*2.1. Europe*

Ongoing vertiport research and regulatory work is driven by EASA's drone and VTOL operation initiative.

In 2020, a first issue of a proposed means of compliance *MOC SC-VTOL* was published focusing primarily on basic VTOL aircraft design topics such as minimum handling qualities and CFP [37]. A thorough definition of a vertiport's role and minimum requirements was missing. EASA's second publication of proposed *MOC-2 SC-VTOL* [38] started to address the airside operation of a vertiport such as approach and departure paths, operating volumes, FATO dimension and climb gradients, for which a final publication is expected in 2022.

Based on those developments, a Prototype Technical Specification for the design of VFR vertiports accommodating manned eVTOL aircraft, *PTS-VTP-DSN*, was published in March 2022 and is leading the way for a first European regulatory framework [22].

#### 2.1.1. Operation Classes

In Europe, UAS operations are grouped in different operation classes based on the performance involved and the operational risk addressed. Its categories are *open*, *specific* and *certified*. Operations in the open and specific category address (leisure) operations with low and medium level of risks for which we already have a European regulatory framework for (Open: [39], Specific: [40]). Lastly, the certified category caters for the highest level of risk, therefore asking for the highest safety standards compared to other operation classes. According to [41], certified operations need to meet aircraft standards for manned aviation requiring a type certificate and a certificate of airworthiness. The dependency between type certificate, risk-levels and operational requirements including the use of designated UAM ground infrastructure was developed in the first issue of *SC-VTOL-01* in 2019 [21]. "VTOL aircraft that are certified in the Category Enhanced would have to meet requirements for continued safe flight and landing, and be able to continue to the original intended destination or a suitable alternate vertiport after a failure. Whereas for Category Basic only controlled emergency landing requirements would have to be met, in a similar manner to a controlled glide or auto-rotation" [21]. In order to better understand the European approach of classiyfing UAS operations, a structured overview of its setup is depicted in Figure 7. European regulation for certified UAS operations is currently under development under the rule making task *RMT.0230(C)* which initially defines three types of operation [42]. Operation type #1, IFR cargo UAS operations in class A-C airspace. Operation type #2, UAS operation in congested environment in *U-space* airspace including unmanned passenger and cargo transport. Completed by operation type #3 following characteristics of type #2 but with pilot on-board and considering also operations outside of *U-space* airspace. For further description of the topic *U-space*, please visit Section 2.1.5.

**Figure 7.** European UAS operation classes, subcategories and types based on [41,43,44].

Later on, when operating volumes and contingency procedures at vertiports are being defined, the corresponding operation class and operation type will determine performance and therefore vertiport footprint requirements.

#### 2.1.2. D-Value

Following former heliport design guidelines such as [12], the D-value has been used to dimension a heliport's airside topology, safety margins and operating constraints. The Dvalue defines "the largest overall dimension of the helicopter when rotor(s) are turning measured from the most forward position of the main rotor tip path plane to the most rearward position of the tail rotor tip path plane or helicopter structure" [12]. Comparing novel VTOL aircraft designs (cf. [45]), ref. [46] found that the smallest enclosing circle being equally to the D-value for rotorcraft can be off by 15%. A thorough mathematical derivation is provided in Appendix 1 of [22]. In order to secure sufficient obstacle clearance, EASA re-defined the D-value for VTOL aircraft by changing it into "the diameter of the smallest circle enclosing the VTOL aircraft projection on a horizontal plane, while the aircraft is in the take-off or landing configuration, with rotor(s) turning if applicable. [. . . ] If the VTOL aircraft changes dimension during taxi or parking (e.g., folding wings), a corresponding *Dtaxi* and *Dparking* should also be provided" [38].

#### 2.1.3. Vertiport Design Guidelines

Taking into account the new D-value definition specifically fitting VTOL aircraft designs, key elements of a vertiport (airside ground) can be dimensioned in order to establish an operating environment. Please re-visit Figure 6 to refresh specific heliport/vertiport design elements and terminologies used.

