Skai Vehicle Basics
Electric vertical takeoff and landing vehicle
On-demand air mobility
Piloted, ground-piloted and fully autonomous flight
Triple-redundant autopilot system
Redundant 6-rotor propulsion system
Redundant hydrogen fuel cell system
Flight duration: up to 2 hours
Payload: up to 1,000 lb.
Speed: up to 115 mph
Passengers: up to 5 people
Refueling time: less than 10 minutes
Cleanest end-to-end transportation system
Hydrogen fuel cell-powered electric
Emissions of pure water
Efficient, long-lasting fuel cells: 10K - 15K flight hours of use
95% reusable, 99% recyclable fuel cells
Hydrogen fuel produced using renewable energy
Significantly lower impact than lithium-ion or hybrid solution
Fault tolerance allows a system to continue functioning properly in the event that one or more of its components fail. This is in contrast to poorly designed systems in which even a small problem can cause complete breakdown. Fault tolerance is particularly important in systems that protect human life, like passenger vehicles, medical technologies, transportation infrastructure and other critical services.
NASA was at the forefront of fault-tolerant design in the 1960s through computing systems with the acronym “LLNM” (long life, no maintenance). The Voyager program, launched in 1977, used a computer that had the ability to detect its own errors and activate backup systems when needed. The system is still working more than 40 years later.
Redundancy is a key aspect of fault tolerance – essentially backup components that keep the system operating in the event of a failure. Skai was designed using a fault-tolerant approach, through multiple levels of redundancy across all major components.
Skai features triple redundancy to ensure safety and peace of mind on every ride. While any single failure is unlikely, true safety is about planning for worst-case scenarios. Here’s how Skai’s triple redundancy works:
Rotors: Any one of the 6 rotor motors or 6 motor controllers can fail, and the vehicle can continue to its destination safely. Even if two rotors or motor controllers fail, the vehicle can land safely.
Fuel Cells: Any one of the three fuel cells can fail and the vehicle can continue to its destination. Even if all three fuel cells fail, the vehicle can land safely using stored power from a large capacitor.
Autopilot System: In addition to autonomous flight, the onboard autopilot system manages the rotor system to ensure a smooth, stable ride. There are 3 redundant flight computers and 3 “voting” computers that analyze the autopilot decision-making and determine the optimal choice. Any two of either set of systems can fail, and the vehicle can safely continue to its destination. Note that component failures are exceedingly rare – nevertheless, the system is comprehensively backed up to handle even the most unlikely failure scenario.
The previous generation of flight controls, called Fly-By-Wire (FBW), sent information to aircraft components via electric signals running on metallic wires. Skai is equipped with next-generation Fly-By-Light (FBL) controls that use fiber-optic cables, which improve on the FBW system in several ways.
Electromagnetic Immunity: FBL controls are not adversely affected by lightning strikes or by electromagnetic interference, such as the type that transmission towers might produce.
Higher Bandwidth and Redundancy: FBL delivers higher bandwidth and signal redundancy, meaning more information can be transmitted more dependably. Unlike Fly-By-Wire, information can be sent “bi-directionally," meaning system control computers can both send and receive information at the same time.
Weight: FBL controls are significantly lighter than the previous generation, crucial when it comes to flight.
Automatic Dependent Surveillance - Broadcast (ADS-B)
Skai is equipped with ADS-B, a key part of the FAA Next Generation Transportation System. ADS-B is a way for an aircraft to determine its exact location and transmit it, allowing it to be tracked in real time by air traffic control (ATC) and other aircraft.
Currently, radars can take up to 12 seconds to update an aircraft's position. ADS-B equipment provides ATC with updated aircraft information almost every second. This increases visibility and reduces any risk of collision, even in regions without radar coverage.
In addition, ADS-B can provide traffic and weather information from governmentally certified sources – essentially, the pilot can see what ATC sees, for improved situational awareness. Beginning January 1st, 2020, all general aviation aircraft in the US will be required to use ADS-B in most controlled airspace.
Notably, Skai Director Dr. Bruce J. Holmes was a member of the team that developed Automatic Dependent Surveillance - Broadcast (ADS-B) technology, earning the Collier Trophy from the US National Aeronautic Association.
Mode-S is a system that allows better tracking of aircraft along with key information including altitude, speed, bearing and flight status. It’s a secondary surveillance radar process, essentially a transponder that communicates with air traffic control (ATC) using a 24-bit code to identify the aircraft.
Mode-S works with ADS-B to deliver information to ATC as well as to other pilots to increase situational awareness. Importantly, it provides coverage in regions without radar coverage, or at altitudes that are lower than the radar ceiling.
