After last summer’s publication of the special condition (SC) for new VTOL vehicles — as opposed to conventional rotorcraft — the European Union Aviation Safety Agency (EASA) is working out the details, such as accepted means of compliance, and preparing the follow-on certification specification (CS).
Agency officials discussed the process at EASA’s Rotorcraft & VTOL Symposium 2019, Dec. 10 and 11 in Cologne, Germany. Recordings of their presentations are on EASA’s YouTube channel.
An accepted means of compliance is decided on a case-by-case basis, said Lionel Tauszig, EASA Senior project certification manager, continuing airworthiness. It means it is accepted in the framework of the SC. However, generally acceptable means of compliance — usually referred to as AMCs — will not be released until the rulemaking process for the CS is complete. The SC is an interim regulation, while a CS is definitive.
EASA has defined 11 accepted means of compliance and is developing more, said David Solar, head of EASA’s rotorcraft department.
With respect to handling qualities, “we are targeting human performance . . . because we want an accepted means of compliance working with any combination of inceptors,” said EASA pilot Hamdy Sallam. A system failure and degradation of handling qualities in severe turbulence may happen simultaneously. However, in the “Enhanced” VTOL category, it should always be possible to continue safe flight or land without excessive workload so the probability of the combination has to be limited to 10-7 (one in 10 million flight hours). “Not all flight conditions have to be flight-tested, but simulation or other evidence is accepted,” noted Sallam.
In flight control systems, the crew should be aware of the flight mode, according to EASA’s approach. A VTOL vehicle will have fly-by-wire controls, mainly due to its distributed propulsion system (more than two vertical lift units are required to qualify for the SC). “Sometimes fly-by-wire degrades and the crew should be aware of where they are in the flight control envelope,” said project certification manager Duncan Jones.
EASA requires flight envelope protection, meaning the system ensures structural limits are not exceeded. The agency has been using its recent work on fly-by-wire rotorcraft with the Leonardo AW609 tiltrotor and the Bell 525 super medium helicopter.
In emergency landing, EASA is taking inspiration from both the CS27 (light rotorcraft) and CS23 (light fixed-wing aircraft) rules. The latter is used because some VTOLs will have conventional take-off and landing capability.
In a novel approach for crashworthiness, limited damage is allowed as long as occupants are protected, Tauszig noted.
The notion of fuel tank crashworthiness is being broadened to “energy storage system” crashworthiness, as many VTOLs will be electrically powered. A drop test has to take place in a representative surrounding structure, from a height of 50 feet (15 meters). The storage system has to be filled or charged to the most critical condition and the test allows no leakage of flammable substance. No harmful substance should leak into the passengers’ egress path.
Developing the SC took 15 months. To create it, EASA used assumptions for “low to medium” growth in market forecasts, David Solar explained. They translate into a requirement for a probability of one accident every billion (10-9) flight hours, said Solar. Forecasts for faster growth would, for the same absolute number of accidents, translate into 10-11, which would not be technically feasible, he added.
A “VTOL Ops” regulation for urban air mobility operations will be created, mixing manned and unmanned vehicles. Guidelines will be established for vertiport design. “The first operations will take place in dedicated corridors,” said Solar.
Under EASA’s philosophy, a single failure cannot cause a catastrophic situation. This is a major evolution from conventional light helicopters, where a single failure in the main gearbox, for example, can cause a loss of control and therefore a crash.
Lightweight recorders are required, which is “not an undue burden, given the benefits,” said Tauszig.
A novelty has been to link the categories of VTOL vehicles to the type of operations.
The “basic” category of aircraft can be used outside congested areas (such as cities). Allowed are private and commercial transport other than passenger commercial air transport, meaning freight operations are authorized. A failure during the flight should be followed by nothing more serious than a controlled emergency landing.
In the enhanced category, commercial air transport of passengers may be performed. The aircraft may fly over congested areas. In case of an in-flight failure, enough performance should remain to continue to the intended destination or divert to an alternate vertiport. Such alternates should be identified before flight.
Another SC is being devised for the certification of electric and hybrid propulsion systems. After a phase of public consultation, the final version is planned to be published by May this year.