The Future of Advanced Air Mobility

Are Parachutes the Right Safety Backstop for VTOL Aircraft Safety Redundancy?

Safety redundancy to protect against possible system failures in new vertical takeoff and landing (VTOL) aircraft will be critical to getting them certified for use and accepted by customers. Some manufacturers are planning to provide an extra layer of protection in the shape of a ballistic parachute designed to land the aircraft safely on the ground in the event of loss of control. It’s an approach that is already in use with Cirrus’s popular single-engine, fixed-wing aircraft.

When triggered, a firing mechanism launches the ballistic parachute to help it deploy more quickly than a passively activated parachute otherwise would. That’s important for VTOL aircraft, which will spend significant time flying at altitudes and speeds where traditional parachutes may not have time to activate before impact. That consideration is especially important in the hover phase around takeoff and landing, during which there is likely to be little or no momentum to speed deployment.

Transcend Air one fifth scale prototype
The Transcend Air one-fifth-scale prototype.

Transcend Air is relying heavily on ballistic parachutes developed by BRS Aerospace to protect its Vy 400 aircraft, according to COO Peter Schmidt. Six years into a 10-year development cycle, Transcend is currently flying one-fifth-scale prototypes of the vehicle that it plans to use primarily for city-to-city business travel.

Schmidt specifically points to Cirrus as having validated the use of ballistic parachutes through its Cirrus Airframe Parachute System. According to Cirrus, 142 people are alive today specifically thanks to the use of the parachute.

“Cirrus has a 20-year operating history with piston singles,” Schmidt said. “After they retrained the pilot community to go to the parachute as an early resort, not a last resort, their fatality rate became half that of the fleet at large. When a Cirrus airframe parachute has been activated in its deployment envelope, no one has ever died or been seriously injured. When used outside [the] envelope as well, looking at all accidents, deployment reduces the risk by a factor of 13.”

According to Schmidt, the parachute supports Transcend’s entire business case. The company believes that it can achieve the same level of safety redundancy with its Vy 400 tiltrotor design as that of Leonardo’s far more complex AW609 tiltrotor. The Vy 400 is expected to be priced at around $3.5 million.

The Case Against Parachutes

However, there’s a counter-argument that the use of parachutes is not appropriate in all circumstances. Specifically, say critics, since an aircraft suspended from a deployed parachute can be blown off course by wind this might pose dangers for VTOL aircraft and cause collateral damage

In its development work for a planned new eVTOL aircraft, Airbus has been using ballistic parachute technology—also developed by BRS—for its single-seater Vahana demonstrator. Zach Lovering, the company’s vice president of urban air mobility systems, says that the use case for the technology is clear at least for the development phase of the program, but maybe not so clear for regular operations.

“We’re doing flight testing, so it’s pretty much [needed for] all fields, so we don’t have to worry about what’s underneath us,” he said. “When we start thinking about urban missions […] you are maybe going to be helping the people on board, but you also have to worry about the people on the ground, buildings; you can imagine hitting the side of a skyscraper and tumbling after that. It makes the conversation quite tricky.”

Airbus has not yet decided which of its aircraft might be equipped with parachutes in the future, or who will provide that technology, but Lovering said it could be “vehicle- and mission-dependent.”

BRS founder Boris Popov thinks it’s pretty simple. “Any structure that’s in the air needs a brake,” he said. “I always tell people, it’s like driving your car down the freeway: 99 percent of the time, you don’t need the brakes, but the 1 percent of the time you do, it can be deadly.”

Parachutes are right “for everyone” as a last resort, he said, because there’s no algorithm or technology that will preclude their need in every situation. “You’d be foolish not to develop as much redundancy as you reasonably can…and then use the parachute to close any gap that remains,” he insisted.

BRS is working on a high-loading, rapidly opening “smart chute” for the VTOL market. Popov said it must sense speed and altitude and know not to open during cruise but be able to deploy quickly in the event of an emergency. Amortizing research and development and paying for the new features will have a cost, he said, but it won’t be “a significant increase.” In his view, the market will support the cost of parachute systems up to around 20 percent of the value of the airplane.

While BRS is working on using built-in technology to activate its parachutes quickly, Popov also advocates avoiding vertical takeoffs whenever possible.

