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Coming to Terms - UAM Maturity Level 5
  • 20 Feb 2024 01:26 AM
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Coming to Terms - UAM Maturity Level 5

By Al Lawless, Aurora Flight Sciences, and VFS E-VTOL Flight Test Council Chair
Vertiflite, March/April 2024

This column addresses vertical flight terminology and proffers descriptions, definitions, or language to clarify and bridge divides.

Vertiflite’s three previous “Coming to Terms” articles in this series broadly captured various kinds of aviation certifications and operational approvals and how they connect to an aircraft’s Concept of Operations (ConOps). This fourth installment extrapolates these discussions to illuminate challenges for an ambitious new ConOps. While technology advances and innovative approaches enable many novel ConOps, the case chosen here is a notional long-term vision for urban air mobility (UAM), an intracity subset of the now-standard advanced air mobility (AAM).

The staggering investments into UAM projects around the globe (with aircraft development alone garnering over $10B) reflect the faith in this new business, so it is prudent to begin with a UAM description. In 2020, NASA (via Deloitte Consulting LLP) called UAM in Ref. 1, an “…air transportation system for passengers and cargo that revolutionizes mobility around metropolitan areas.” NASA included “everything from small package delivery drones to passenger-carrying air taxis that operate above populated areas.” Note that regulators do not include small uncrewed aircraft systems (sUAS) in UAM, so NASA’s inclusion caused some confusion that continues today with “AAM” as well.

The commonly used term “air taxi” as used above requires a brief mention. This is a misleading sobriquet because it implies a capability that is likely unworkable in urban or suburban environments. Any VTOL aircraft large enough for people must lift off with a substantial air momentum change. Such aircraft will not be allowed to pick passengers up at their doorstep any more routinely than can be done today with a helicopter or a personal jet pack. The safety concerns of power lines, obstructions and pedestrian hazards, compounded by noise and devastated rose bushes, would render this unacceptable. At best, passengers would hail a service that includes a car ride to and from the vertiports. Should true air taxi service arise, especially for rural areas, then this term makes sense for that ConOps. Considering UAM flights will operate to and from fixed platforms (vertiports, heliports, airports), more accurate analogies would be to bus stops and train stations. One might consider “air shuttle” the more appropriate term.

As described in Ref. 1 and 2, and widely promulgated around UAM circles, NASA developed a framework for evolving UAM Maturity Levels (UMLs). This captures an evolution vision as far as UML-4, which entails hundreds of simultaneous operations, expanded networks with closely spaced high-throughput aerodromes, numerous air traffic management services, simplified aircraft operations and low-visibility operations. Presently beyond the FAA’s and NASA’s focus is UML-5, which could have about ten times the throughput and adds remotely flown, autonomous (see info box, “What is ‘Autonomous’”) and M:N operations (i.e., more airborne aircraft than humans responsible for their safe flight). UML-6 could include another order of magnitude greater throughput and adds ad-hoc landing sites, truly ubiquitous use with high throughput, and supporting societal expectations (i.e. the level seen in “The Jetsons”).

The NASA papers (Ref. 1 and 2) did not detail what would be involved to reach the mature state (UML-5 and 6). Nevertheless, we can extrapolate this work to propose a UAM ConOps at the end of this evolutionary line. Doing so not only illuminates challenges but guides us in laying the tracks for the lower-level goals. Ideally, we’d avoid developing dead-end capabilities that can’t expand to the ultimate UAM vision, although the reality of vastly increased throughput might require radical changes.

Towards this purpose, consider a notional UML-5/6 ConOps with the following features that rely heavily on automation. Note that this vision avoids the word “autonomy” (or the “Level 5 Automation” of Ref. 2) because humans are ultimately responsible and can override the automation to some degree. Also note this vision is deterministic; we have yet to develop a path to certify non-deterministic decision making.

