taken into consideration.

From the U.S. Federal Aviation Administration’s Urban Air Mobility (UAM) Concept of Operations document from last June, we see that fixed UAM corridors are in the pipeline. This would mean that an aircraft would probably not always be able to fly direct routings and might have to fly lower or higher than its optimum cruise altitude for that specific mission length. This would affect the endurance as well as reserves remaining on arrival. If the departure and arrival points are already at the edge of the nominal range circle, any de-optimization of the mission profile could make it unattainable. Prevalent wind history must also be considered and slower eVTOLs, such as multicopters, could be impacted significantly even for relatively short mission distances.

Performance for turbine and internal combustion engines is also dependent on atmospheric conditions but another element is the fuel used. Some contracts will even specify the fuel lower heating value and the fuel density when establishing contracted performance. For fully electric VTOL aircraft, the power source is of course the batteries. Hybrid applications will have a turbine or internal combustion engine component to supplement these, but the pure electric aircraft will need very well-thought-out contracts because of the nascent technology.

Although a lot of testing is  being done, there is still very little experience for how battery life and state of charge will respond in a real-world urban aviation application. Battery life will depend on charging algorithms, where a charge sustaining model might be more beneficial than a fast charging profile, but this must also match the mission requirements of the operator. Other factors such as ambient temperature also need to be factored in. Determining the end-of-life of the battery is also critical. Some of this may be mitigated by battery swapping, but even these units must still be able to deliver an economically viable lifetime.

Airbus Vahana eVTOL battery swap
Ground crew load batteries into the Airbus Vahana Alpha One eVTOL demonstrator. Vahana’s engineers designed its batteries to be swappable in less than 10 minutes. A^3 by Airbus Photo

Before signing on the dotted line…

Airlines define technical dispatch reliability as the percentage of flights that depart within a specified period (typically 15 minutes) of the planned departure time. Typically, airlines see figures around the 99% mark, although these may be slightly lower at a model’s entry into service. This would of course only cover those delays that arise due to technical faults. The 15-minute grace may have to be adjusted for UAM operations, but dispatch reliability would still be an important metric that may find its way into a sales and purchase agreement for eVTOL.

Unit costs for eVTOL will probably be lower than for mainstream aircraft, yet some urban operators may have sizeable fleets that will represent a considerable investment, so it is crucial that this is protected by a rock-solid agreement. This article barely scratches the surface of what needs to be considered by the technical and legal teams as then there are also very comprehensive contracts that need to be agreed upon for after-sales service, maintenance and warranties.

The manufacturer must of course also be protected. For example, if some purely operational rules in a specific jurisdiction were to change without notice, it would be unrealistic to expect that any penalties would be carried only by the airframer.

With a new technology such as eVTOL, these contracts must clearly outline what each party has promised to deliver. This will establish a sound relationship between the operators and the manufacturers that will promote trust and ensure a mutually beneficial growth of the industry.

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  1. Thank you for this very comprehensive, knowledgeable and insightful article. In fact, we need a much more realistic approach to the UAM / AAM / DAM topic. Any hype and profit-asap fantasies are counter-productive.

    There are 2 (small) weaknesses:
    – The taxi-service focus, not mentioning e.g. medical services and other urgent applications
    – The reduction on batteries as power source, not considering fuel cell – battery combinations

    Nevertheless – the best I´ve read since months. Thx again.

  2. Bring back the rigid dirigible. Slow, yes, but capable of great carrying capacity, extremely fuel efficient and somewhat labor intensive (jobs, jobs, jobs).

  3. Great information !
    I would like to sign an NDA. I have 50 landing sites in Florida. I want to be operational in 36 months. My business model is profitable. Looking for the wisdom to accomplish my goal.
    Respectfully Requested

    Elliot Kahana
    Air Taxi Services of Florida, LLC
    Global Cell

    1. Hi Michael, thanks for the comment. Certification is what takes cares of the safety aspect, hand in hand with the operational rules that will be developed.

      FAA ad EASA are approaching this differently as described in my previous article,

      Once the aircraft meets these safety objectives and the operational rules are set, insuring these aircraft won’t be too different than what is happening today but the particular urban operation will of course be a consideration.

      I agree that it is an important aspect even when looking at how to factor in insurance costs.

  4. What really triggered me to write this opinion piece for the community
    was a comment made by a eVTOL platform founder. He bragged that after a 20 second flight where his eVTOL platform rose to several hundred feet, that they had completed 10 hours of testing over the last two years. That amount is nothing in comparison to what the major rotor developers participate in. Your excellent article embellishes my point on many related topics.
    Given the fluid eVTOL landscape it’s hard to fathom how any platform developer or Uber will achieve any type of insurance underwriting at all without demonstrating a rigorous flight test paradigm that mirrors that of rotor suppliers.

  5. I am looking for hybrid electric motor system for the Hopper. The Hoppers is a new flying craft design with no exterior moving parts. The Hoppers rotor blades are enclosed making it faster and quieter than slower flying multi rotor craft with there propellers exposed to the atmosphere. If interested in learning more go to
    A helicopter is called a Chopper>Now take the C off of cHopper and you have Hopper>

  6. Great article, very insightful. Indeed operating one of these machines from DXB to an skyport somewhere in the city at 45 degrees C will be a challenge. The weight and balance issue will be intruiging to see unfold: will pax have to divulge their own weight at the time of booking? Lots to unpack here, cheers!

  7. FARPart 23 requirements state that any aircraft NOT seeking approval for flight in known icing must demonstrate the ability to conduct a safe landing after encountering inadvertent icing.
    Many of the current evtol configurations will require their rotors to be exposed to actual ice accretion testing to demonstrate the ability to produce adequate thrust levels all the way to a safe landing and in addition the ability of critical systems to withstand the associated increase in vibration levels.

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