“There’s no reason to take off vertically if you don’t have to,” he said. “Just the fact that you’re getting some forward direction, some glide angle…a lot of people are converting from VTOL to almost-VTOL, and if you can take off at a 45-degree angle, you’ve significantly increased your chances [of survival.] There’s a gray area there that you and I can probably survive a malfunction at 50 feet off the ground, without a parachute, with airbags…Between 50 and 150 [feet], there’s no way the parachute opens.”

Bell Nexus at NBAA 2019
A life-size mockup of the Bell Nexus at NBAA-BACE 2019. (Photo: Mariano Rosales)

Bell decided not to use a parachute for its Nexus eVTOL aircraft. Paying a weight penalty for something that will only help in certain situations—and even then, is a risk given the urban environment the company wants to operate in—just doesn’t make sense, according to Scott Drennan, the company’s vice president of innovation.

“With vehicles at air taxi size, 6,000 to 7,000 pounds, you couldn’t put in a parachute system that allowed full controlled descent into that context that would ever really be safe for the people down below,” he said. “I know there are systems out there that claim it’s powered and controlled. They use thrusting mechanisms to keep the flight path they want, but—gosh—tall buildings, cars, people—I just don’t feel comfortable with it.”

Drennan also said that passengers who are on a vehicle during a parachute landing would “never return and get back on the aircraft,” regardless of injury, so Bell wanted to find gentler ways to assure safety.

Instead, the Nexus reserves a portion of battery power—enough for about four minutes of flight, equivalent to autorotation on a Bell 407 helicopter—to use only in the event of an emergency landing. Thanks to increased automation, Drennan said the onboard computer and pilot should know where its emergency landing spot would be at any given point during a route so that contingency plan simply kicks in in an emergency.

If Bell had used a parachute on the Nexus, it would have been an additional redundant system—with a weight penalty—that didn’t allow the company to remove any parts from its criticality list. Drennan cautioned against using that “get out of jail free card” to strip away systems that are important for reliability and safety. “It can be dangerous if you’re looking at redundancy as a panacea to what is fundamentally a reliability question,” he said.

Among Uber partners, Bell is relying on the Nexus’ batteries, Pipistrel will use the 801’s battery packs and the fixed-wing aircraft’s ability to glide and Jaunt will use the slowed-rotor capability it acquired from Carter Aviation to facilitate safer autorotation on its ROSA design. That said, Uber engineering eirector of aviation, Mark Moore, is a fan of ballistic parachutes and has suggested that Uber’s partner companies look into their applicability.

“We’ve gone to makers of systems as well as companies who don’t currently and are encouraging them to make them—encouraging them to find ways to make them workable across the entire envelope of our mission,” he said.

According to an Uber study of five different aircraft developers, parachutes can currently be effective at 200 feet of altitude with no airspeed, Moore said, though he wants to beat that performance. To that end, Uber is also looking at alternatives to parachutes, including a retrorocket solution.

“I know it sounds farfetched, but some large aerospace companies are starting to think it’s feasible,” Moore said. The idea is that several small, “smart” rockets positioned around the aircraft would use the vehicle’s sensors and flight controls. In the event of an emergency, they would fire to enable a safe landing reminiscent of Space-X’s reusable Falcon 9 boosters—ideally, when they land correctly, and not when they crash.

A good retrorocket solution would need to be lighter and provide better survivability than a parachute, Moore said. The bigger issue might be price, but since the new parachutes would need to be smart and have active-control devices, retrorockets that tie into existing sensors might not cost much more.

At the Revolution Aero conference held in San Francisco in late September 2019, U.S. company Aviation Safety Resources won a "shark tank" style competition for its pitch to develop the Xtreme Rapid Deployment parachute system for eVTOL aircraft. The concept involves rockets and three separate chutes and ASR believes the system could weigh no more than around 100 pounds.

At this stage in the development of the urban air mobility industry, there is no clear consensus solution for different problems. Manufacturers are trying different things and suppliers are attempting to create or adapt technologies that better fit needs. They’ll have to work together with the Federal Aviation Administration, the European Union Aviation Safety Agency and other regulators to determine the favored way of keeping people and vehicles safe. One thing industry experts do agree on is that safety is paramount, and early missteps on that front may permanently undermine efforts to win public acceptance.