If they're not aeroplanes or helicopters, what are they exactly? And how should they be certified as being safe for passenger carrying commercial flight operations?
EASA's VTOL Special Condition
The European Aviation Safety Agency (EASA) is, along with the Federal Aviation Administration (FAA), one of the key certifying bodies globally for aircraft. For all intents and purposes a Type Certification from the body means that your aircraft is good to fly almost anywhere in the world. Recent issues with the Boeing 737 MAX aside, a rubber stamp of approval from either organisation is accepted to mean that a given aircraft adheres to the highest safety standards and can safely operate in the skies almost anywhere.
It's fair to say that the explosive growth of the consumer drone market caught regulators like EASA napping - the pace of change and rates of adoption in an entirely new aircraft market far outstripped the rate of appropriate new regulations being introduced. They've therefore been keen to avoid a similar situation in the emerging Urban Air Mobility (UAM) market.
It should be noted however that aviation regulations have always been far more stringent with respect to aircraft carrying out commercial air transport operations than with small, unmanned aircraft like consumer drones, as you would expect. So whilst it was never likely that we'd see hybrid or electric 'air taxis' ferrying people around in the sort of Wild West unregulated manner that we've seen with consumer drones, EASA also wanted to engage with industry early on and to innovate their means of helping aircraft manufactures reach Accepted Means of Compliance (AMC), and indeed in defining what constitutes compliance for an entirely new category of aircraft.
Not planes, not helicopters, then what?
The challenge with regulating any industry is deciding where to draw the lines, and with everything from small pleasure craft, cargo drones and passenger carrying air taxis all potentially falling under the category of Electric Vertical Takeoff and Landing (eVTOL) aircraft, the first challenge for EASA was to establish some clear guidelines for the UAM market.
Aircraft are typically categorised by weight, as defined by Maximum Permitted Takeoff Weight / Mass (MTOW / MTOM), the number of seats for passengers and aircraft performance (eg. as defined by their maximum operating velocity or 'Vno'). EASA certifies fixed wing aircraft of this type under CS-23, the fifth amendment to which was issued in March 2017 and deals with aeroplanes that have up to 19 seats and a MTOM of 8,618kg / 19,000 lbs. These are further subdivided as low or high speed and by Levels 1 to 4 depending on the maximum number of passenger seats.
Likewise there is legislation in place for certifying helicopters, CS-27, the 6th amendment to which was issued in December 2018. This deals with rotorcraft weighing up to 3,175kg / 7,000lbs and with nine or less passenger seats.
However neither CS-23 nor CS-27 are fit for the new breed of VTOL aircraft seeking certification at present. They typically have electric or hybrid powertrains vs their piston- or turbine-engined predecessors. Whilst existing aircraft will typically have one to four engines, eVTOL aircraft might have dozens, making use of Distributed Electric Propulsion (DEP) for example.
To further add to the complexity eVTOL craft typically make use of novel flight control mechanisms that make use of differential power being applied to the propulsors, more akin to the way a small consumer drone is flown than the collective in a helicopter or control surfaces found on a fixed wing aeroplane.
And if that were not enough EASA were being tasked to review 150+ VTOL concepts from myriad manufacturers, both industry veterans and entirely new startup outfits. Most are radically different from one another, with few commonalities.
The VTOL Special Condition
EASA response? Special aircraft required special measures - specifically the Special Condition for VTOL published in July 2019, after much consultation (see the 227 pages of industry comments and responses here for example).
This new regulatory instrument is based on parts of CS-23 and CS-27 with entirely new elements applicable to VTOL aircraft with distributed propulsion and together with similar legislation from the FAA (which we'll examine in a later post), has started to define the playing field in which all players in the Urban Air Mobility (UAM) market will have to compete. If your 'flying taxi' doesn't meet the criteria EASA set out, they simply won't be certifying it commercial passenger carrying operations in they jurisdiction - no matter how good your CGI renderings are and regardless of how many millions of Venture Capital funding or corporate cash you have behind your project!
The first thing to note is it's short (in aviation regulation terms at least), at just 30 pages, and leaves lots of scope for further definitions and additions as the aircraft, the market and certification requirements evolve. Expect some concerted and prolonged lobbying efforts from everyone with a stake in the UAM market.
Who's it for?
This Special Condition prescribes technical specifications for:
Vertical take-off and landing (VTOL) aircraft that are 'heavier-than-air' (ie not balloons / blimps etc)
That generate powered lift and control from lift/thrust units
Can take-off and land vertically, unlike conventional fixed wing aeroplanes
Have two or more lift/thrust units, unlike conventional rotorcraft (eg. helicopters)
With 9 or fewer passenger seats
And a Maximum Take-off Mass (MTOM) of 3,175kg / 7,000lbs or less
There's no specification yet with regard to remote piloting or different levels of autonomy, though a note that it's intended to be compatible with these as additions become required in future.
Two categories for certification are established - Basic and Enhanced. The latter being more stringent and in line with requirements foreseen for craft engaged in passenger carrying operations eg. over congested areas (eg. cities). The requirements for the categories differ in relation to what the aircraft must be capable of doing in emergency situations - Basic category aircraft needing to perform a controlled emergency landing for example, whereas Enhanced category aircraft need to be able to continue flight to an alternate airfield for example.
In most emergency situations a fixed wing aircraft has some sort of glide capability and (contrary to popular belief) a helicopter does not simply drop like a stone but can 'auto-rotate'. Catastrophic failures in commercial aviation are rare, because the certification of airliners has for years demanded reams of test data to prove that the likelihood of failures are below prescribed levels, and that there's sufficient redundancy in aircraft systems to maintain safe operations in the event of failures.
Aviation regulators are strictest when aircraft are carrying paying passengers, and when they operate in or near populated areas - the industry works on the assumption that the risk of harm to people going about their lives on the ground, as well as to members of the general public who've paid for an air transport service should be As Low as Reasonably Practicable (ALARP). Air taxi operations are therefore focused on operating in an environment with the most stringent possible regulatory requirements. Certification will be expensive - test flight programmes are not cheap for any aircraft, let alone one of an entirely new type.
We can already see requirements being introduced that some manufactures weren't expecting, an example being the need for Cockpit Voice Recorders (CVRs) and Flight Data Recorders (FDR) which are not normally found in smaller aircraft. Requirements for combating aircraft icing as well as dealing with lightning and bird strikes are also proving a surprise / challenge for some manufacturers.
But it might just be that this 'Special Condition' for these innovative aircraft end up making us all safer - engine failure on an aircraft with 30+ engines could prove to be far less of a problem than on an airliner with just two - the potentially enhanced safety margins of distributed propulsion have been suggested for some time.
Similarly flight data recorders on commercial aircraft are often only examined in detail after a disastrous incident - the next generation of them may well be transmitting richer streams of data in realtime, allowing for ongoing diagnosis and machine-learning aided predictive maintenance. The VTOL Special Condition already has an eye on this and expects elements of data to be transmitted on-the-fly.
Expect air taxis to be connected to a raft of communications systems - from 5G and low orbit satellite networks to as-yet-undefined Universal Traffic Management (UTM) systems that replace the reliable but antiquated push-to-talk service airliners use today.
One thing is for sure - Urban Air Mobility and eVTOL will push the boundaries across the global aviation and aerospace industries, and early indications are that they're bringing the regulators along with them. EASA's VTOL Special Condition is 30 pages of new rules that are defining the boundaries of the next generation of aviation technology and preparing us all to welcome a whole new class of flying machine into our skies.