According to [22], a vertiport has to offer at least one FATO, in order to provide a designated area free of obstacles and with sufficient surface and load-bearing qualities. The dimension of a FATO is driven by the vehicle with the largest D-value intending to operate on the designated ground infrastructure. Furthermore, at least one TLOF needs to be provided at a vertiport. It can be located within a FATO or co-located with a stand. An additional safety area (solid/non-solid) exceeding the FATO and a protection side slope should protect the operation from penetrating obstacles. The vertiport might also offer taxiways and stands for additional operation. Both can be designed to meet either ground or hover movement capabilities of the VTOL aircraft resulting in higher footprints for the latter. Stands can be used simultaneously, sequentially, by turning in a hover or by taxiing-through without a need to turn. Depending on the intended operation, different requirements need to be met. Furthermore, EASA's *PTS-VTP-DSN* proposes a lightning vertiport identification marking of a letter "V" inside a blue circle, a D-value marking

to clearly state those aircraft designs being able to be accommodated at the vertiport, a FATO identification number, as well as a marker for the maximum allowable mass. Additional proposals for approach lighting systems and flight path alignment guidance markings and lights were elaborated, defining the location, characteristics, and configurations of each system. It is expected, as a second step, that a full regulatory framework will be developed in the context of the rule making task *RMT.0230* "Introduction of a regulatory framework for the operation of unmanned aircraft systems and for urban air mobility in the European Union aviation system" [42] in the near-term.

For further details, the reader is pointed to EASA's certification specification for VFR heliports *CS-HPT-DSN* [12] and VFR vertiports *PTS-VTP-DSN* [22].

#### 2.1.4. Proposed Reference Volume for VTOL Procedures

After examining the design requirements for a vertiport's airside ground topology, the airspace directly attached to the vertiport accommodating among others approach and departure paths (airside air) needs to be structured. Reviewing different regulatory proposals and guidelines, in the second publication of the proposed *MOC-2 SC-VTOL* [38], VTOL take-off and landing procedures are building on existing regulations for helicopters of category A. "Category A with respect to helicopters' means a multi-engined helicopter designed with engine and system isolation features specified in the applicable airworthiness codes and capable of operations using take-off and landing data scheduled under a critical engine failure concept that assures adequate designated surface area and adequate performance capability for continued safe flight or safe rejected take-off in the event of engine failure" [47]. Novel VTOL aircraft designs are expected to offer advanced vertical take-off and landing capabilities in order to meet the needs of emerging VTOL operations in urban environment. Therefore, a novel take-off path was elaborated addressing explicitly vertical take-off. It consists of a significant vertical climb segment until the take-off decision point is reached. Additionally, at least two take-off/climb and approach surfaces with a separation of at least 135◦ (ideally 180◦) should be provided. Furthermore, obstacle clearance in terms of protection surfaces apply with respect to the virtual elevated vertiport which describes the top of the vertical climbing segment until positive rate of climb is achieved and the VTOL aircraft is starting the acceleration into forward flight. VTOL aircraft can either follow conventional landing or a newly developed vertical landing procedure while complying with the requirements of obstacle separation. For this purpose, vehicle performance as well as navigation and communication performance requirements need to be elaborated in order to define the maximum allowed deviation from the nominal landing path. The required landing distance provides a safe environment if a CFP event is recognized at the landing decision point (LDP). For additional details please refer to Figures 1 and 2 of [38].

Due to the variety of VTOL designs, a first "Reference Volume Type 1" was proposed by *MOC-2 SC-VTOL* providing standardized parameter values for vertical take-off and landing procedures [38]. This proposed reference volume for VTOL procedures led into EASA's so called obstacle free volume (OFV) proposed in [22]. It describes a protection volume above take-off/landing pads in order to create a safe environment for UAM operations especially in congested and obstacle-rich environment (see left visualization in Table 4). In order to qualify as a OFV, certain criteria and dimensions must be met. Considering different accumulations of approach and departure surfaces to fit different obstacle characteristics can lead into bi-directional or omni-directional OFVs. A standardized reference volume Type 1 was developed and is displayed in Table 4. Manufacturer of VTOL aircraft may voluntarily comply with the reference volume type 1, and if required, additional reference volumes can be defined. It needs to be highlighted that the reference volume type 1 displayed in *PTS-VPT-DSN* [22] was enlarged compared to what was proposed initially in *MOC-2 SC-VTOL* [38].

