Joby Aviation, the S-four eVTOL, and the FAA Certification Path Nobody Had Ever Walked Before

Joby Aviation is closer to commercial passenger operation than any other electric vertical takeoff and landing company. The company has completed Stage 4 of a five-stage FAA type certification process for its S-4 aircraft and holds a Part 135 Air Carrier Certificate issued in 2023. What remains is Stage 5 - final flight testing under direct FAA oversight - and the infrastructure required to actually run the service.

The Engineering Background: Why eVTOL Is Possible Now

The idea of combining vertical takeoff with efficient cruise flight is not new. Tilt-wing and tilt-rotor concepts were already under development in the 1950s and 1960s, and the Bell-Boeing V-22 Osprey proved the concept could reach military service, entering operation in 2007. But the V-22 is built around two large turboshaft engines and a complex mechanical gearbox - large, loud, and expensive to operate.

What changed between the V-22 era and today is the electric motor. Brushless DC motors became dramatically more powerful, efficient, and affordable. Battery energy density improved. High-speed flight controllers capable of managing multiple independent power channels became commercially available. Those three developments together opened a design space that simply did not exist in 2000, and they explain why serious eVTOL programs emerged in the 2010s and not before.

Joby Aviation was founded in 2009 by JoeBen Bevirt, operating out of a farm near Santa Cruz, California. The company spent its early years largely self-funded, attracting little outside attention while developing the foundational engineering question: can you build an aircraft that takes off vertically, cruises at meaningful speed, uses electric propulsion, and can be certificated to carry paying passengers under federal rules?

What the S-4 Actually Is

The S-4 seats five people - one pilot and four passengers. The airframe uses a conventional fixed wing of roughly 12 meters in span. Six rotors are mounted on tilting arms extending from the wing and fuselage. For takeoff, all six rotors point upward and the aircraft operates as a large multirotor. Once at altitude, the rotors tilt forward, the wing takes over lift duties, and the aircraft cruises like a conventional airplane.

The transition between these modes is aerodynamically non-trivial. As airspeed builds and the rotors tilt, the aircraft passes through an intermediate state where neither the wing nor the rotors are providing full lift authority. The flight control system continuously apportions control between the wing surfaces and individual rotor thrust vectors during this window. Joby reports the transition takes only seconds and is handled automatically - the pilot selects cruise mode and the aircraft manages the rest.

Performance Numbers, With Context

Top cruise speed: approximately 200 mph. The existing fleet of certificated electric general aviation aircraft typically cruises in the 90 to 110 knot range. Joby’s speed comes from the aerodynamic efficiency of the fixed wing combined with the reduced drag of tilted rotors in cruise - where they function as efficient propellers rather than dragging a large disk broadside through the air.

Noise on approach: approximately 65 decibels at the standard measurement distance. A conventional helicopter on approach typically measures 85 to 90 decibels. Because the decibel scale is logarithmic, that 20 dB reduction represents roughly a 75% decrease in perceived loudness - approximately the difference between a normal conversation and heavy traffic. Smaller rotors at lower rotational speeds generate less aeroacoustic noise, and electric motors can be speed-managed in real time for acoustic optimization rather than purely for thrust. The 65 dB figure warrants independent verification across all flight phases and operational conditions, but the directional advantage over helicopters is grounded in physics.

Demonstrated range: approximately 150 miles. That figure reflects specific test conditions - optimal battery state, specific payload, minimal hover time. Hovering consumes energy at a far higher rate than cruise for an aircraft that depends on a wing for lift. Accounting for required reserves and landing with meaningful charge remaining, operational range in typical service will be lower. For Joby’s target routes - 30 to 60-mile hops, airport connections, city-center-to-city-center - 150-mile demonstrated capability is sufficient. The company is not claiming this aircraft serves longer routes.

The Certification Path Nobody Had Walked Before

No existing part of the Federal Aviation Regulations cleanly describes the S-4. The FAA’s established categories - helicopter, fixed-wing airplane, powered lift, lighter-than-air - none of them fit a six-rotor, all-electric, five-seat tilt-rotor aircraft. The powered lift category is closest, but those airworthiness criteria were developed with the V-22 in mind and do not map cleanly onto this design.

The mechanism Joby and the FAA used is a Special Class designation under 14 CFR §21.17, Subpart B. This provision allows a company to petition the FAA to establish airworthiness criteria specifically for a novel aircraft - criteria negotiated between the applicant and the agency, drawing from existing standards where applicable and writing new requirements where existing rules don’t address the novel aspects of the design.

Joby and the FAA have been engaged in that process since approximately 2015. The FAA does not sustain multi-year Special Class negotiations without a substantive engineering program behind them.

The Five Stages of Joby’s Type Certification

The FAA structured Joby’s type certification in five stages:

Stage 1 - Qualification. The agency familiarized itself with the design and the two parties agreed on the evaluation framework.

Stage 2 - Design and systems review.

Stage 3 - Flight test data collection and initial conformance assessment.

Stage 4 - Continued flight testing using conforming aircraft - meaning aircraft built to the actual production specification rather than development prototypes. Joby has completed Stage 4.

