By Philip Butterworth-Hayes and Beth Stevenson
On 10 June a helicopter crashed in flames onto the roof of an office building on Seventh Avenue in New York, killing the pilot Tim McCormack.
It was another salient reminder, if any more were needed, of the complex challenges facing urban air mobility vehicle designers and operators, safety regulators, local authorities and environmental agencies when it comes to ensuring the safety and well-being of passengers and people when planning new concepts of flying in urban environments.
According to Uber Elevate’s plans for aerial ride-sharing in cities around the world – outlined in the Uber Elevate summit earlier this week – the company will start conducting demonstration flights in 2020 to measure eVTOL noise, assess community acceptance, and quantify vehicle safety and performance. With the first operations planned for 2023.
The problem is this timetable looks, at first sight, to be well in advance of the regulators’ own timetable for developing regulations to support electrically-driven air taxi operations in heavily populated urban areas.
As Federal Aviation Administration (FAA) Acting Administrator Dan Elwell pointed out to delegates at the Uber Elevate event in the past it has taken around 10 years for the FAA to certify a new aircraft type. And although the agency has adopted a speeded-up performance-based approach to developing new safety rules – based on assessing data from multiple trials and evolving the certification system along crawl/walk/run processes, using existing regulations wherever possible – it is not just the certification agency which will have to give the thumb’s up to the UAM revolution, it is the public, too.
NASA – the FAA’s partner in developing data to form the regulations – plans to advance progress on UAM with a series of grand challenges, with GC1 planned for January 2020 when industry partners will be asked to address the foundational airworthiness UAM vehicle design readiness and robustness for UAM operations, followed in January 2021 by GC2, which will address key safety and integration barriers.
The problem facing certification agencies when addressing UAM concepts is that they are being asked to certify not just a new platform but new types of engines, power storage and transmission systems, communications systems and traffic management systems, all integrated within a new automated and eventually autonomous operating system.
It might not take ten years for certification agencies in north America and Europe to certify all these but it is almost certain to take more than the three envisaged by Uber.
In Europe the European Aviation Safety Agency (EASA) says it has received several applications for urban air mobility-type vehicles, adding that the certification process is underway and the authority is “working actively on these projects” alongside industry. EASA told Urban Air Mobility News that it has been working with industry for a couple of years on this, however they noted that “we are still at the beginning of the journey”. It is supporting these developments through the introduction of a so-called special condition, as it considers that existing requirements for both fixed- and rotary-wing aircraft are not adequate for these new concepts to fly under, and this is expected to be finalised in the coming weeks. A draft version of this VTOL special condition was published for consultation in October 2018, a spokesperson noted, adding: “EASA received a large number of comments from a large number of stakeholders, including non-traditional aviation stakeholders.
“Comments have been addressed and EASA will publish the final special condition by June 2019.” This VTOL special condition will be the backbone of the certification basis for VTOL vehicles that are not considered to be conventional rotorcraft, and this will be centred on performance-based criteria, blending existing requirements from CS-23 (Normal-Category Aeroplanes), CS-27 (Small Rotorcraft), CS-25 (Large Aeroplanes) and specific requirements.
It will also define the different safety objectives depending on the type of operation, and typically urban air taxis will be required to meet the highest level of safety, the spokesperson noted.
From the regulator’s point of view, the challenges associated with these projects vary between them, due in part to the number of different technologies and approaches being introduced. “”These vehicles are introducing simultaneously several innovations and integration of these innovations may be a difficult point. Recent experience showed this is probably the most demanding aspect even for very experienced companies.”
The electric/hybrid propulsion of these systems is new, EASA adds, but noted that in most of the proposed designs it has been presented with, propulsion/lift and flight control systems are more integrated than with conventional aircraft. “Other aspects such as batteries integration, handling qualities and so on are also challenging, but as said, technical aspects are manageable,” they added.