  • Passenger and cargo transportation, including scheduled commuter, tourism, on-demand flights within the UAM ecosystem
  • Precisely planned, tightly choreographed (four-dimensional flow control) flight execution with landing tentatively approved before takeoff.
  • Automated flying without an onboard or remote pilot having direct flight path control.
  • Flights strictly between qualifying vertiports or other platforms.
  • Flight along surveyed routes in heavily trafficked areas with identified precautionary and emergency landing locations.
  • Common-use all-weather navigation equipment supporting UML-5/6.
  • Realtime (database) sharing of UAM environment air traffic information. Includes awareness of ConOps operators and non-participants (cooperative and non-cooperative).
  • Realtime sharing of ground and facility information, vertiport status, temporary flight restrictions.
  • Realtime sharing of UAM environment weather including 21st century windsock data.
  • Realtime sharing of other UAM operators’ communications and instructions.
  • Realtime sharing of ground-based radar or other detect-and-avoid methods.
  • Aircraft command and control (C2) datalink handling post-takeoff flight plan changes.
  • Aircraft capable of self-monitoring and deterministically executing normal and abnormal procedures.
  • Human over-the-loop M:N supervision with limited “re-direction” options and responsibility to ensure safe flight.
  • Passengers continually informed of flight progress and enabled to communicate with the responsible operator to convey distress or observations.
  • Passengers enabled to conservatively override the plan via limited triggering options (e.g., divert to hospital or alternate route).

Such a detailed ConOps needs a name for easy reference and to distinguish it from other UAM approaches. Contrasting sharply with the familiar open airspace construct used for piloted visual flight rules, the key defining features of this ConOps are supervised flights with limited override options and controlled 4-D flights along approved routes.

Sanctioned routes are critical to this vision for two reasons: (1) noise approval and public acceptance, and (2) surveyed routes and alternate landing sites enable deterministically planning and executing normal and abnormal flying without a traditional pilot. Note that this ConOps is more contained than the earlier analogy of a taxi that can quickly reroute along anything drivable and relies on the driver to deconflict. It is more comparable to a train that deconflicts by traveling between platforms along assigned tracks and elevations, while retaining backup capability to pass, divert, and evade. The E-VTOL Flight Test Council recognized this airborne parallel to big-city elevated (El) trains, coining the term “AeriaL UAM” to capture this ConOps (see “The E-VTOL Flight Test Council’s Pioneering Year,” Vertiflite, Nov/Dec 2021).

The other features itemized above are also generally required for public acceptance and to make an AeriaL UAM system work. There may of course be different solutions, such as onboard detect-and-avoid systems, data sharing, distributed traffic control or not enabling passenger override, but the intent here is to show the complexity of a mature ConOps vision.

Recalling the previous three installments in this series connects these AeriaL UAM features to the various certification and operational approval requirements. Some will be simple evolutions of existing rules. For example, we know how to certify liquid fuel systems, so solid or hybrid fuel certification should be a variation of that theme. Similarly, we know how to certify traditional mechanics, dispatchers and pilots, so only the details would change for UML-5 approvals (see “A Small Step for EHang, a Giant Leap for eVTOL,” Vertiflite, Nov/Dec 2023).

On the other hand, numerous features have yet to be fully defined, let alone have a clear path to certification. A compounding challenge is that the whole system must evolve in a coordinated manner beyond the span and capabilities of any single organization. For example, how will novel realtime data-sharing interfaces and services be approved? What new standards and processes will certify a ground control station and supervisors for this ConOps? How will we deal with an earthquake or other major disruptions? How can we certify simulators so they may be used to show compliance for failure or highly non-linear flight conditions? Can we offer 21st century passengers the equivalent of an emergency stop pull chain once available on 19th century trains? How can we certify C2 link reliability in reflection-rich urban areas?

Digging deeper into this list (and more) reveals the work ahead.

Check out previous editions of the “Coming to Terms” column online at www.evtol.news/terms.


1) UAM Vision ConOps UML 4 (Version 1.0), NASA, Dec. 2020

2) Description of the NASA Urban Air Mobility Maturity Level (UML) Scale, NASA, Jan. 2021

3) JARUS Methodology for Evaluation of Automation for UAS Operations, JARUS, April 21, 2023


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