Synthetic Vision System (SVS)
Skai’s avionics include synthetic vision capabilities. SVS uses computer graphics to represent the real-world environment that an aircraft is flying through – in other words a 3D display system that shows the terrain outside of the aircraft. SVS increases the pilot’s situational awareness and accurately depicts the environment when visibility is low. It’s particularly important for ground-based Pilot-In-Command (PIC) flight.
The system enhances flight safety by combining accurate topographical maps (as an animated image) synced with GPS to give the pilot a clear visualization of the world around them. The image is typically rendered as a simulated “Highway in The Sky” (HITS) pathway, the same perspective one might see on any computer flight simulator.
Like any modern vehicle, Skai is equipped with a combination of sensors to ensure safety and maintain persistent, proactive awareness of any potential risks. A key system is the “sensor fusion” of LIDAR and cameras.
LIDAR: In addition to a range of safety avionics that reduce the possibility of a collision, Skai is equipped with LIDAR (Light Detection and Ranging) to detect anything in its path. The system works by firing thousands of laser pulses every second, and then measuring how long it takes for the pulses to bounce back. In this way, the system can “see” a 3D image of an object. A key advantage of LIDAR over RADAR is its ability to detect smaller objects, such as power lines.
Camera Sensors: Camera sensors are used in most modern vehicles to detect a variety of potential hazards. Many autonomous vehicles have camera systems that can identify different signage and landmarks. Skai’s camera sensors are primarily designed to identify obstacles in the vehicle’s path and in potential landing areas. Advanced algorithms interpret video input to identify any anomalies that might adversely affect the safety of the vehicle.
Fuel System Safety: Sensors
Pressure relief devices (PRD) are designed to protect fuel systems from burst failures caused by overpressurization.
While the liquid hydrogen in Skai’s fuel tanks is at a low pressure (approximately 50 PSI), the PRD is in place to safeguard against hydrogen gas forming and creating overpressurization in the tanks. Because hydrogen can embrittle some metals, the tanks and PRDs used for hydrogen applications are specially made. In the event that the pressure in the tank exceeds safe levels, the PRD valve opens to safely release non-toxic, non-corrosive gas. In fact, hydrogen is so light that it will ultimately escape the earth’s atmosphere into space.
Skai’s safety-first approach means planning for highly unlikely, worst-case scenarios – enter the airframe-mounted parachute. For its deployment to be necessary, multiple redundant systems would need to cease operation.
The airframe-mounted parachute is a trusted fail-safe. This system has been in use for decades, and has been installed on more than 35,000 aircraft.
The system is straightforward – in the event of an emergency, the chute is immediately deployed as the rotors wind down. Note that the placement of the parachute and cables prevents contact with the rotors. A speed-sensing slider controls how quickly the chute is deployed to prevent undue stress on occupants or the chute itself. The chute then fully inflates, and the vehicle is lowered to the ground. A particular goal of the parachute system is to prevent back injuries – even in the harshest test conditions, load on touchdown has been shown to be well within tolerances to avoid spinal compression injuries.
Skai can be flown by a pilot, and is currently fully capable of safe autonomous operation. However, until regulations catch up, the system will be piloted by a human being. In some cases, the pilot could be flying from a ground-based station.
This "Pilot-In-Command" (PIC) communicates with the avionics suite and flight computer over a tactical data link that uses military-grade encryption. This data link is highly secure and delivers ultra-low latency for responsive, immediate pilot control. PIC is itself a redundant safety system. In the event that an onboard pilot loses consciousness, the remote PIC can take command and safely land the vehicle to safety. In the event of multiple data link failures, the flight computer will take over and fly the vehicle autonomously to a safe landing area.
Skai is designed for fully autonomous operation, allowing an unprecedented level of freedom for everyone to go from A to Anywhere. Without the need for certified pilots, Skai becomes more accessible for broader utilization around the world.
Autonomous flight is a significantly more mature technology than automotive autonomy (self-driving cars), with decades of autopilot use and optimization. In addition, airborne autonomy benefits from a less challenging environment than roads. Air mobility vehicles operate in “3D” space, with more room to maneuver than cars that are locked into a dynamic and dangerous “2D” road environment. There are also fewer unexpected hazards in the air than on the roads – for example, there are no pedestrians wandering into traffic.
Despite Skai’s autonomous-ready status, it may take some time for regulations to be developed by the FAA to allow this capability to be used by the general public. In the meantime, both onboard or remote ground-control pilots can control the vehicles.