Joby Aviation's S Four and the Type Certificate That Had to Be Invented From Scratch

Joby Aviation’s S4 electric aircraft is in the final stages of an FAA type certification process so novel that neither the aircraft nor the rulebook existed before this program began. The S4 fits no existing regulatory category, so Joby and the FAA built a new framework together - one that will serve as the template for every eVTOL that follows.

What Is the Joby S4?

Joby Aviation was founded in 2009 by engineer JoeBen Bevirt in Santa Cruz, California. For most of the following decade, the company worked without press releases or venture capital headlines, accumulating actual flight data and test hours. When Joby emerged publicly around 2020, it brought real hardware and real test records, not renderings.

The S4 is a six-motor, six-rotor electric vertical takeoff and landing (eVTOL) aircraft. It can take off and land like a helicopter and cruise like a fixed-wing airplane - which is both the engineering achievement and the certification challenge.

How the S4’s Tilting-Rotor Design Works

Each of the S4’s six electric motors is paired with a propeller that can rotate from pointing upward (for hover) to pointing forward (for cruise). In hover mode, all six rotors generate vertical lift. Once airborne, those rotors tilt forward, the fixed wing takes over generating lift, and the aircraft transitions to conventional winged flight.

The efficiency gain in that transition is the core engineering argument. Hover is energy-intensive - every pound of lift comes from rotors pushing air downward. In cruise, the wing does the lifting and the rotors only provide horizontal thrust. The energy math improves dramatically.

Why the S4 Is Quiet Enough to Fly Over Cities

Noise is arguably the most important number in the eVTOL business case, and the S4’s design addresses it structurally. When the wing carries the lift load in cruise, the rotors work at lower power - lower power means lower tip speeds and lower acoustic output.

Joby has demonstrated approximately 65 decibels overhead in cruise testing. A helicopter at comparable altitude and distance typically produces 80 to 85 decibels. Because decibels are logarithmic, that 20-decibel gap represents the helicopter producing roughly 100 times the acoustic energy of the S4 in cruise. The S4 is quiet enough to fly over residential neighborhoods without generating noise complaints - which fundamentally changes which urban routes are commercially viable.

How Far Can the Joby S4 Fly?

The S4 uses a lithium-ion battery architecture, and Joby has published a maximum range of approximately 150 miles. Commercial routes are designed to stay under 100 miles to preserve energy margins for contingencies.

For a battery-electric aircraft, 150 miles is genuinely impressive. It comes at a cost in battery weight - the S4’s maximum gross weight is approximately 4,400 pounds, and a meaningful fraction of that is battery pack.

Why Six Motors Is a Safety Argument, Not Just an Engineering Choice

The distributed electric propulsion system is the safety case.

There is no hydraulic system, no gearbox connecting a single engine to a rotor, no single mechanical failure point that can bring the aircraft down. With six independent motors, the loss of one leaves five operating. The loss of two still leaves the aircraft capable of controlled flight and a safe landing. The architecture tolerates failures that would be catastrophic in a single-engine helicopter, and that failure tolerance is the central thesis of the entire certification campaign.

Why the FAA Had to Invent New Certification Standards

The S4 doesn’t fit any existing regulatory category. It isn’t a normal-category airplane (it takes off vertically). It isn’t a rotorcraft (it has a fixed wing and transitions to winged flight). It isn’t a powered-lift aircraft under the traditional rules, which were written in the 1980s with tiltrotors like the V-22 Osprey in mind and don’t map onto a six-motor electric aircraft.

So the FAA and Joby built the certification basis from scratch.

What Is a G-1 Issue Paper and Why Does It Matter?

The document that establishes what an aircraft must prove to earn a type certificate is called the G-1 issue paper. It defines which standards apply, how they’re interpreted, and what acceptable means of compliance look like for the specific design. Joby’s G-1 was formally accepted by the FAA’s Aircraft Certification Service in 2021. That was the starting gun.

The G-1 was not a single document resolving a single question. It opened more than 130 individual issue papers, each addressing a specific regulatory question that had never been formally posed before. Questions like: what performance margins are required after losing two motors simultaneously? How do you define “continued safe flight and landing” for an aircraft that can hover, transition, and cruise? What does flutter analysis look like for a tilting rotor that changes its aerodynamic role between flight phases?

Each issue paper is a small regulatory negotiation - not passing a test that already exists, but helping write the test, then demonstrating compliance with the standard just defined. Every answer Joby and the FAA reach becomes part of the regulatory foundation that the next eVTOL applicant will inherit. Joby has largely been funding that infrastructure at its own expense.