Stage 5 - Final flight test program conducted under direct FAA oversight, leading to full type certificate issuance.

Reaching Stage 4 matters because it means the FAA has verified the design is stable, the testing methodology is sound, and the program is proceeding against the agreed certification criteria. Fundamental design questions do not survive to Stage 4 unresolved.

The Safety Architecture: Distributed Electric Propulsion

The core safety argument for the S-4 rests on distributed electric propulsion. In a conventional single-engine piston aircraft, engine failure leaves the pilot flying a glider. In a helicopter, the pilot enters autorotation. In a twin-engine airplane, a single-engine failure triggers immediate emergency procedures involving asymmetric thrust management. In every case, a single failure demands immediate pilot action.

The S-4’s six independent electric motors - each on a separate power channel, each separately controllable by the flight management computer - are designed to change that calculus. The stated design goal is that the aircraft remains fully controllable after losing any single motor, without pilot intervention. The flight control computer detects a rotor speed deviation and compensates with the remaining five motors in milliseconds - faster than a pilot can register the anomaly. Joby has demonstrated controlled flight following deliberate in-flight motor shutdowns in testing, including multi-motor scenarios.

Electric motors make this achievable because of response time. An electric motor transitions from idle to full power in milliseconds. There is no turbine spool-up lag, no fuel system delay. The control loop runs fast enough to exploit the distributed architecture before the aircraft has measurably deviated from its intended flight path. Whether this holds across all edge cases - all weights, all wind conditions, all failure combinations - is precisely what the certification testing is designed to verify.

The Part 135 Certificate and Commercial Partners

Separately from the type certificate, Joby received a Part 135 Air Carrier Certificate in 2023. Part 135 governs on-demand air taxi and charter operations. Obtaining this certificate before the type certificate is finalized is deliberate strategy: when the type certificate issues, the operational approval is already in place. Pilot training programs, maintenance requirements, dispatch procedures, and crew rest rules are already approved.

Delta Air Lines has a commercial agreement with Joby to operate airport connection and urban routes. Toyota has invested substantially in the company - reported figures exceeding $800 million - and brings manufacturing scale and process discipline that most aerospace startups cannot independently access. These relationships represent operational and manufacturing credibility, not just capital.

What Remains Unresolved

Infrastructure is the problem that engineering alone cannot solve. Vertiports require significant electrical supply for rapid charging, approach and departure procedures, air traffic coordination, and physical space in dense urban environments where real estate is expensive. Some sites have received planning approval. The full network required for viable commercial-scale urban air taxi operations does not yet exist.

Airspace integration is genuinely open. The FAA’s Urban Air Mobility concept of operations describes how eVTOL aircraft will share low-altitude urban airspace with general aviation and helicopter operations, but the digital air traffic management tools required are still under development - partly through NASA research programs, partly by commercial providers. They are not deployed at operational scale.

The economics remain uncertain. Early market estimates range from roughly $3 per mile down to under $1 per mile as scale improves. Early operations will almost certainly target business travelers at a premium before costs come down. Whether the economics eventually support broader access depends on battery cost trajectories, infrastructure amortization, and operational experience no one has yet accumulated.

The Competitive Landscape

Archer Aviation’s Midnight is pursuing a similar timeline and mission profile, backed by United Airlines. Wisk Aero, backed by Boeing, is developing an autonomous eVTOL without a pilot in the seat - a longer-term bet that the FAA will eventually write a regulatory framework for certifying passenger-carrying autonomous operations, a framework that does not currently exist.

Why This Matters for Working Pilots

A new certificated aircraft type operating in commercial air service requires a new type rating. The training infrastructure for the S-4 - simulation standards, checkride requirements, abnormal and emergency procedures - does not yet exist. When the type certificate issues, the FAA and Joby will build it. That represents a real professional opportunity: new category, new aircraft type, new trained pilots required.

The overall picture as of late 2025: Joby Aviation is further along than any other company in this space. The certification progress is documented and real. The Part 135 certificate is real. The industrial partnerships are real. The engineering problems solved - transition flight stability, distributed motor failure management, acoustic optimization - were genuinely hard.

The remaining problems are also real. Stage 5 testing. Vertiport infrastructure. Low-altitude airspace management. Operational economics under airline utilization cycles. None are small. The S-4 has moved definitively past concept renders and investor presentations. It has not yet entered service. The distance between those two points is where the actual work is happening now.

Key Takeaways

Joby Aviation completed Stage 4 of five in its FAA type certification process; Stage 5 - final testing under direct FAA oversight - remains before the type certificate issues.
The S-4 uses a Special Class certification under 14 CFR §21.17 Subpart B, with airworthiness criteria negotiated directly between Joby and the FAA since approximately 2015.
Joby’s Part 135 Air Carrier Certificate (issued 2023) means the operational infrastructure will be ready the moment the type certificate arrives.
The safety architecture depends on six independent electric motors and a flight management computer that compensates for single-motor failures in milliseconds - before a pilot can register the event.
Infrastructure (vertiports, airspace management, charging networks) and operational economics are the unresolved challenges that engineering alone cannot close.