Which all suggests that even with no technical hitches – and without taking into account the views of the public – it is unlikely that the regulations for high density UAM operations will be in place much before 2025. The crawl/walk/run data driven process underway by the FAA and EASA has not yet begun. It will take the regulators considerably more than a year to digest all the data from planned trials and propose the complex new sets of regulations required for operating air taxis in the vision set out by Uber.
| The operating requirements for Ubair Elevate air taxis
Uber has defined a set of missions and requirements for Electric Vertical Takeoff and Landing (eVTOL) aircraft operating on the Elevate network. • VTOL: Vehicles must have Vertical Takeoff and Landing capability with short duration hover. • Safety: Certified vehicles must be able to perform a safe vertical landing in the event of a critical failure, including a collision with a bird, in any phase of flight. • Noise: Uber is developing noise standards for UAM eVTOL aircraft that will use detailed time integrated annoyance metrics to ensure community friendliness. This involves designing the aircraft to achieve specific signatures that can map into background city noise soundscapes. Simplified comparative sound levels experienced by observers should be on the order of 15 dB quieter than existing light helicopters, or about 70 dB SEL at 500 ft vs 85 dB for a typical helicopter of similar weight. • Energy Storage: Vehicles should operate entirely on battery-electric energy storage. Vehicles utilizing liquid hydrocarbon fuels, such as hybrids, have severe logistical and economical challenges for successful integration into the network. • Skyports: Vehicles in the network will operate only from Skyports, which are VTOL hubs with multiple takeoff and landing pads and include charging infrastructure. Vehicles should not require ground equipment that is specific to the vehicle model. Passenger ingress/egress should be optimized in support of high throughput Skyport operations. • Charging: The company is working with partners to develop a rapid charging capability (up to 600 kW); the vehicles should be able to interface with this infrastructure. Vehicles are expected to charge for less than 7 minutes during 3-hour sprint windows and less than 15 minutes otherwise. • Pilots: Uber expects on day one that vehicles will have the avionics and sensors needed for autonomous flight. However, they will be piloted in the first years while data is collected to prove the safety of autonomous systems. While not required by FAA, Uber will require the pilot to be physically separated from the passengers to achieve maximum safety. • Network Communications: Uber will work with partners to develop a secure bi-directional API for network communication. For example, the vehicle must be able to receive precise flight plans and report position and battery state. Vehicles must have the required equipage to fly in Class B airspace. • Payload: Vehicles must have space for a pilot and 3 or 4 passenger seats with a max payload weight of at least 980 lb, including luggage. Ground Taxi: Vehicles must be able to perform a powered, wheeled ground taxi without spinning propellers or rotors. The company intends for vehicles to travel on the ground for up to 300’ at a ground taxi speed of approximately 5 ft/sec. • Size: Due to the limited size of the Skyports in urban environments, the vehicle footprint must not exceed 50’ in its max dimension, including fully extended rotors. Max height should not exceed 20’. Missions Range: Vehicles shall be able to fly 60 miles while maintaining enough energy to fly the reserve mission. This must be achieved at the battery’s end-of-life state, which is determined by the vehicle developer. • Takeoff: Vehicles shall be capable of takeoff and hover climb at 5,000 ft DA. This is sufficient to operate in a majority of cities. Over time, vehicles with increased capability may need to exceed 5,000 ft DA takeoff requirements to generate network productivity in select markets (e.g. Mexico City). • Reserves: The aircraft shall have sufficient energy at the end of the Sizing or Repeated missions to perform a balked landing at the original destination, divert 6 miles to an alternate landing site, and land vertically at the alternate. The actual reserve energy required to execute a flight will be determined operationally and through working with regulators. Note that the effect of battery aging (decreased capacity and increased internal impedance) must be accounted for in evaluation of battery reserves. • Cruise: Network analysis favours faster vehicles capable of cruise speeds of 150 mph, with additional ±15 mph capability required for airspace sequencing and headwinds. Cruise should be performed at 1,500 ft above ground level (AGL). Initially, vehicles will operate only in visual meteorological conditions (VMC), with a path to autonomy enabling near-all-weather operation. • Repeatability: The vehicle must be able to operate continuously for at least 3 hours while flying 25 mile missions at the battery’s end-of-life state. Uber defines continuous operations as needing to charge for less than 7 minutes between flights, such that the limit on throughput is only the time to unload and load the passengers. Uber believes that this repeatability will be key to maximizing throughput. • Battery Rapid Charge Ceiling: Above an upper state of charge (SoC), the charge rate will begin to decrease from maximum capability. Minimum State of Charge for Dispatch: Battery must be above this SoC to take off. This includes energy to complete the next mission with reserves, without dropping below the battery floor. Minimum Reserve State of Charge: Battery SoC should not pass below this line in nominal operations. The battery may operate below this threshold during a reserve mission, but the SoC must remain above the battery floor. Battery Floor: At low SoC conditions, the current spikes and voltage drops precipitously. This section of the battery should not be counted as battery reserves.
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