eVTOL safety from an operational perspective

November 7, 2020

Electric aircraft are here, and they’re changing aviation as we know it.

The advantages are clear. We could see and feel that with our aEro 1 – the first electric aircraft that was suitable to fly regular aerobatic programmes. Many times a day, we could recharge the batteries, get in and go flying. Safer, simpler, less polluting, less noisy, and much less expensive: a flight hour in aEro 1 costs about 3 CHF in electricity, with total direct operating costs about 5% that of a conventional aerobatic plane.

I am personally very happy that more and more pilots can now experience the magic of electric flight as electric aircraft such as the Pipistrel Velis come to market. These are changing the perception of aviation: from a necessary but disliked industry to a more sustainable and efficient way to travel (for example, our aEro 1 aircraft consumes about half the energy of a Tesla Model 3 for the same distance).

Pipistrel Velis Electro

As a helicopter rescue pilot, I’m naturally interested in applying this technology to electric vertical take-off and landing (eVTOL) aircraft. And with more than 10,000 flight hours over 3,000 rescue missions, I can see that eVTOL aircraft will play an important role in these operations.

Electric propulsion technology is making vertical flight cleaner and more affordable. Many designs are being developed,  and the question of multicopter, tiltwing or tiltrotors, fans or any other propulsion system, hybrid or pure electric is a very emotional discussion. It is always accompanied by many wishes, certification requirements and many more elements where everyone has the belief he has the right concept and the right approach to the holy grail of Urban or Regional Air Mobility. All of these projects in the run advance aviation and vertical transport to become cleaner and more environmentally friendly. We are living in exciting times at the beginning of the next aviation revolution. Regardless if it is an electric aerobatic aircraft to have fun with, a cargo drone who provides goods or manned transportation in urban areas.

Safety and certification are the most important focus to get such aircraft in the air and enabling them to fly and operate commercially. For everyone in aviation – including all the new startups – safety is paramount, and for good reason. Aviation has an excellent reputation of safety, and commercial air transport is one of the safest ways to travel. No one wants to compromise this.

That is as well the goal of the certification authorities around the world. In Europe EASA defined the standard of future electric aircraft in CS 23 amendment 5 and of VTOLs in SC-VTOL. The standard is demanding: a catastrophic failure is allowed to happen no more than once in every billion flight hours. The rationale behind is that Urban and Regional air mobility will become a significant market and these aircraft will fly much more above our heads than helicopters and planes today.

Being very early in this eVTOL and electric plane industry and discussing all of these concepts thoroughly, I have never heard about the most important part of the safety chain. Looking at the accident statistics shows something interesting: only 20% are caused by mechanical failure. 80% of accidents have an operational cause, and the ratio is even more extreme if we look at near accidents and incidents. Note that I am not calling these pilot mistakes. In helicopter operations, there are often unknown situations that lead to an accident. You often get into difficult situations that are almost impossible to predict. You can prepare your flight as thoroughly as possible, but at the site of the mission things are not as you expect, and you suddenly find yourself in a situation that you were not prepared for.

Flying in the Swiss Alps

That is where training, experience and a good company culture comes in. It takes years to get your team up to be able to work at the edge of the possible and still provide a safe operation. Keep in mind that the training process for a VTOL pilot is different than for an airline pilot. In a helicopter (and also with future eVTOL aircraft) it is not economical to operate all flights with a second pilot (an experienced captain) on board. So you have to train your pilots gradually up to the required standard.

Since 80% of accidents are due to operational conditions rather than aircraft failure, we are focusing not only on type certification safety requirements, but also what we call the operational safety margin. This margin is critical for pilots to have in reserve in case things get tough.

In aircraft systems it is obviously great to have redundancy. But this helps only in failure cases, and not in normal operation. When you are out at night to save someone’s life in the mountains in bad weather you want to be able to trust your aircraft to have enough power reserves, good flying behaviour, good C of G range, and a rock solid aircraft design. In the early days of my flying career, no one would take the AS 350B1 to go for a sling loading mission – each of us preferred the good solid and powerful SA 315 B Lama. In case of an unexpected situation we called it our insurance policy. This only changed when Airbus brought in the AS 350B3. Since then, every mountain and sling loading pilot is happy to go flying with it. Why? Because of the power reserves, because the governor keeps your rotor RPM rock steady, because you have a very smooth and predictable power behaviour at the red line of your FLI – not simply because it is certified.

What does all of this have to do with tilt-wing aircraft? Well, of all of the concepts that are now in construction, this is the concept that almost a hundred pilots have been able to fly and could judge about its flight behaviour, and all of them loved the aircraft. They flew super well, had a huge performance reserve, a wide transition corridor, and great CofG reserves and safety margins in case of a propeller failure during transition.

Why does it fly so super well? Because the wing is immersed in the prop wash and always produces lift. This makes the aircraft respond super well in all conditions, from hover through transition to cruise and back. The airflow around the wing has significant performance advantages in transition and in hover. The propwash also protects the wing from gusts and side winds in hover (regardless of the direction), and as soon as you start to tilt the wing forward, the aircraft can achieve a climb rate comparable to the best high performance aerobatic aircraft. The result is less noise impact on the ground for less time. Even more important is the huge power reserve if you need it. You will for sure come into a situation where you will be very thankful for it – ask any utility or mountain helicopter pilot in the world.

Tilt wing aircraft also have other significant advantages:

-> Huge yaw efficiency in hover due to the ailerons immersed in the propwash.
-> Huge pitch efficiency and subsequently a large CofG range as the thrust device is on the tail and therefore very effective.
-> Very good slow flight characteristics as the wing and the tail is in the propwash and therefore the rudders are effective.
-> Very good STOL capabilities that allows for more payload and conventional take off and landing capability.
-> Simple proven systems that are known in aviation since the beginning.
-> Inherently stable flight behaviour including in transition (proven by the CL-84, which could be flown completely manual without any advanced stability system)

Dufour eVTOL demonstrator

We have been able to confirm these advantages for a tilt-wing with electric propulsion in all wind conditions and over 500 test flights with our large scale eVTOL demonstrator. We are now applying this technology to build a new generation of safe, sustainable and efficient eVTOL aircraft.

My goal as a helicopter rescue pilot is to build the future aircraft for my colleagues around the world, giving them an aircraft that will be safe not only for the 20% of technical incidents but also gives enough operational reserves to protect against the other 80% of operational incidents: that moment when you need the additional power or control range that keeps your a** in the air.

Thomas Pfammatter, CEO and Co-Founder Dufour Aerospace

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With over 30 years of experience in Swiss commercial helicopter operations, we understand the real world requirements of VTOL aircraft. Our engineering team with years of aircraft construction experience is building aircrafts for rugged operations in harsh environments. Our patented combination of proven aerodynamic concepts with the latest technology makes our aircraft rock solid with unparalleled performance.