Where Joby’s FAA Certification Stands Right Now

The FAA organized Joby’s certification into five stages:

Stages 1–2: Establish the certification basis and conduct design review
Stage 3: Ground testing, including structural validation and systems integration
Stage 4: Formal flight test compliance (current)
Stage 5: Final conformity inspection and certificate issuance

Stages 1 through 3 are complete. Stage 4 is the systematic, test-point-by-test-point demonstration that the S4 meets every requirement in the negotiated certification basis. The aircraft conducting Stage 4 testing is a production-conforming prototype - built to the same manufacturing processes and tolerances a production line would use, because the FAA is certifying the design, not just one airframe.

Joby has accumulated more than 30,000 miles of test flights across multiple aircraft over the course of the program.

External Validation: The Air Force and Toyota

Two external parties have made substantial commitments that carry their own weight as technical endorsements.

The U.S. Air Force, through its Agility Prime program, funded early-stage development and later took delivery of Joby aircraft for operational evaluation at Edwards Air Force Base in California. Getting on the ramp at Edwards requires passing a military technical review - Joby passed it.

Toyota has invested over $400 million in Joby. Beyond the capital, Toyota brings manufacturing expertise and supply chain management at industrial scale. The question of whether Joby can eventually produce these aircraft at volume is partly answered by who is in the room helping design the production system.

When Will Joby Begin Commercial Operations?

The honest answer involves some date slippage worth naming directly.

Joby originally targeted a type certificate and commercial operations in 2024. That didn’t happen. The target shifted to 2025. As of the most recent available information, the type certificate looks achievable in that general timeframe - but commercial revenue service requires considerably more than a type certificate.

After the type certificate, Joby needs an operating certificate under Part 135, which governs air taxi and on-demand commercial operations. That is a separate FAA approval process covering operations specifications, dispatch procedures, maintenance programs, and crew training. There are currently no type-rated Joby S4 pilots anywhere in the world - the training curriculum, simulator program, and evaluation standards all have to be built and approved from scratch.

Physical infrastructure also doesn’t yet exist at commercial scale. eVTOL operations require vertiports - charging stations, passenger facilities, operations centers - that are in development but not yet operational.

The aircraft is real. The flight test data is real. The certification progress is real. Joby has been building and flying hardware toward the same stated mission for 16 years without changing course. In a field where several high-profile competitors have filed for bankruptcy, pivoted their technology, or quietly stopped flying, that consistency is notable. It is not a guarantee of any specific commercial launch date.

What Joby’s Certification Means for the Broader eVTOL Industry

The regulatory framework Joby has been building with the FAA is not exclusive to Joby. The 130-plus issue papers, the means of compliance for distributed electric propulsion, the performance standards for multi-mode aircraft - these are becoming the template for every subsequent eVTOL type certificate application. The next company applying in this category will have a shorter path, in part because Joby walked the first one.

What This Means for Pilots

The production S4 is a piloted aircraft. The pilot manages the mission, monitors systems, communicates with ATC, and holds pilot-in-command authority and responsibility. The flight control computers manage individual motor outputs continuously in real time - a task beyond any human’s ability to execute manually - but the automation is not flying autonomously. It is managing the physical complexity that makes the aircraft controllable. The pilot is still making the decisions that matter.

The pilots who first build type ratings in this aircraft will be working from a blank sheet. Energy management discipline is different from anything in the current fleet. Building a mental model for a vehicle that operates in fundamentally different modes across flight phases - hover, transition, cruise - represents a new kind of professional knowledge. The people who develop it first will have outsized influence on how this category of aviation develops over the next several decades.

Key Takeaways

The Joby S4 is a six-motor, tilting-rotor eVTOL with a maximum range of approximately 150 miles and a cruise noise level of roughly 65 decibels - the helicopter at comparable distance produces roughly 100 times the acoustic energy.
Because the S4 fits no existing FAA certification category, Joby and the FAA have jointly authored more than 130 individual issue papers since the G-1 basis was accepted in 2021, creating regulatory precedent for the entire eVTOL industry.
Joby has completed FAA certification Stages 1 through 3 and is currently in Stage 4 flight test compliance, with more than 30,000 test miles accumulated across the program.
A type certificate does not equal commercial service - Joby still needs a Part 135 operating certificate, approved crew training, and vertiport infrastructure before revenue flights can begin.
Every regulatory standard Joby negotiates with the FAA becomes the foundation for the next eVTOL applicant, meaning Joby is effectively building a new category of aviation certification at its own expense.