Flying taxis in the dark

These issues were recently addressed by Julian Lee MRAeS from test flying, capability management, safety and regulatory specialists, Nova Systems at a presentation at RAeS HQ on 9 May (and also at an earlier talk on 12 February at the RAeS Oxford Branch).

Lee began by outlining the advances in composite construction, electric motor, battery and control and navigation system technology which have enabled the development of eVTOLS which can either be piloted or automated to operate as autonomous passenger-carrying drones.

The market for urban air mobility is expected to be big business and many companies have become interested in its potential, including mainstream manufacturers such as Boeing, Airbus, Embraer and Bell, plus a large number of start-up companies. Lee explained how multinational transportation ridesharing service company Uber Technologies, which has already shaken up traditional concepts of ground transportation, is keen to encourage manufacturers to develop electric flying vehicle designs which would enable the company to spark a revolution in urban air transport. To this end, Uber has created the annual UberElevate conference to bring together eVTOL designers, operators and regulators to discuss how the flying taxi vision can be realised. The 3rd UberElevate conference is due to meet later this year in Washington and will include speakers from technology researchers, manufacturers, airlines, air traffic control providers, civic authorities and financiers.

Lee estimated that there are currently around 130 urban air transport projects in development, many of which are funded by Californian Silicon Valley billionaires. “There is currently a race to see who will get a design developed and certificated,” he remarked. However, Lee also predicted that only around 10% of these projects was likely to achieve commercial success, saying: “The first ones to develop and mass produce flying taxis will be the ones who achieve market dominance.”

CityAirbus VTOL flying taxi design concept. Airbus

Operational challenges

In order to maximise safety and build public confidence, the initial generation of flying taxis are expected to be piloted. Once the concept is accepted and proven to be safe, then autonomous vehicles, which will effectively be large UAVs with passengers, will follow. These flying vehicles will fly along narrow airspace corridors between dedicated landing pads at air taxi ports located at airports, railway stations, open areas or on top of tall buildings. These ports will act as hubs for air taxi routes and will include facilities for arriving and departing passengers, as well as recharging, storage and maintenance services for the air taxis. Not long after the lecture on 23 May, plans were announced by vertiport specialists, Skyports and eVTOL manufacturer Volocopter, to build the first eVTOL landing pad in Singapore.

However, before this vision is realised, there are a number of challenges yet to be overcome. “This concept will only work if the aircraft are as reliable as an Uber car or a black cab,” cautioned Julian Lee. “Otherwise, you’ll return to the old adage ‘Time to spare, go by air’.

Because the eVTOL taxis will operate in cities, the vehicles will have to fly much lower and closer to buildings than aircraft (and drones) are currently permitted. “These vehicles are going to have to operate in an extremely challenging environment,” warned Lee. “Using them in California where it’s sunny and there’s plenty of space seems realistic but I’m not sure the designers have thought about what it might be like in London in February when it’s freezing and it’s foggy.”

Hazardous environment

An air taxi high-rise hub, as envisioned by German eVTOL designers VoloCopter. Air taxis are expected to have to fly much lower and closer to buildings than conventional aircraft or UAVs. Volocopter

Lee explained how, even though air taxis will be flying along narrow airways, there will be risks due to higher speeds,” he said. “These are high-performance vehicles capable of speeds of up 150mph. If they fly down a street surrounded by high-rise buildings, you’ve got congested airspace with potential closing speeds of 300mph, If you’re flying near buildings, you’ve also got potentially high levels of air turbulence (for example, in the lee of the Shard) – as any helicopter pilots who have flown near mountains can tell you. Not only that but the eVTOLs will be flying through highly congested terrain which changes quickly as new infrastructure is added, such as masts, wires, scaffolding and cranes, etc. Although the vehicles will be fitted with precision navigation systems, what happens if the inertial navigation system fails or if you have GPS jamming?”

Degraded vision environment

Another factor to be considered is that the air taxis will not always have clear weather and good visibility but will have to operate in degraded vision environment (DVE) conditions, such as at night, in mist, fog, drizzle, rain, snow and freezing conditions. However, Lee reminded his audience that the challenge of operating aircraft in bad visibility has been around ever since aircraft started to fly. The issue of DVE is a familiar one to helicopter pilots who have to fly or land both day and night in rain, fog, storms, snow or even sandstorms – a problem which was highlighted in recent military operations in Iraq where it was very dusty and also when pilots were trying to land at night in low ambient light levels.

The issue of poor visibility and loss of situational awareness has been a contributory cause in a number of recent fatal helicopter accidents, including a Sikorsky S-76 in Ireland which crashed into the side of a mountain during a night time GPS approach into a valley in 1996, an Agusta A109 which collided with a crane in Vauxhall in London when flying in poor visibility in 2013 and an AgustaWestland AW139 crash in Norfolk in 2014 which involved a loss of situational awareness after a take-off on a night flight in fog from a confined area.

Images from left to right: The Second UberElevate conference met in Los Angeles in 2018. What happens to eVTOLs when it gets dark or starts raining? Will flying taxis need to be fitted with wirecutters similar to those on helicopters?

The helicopter experience

To tackle the problem of flying in degraded vision environments, the helicopter community has identified ‘three legs of the stool’ which are needed to fly a helicopter (or any VTOL aircraft) close to the ground when the pilot cannot see what is going on. Lee explains: “The first thing that you need is something which gives the pilot situational awareness as to what aircraft is doing to make sure that it is the right way up and in the right place. You also need something which can detect obstacles and, thirdly, something to help the pilot fly the aircraft – which could be an automated autopilot or just something which makes it easy to fly the aircraft in that situation.

Lee also explained some of the equipment which has been used in helicopters to help pilots fly in DVE conditions. In the 1970s, Royal Navy Westland Sea King helicopters on anti-submarine missions flying in degraded vision environment over water were fitted with a hover meter, altimeter, radar and automatic flight control system. Technology has since become ever more sophisticated and the latest model of the Sikorsky HH-60 Pavehawk has now got night vision goggles for the pilot, terrain-following radar, an electro-optic system, an embedded GPS-INS system, a radar to detect obstacles and a digital flight control system to keep the aircraft steady.

Challenges for eVTOLs ​

The city skies of the future? Air taxis will need to be fitted with sensors capable of identifying all potential hazards – including each other.

The sensor challenge

Lee went on to explain how air taxis (either piloted or autonomous) would need to be fitted with systems capable of detecting both large and small objects which might cross their flight paths. These systems would have to work not just in daylight and clear weather but also in bad weather and night conditions. “You will need to be able to detect a wide variety of obstacles, such as a construction crane, bird, drone, another eVTOL or a vehicle driving across landing site,” he said. “Even helicopter technology has not yet achieved this level.”