Next to the design dimensions of a VTOL-specific operating volume, VTOL aircraft manufacturer and certification authorities need to agree jointly on an operating procedure and minimum performance requirements. This also includes strategies and measures if non-nominal situations occur during different flight phases.

During the flight, ref. [22] introduced the concept of alternate vertiports assigned to the flight prior take-off in cases of a critical failure. Whereas, if an individual take-off procedure needs to be aborted, the vertiport needs to provide a suitable FATO extension (rejected take-off distance) for the VTOL aircraft to complete a rejected take-off under a CFP at the take-off decision point. This results into bigger vertiport footprints in order to accommodate those contingency procedures. Similar to the aborted take-off procedure, a vertiport needs to offer a safe operating volume when balked landing is conducted due to CFP and a go-around procedure needs to be in place guiding the VTOL aircraft from LDP back to LDP in order to start a second approach.

**Table 4.** VTOL reference volume type 1 according to *PTS-VPT-DSN* [22]; visualization (left) extracted from [22], ©EASA.


#### 2.1.5. Airspace Structure and Traffic Management

Latest European UAM development show, that urban passenger-carrying operations are considered to operate first under current ATM procedures and most probably under visual flight rules, but are targeting an operation inside the European UTM system *U-space* in the mid- and long-term. *U-space* was elaborated initially in form of a ConOps (see [48,49]) providing a first set of operational practices and rules, predominantly addressing drones and small UAS. Those insights contributed to the recent regulation describing the *U-space* framework, its foundational structure and mandatory services [50]. Furthermore, a corresponding draft of acceptable means of compliance and guidance material was developed in accordance with the *U-space* framework [51]. However, the peculiarities of passengercarrying operations were not considered during the initial *U-space* ConOps, consequently a vertiport's role, responsibility and participation in *U-space* is not defined yet on a ConOps or regulatory basis. In addition, *U-space* is currently limited to very low-level airspace up to 500 ft (150 m) AGL which might be re-evaluated considering passenger-carrying UAM traffic. As UAM is considered to grow over time, the *U-space* system is assumed to mature in levels of connectivity and automation as well (*U-space* services U1 to U4). Starting from foundational services like e-identification and traffic information, it targets a full set of strategic and tactical operating *U-space* services in order to accommodate the complexity and dynamic behaviour of UAM including passenger-carrying VTOL operation. The basis of the *U-space* framework and its corresponding ConOps asks for a detailed analysis of stakeholders, roles, required services and a thorough ground and air risk evaluation. In 2021, the European standardization organisation *EUROCAE* published the second volume of an eVTOL ConOps *ED-293* [24], in which the vertiport was highlighted as an essential stakeholder and operational procedures such as ground handling processes were proposed. Further details including the distinct definition of roles and responsibilities within a vertiport's organisation are currently finalized in ED-299 [23] and are expected to be published this year.

For vertiport operations, a thorough traffic management analysis is still pending. What information is required by the *U-space* community during the course of different flight phases? How is a vertiport integrated into urban airspace? Who is responsible for the air traffic management at a vertiport and how do multiple *U-space* service provider interact in the vicinity of a vertiport In the next years, *U-space* will be re-evaluated and expanded in order to fit UAM demands in the mid-and long-term. The completion of several European *U-space* research projects including but not limited to CORUS-XUAM developing an extended *U-space* ConOps [52], TINDAiR investigating the safe integration of UAM as an additional airspace user [53], DACUS developing demand and capacity balancing strategies [54] and PJ34-W3 AURA developing a ATM *U-space* interface [55]) will support essentially this development.
