Can these self-flying planes transform the skies?

Recently two staff members watching on monitors tracked the flight path of a small airplane flying itself above northern California, about 40 miles away. The Cessna Caravan had a pilot aboard on standby, along with a flight-test engineer, but was otherwise on its own. They were testing a new collision-avoidance system and new radar that helps the plane see and avoid other aircraft.

If all goes as planned, Reliable Robotics eventually won’t need any pilots on board its planes. It is among startups vying to revolutionize air travel by doing away with the need for human pilots on cargo, military and maybe even passenger aircraft. Autonomous flight is being tested at companies from giant aircraft makers to small startups.

For now, cargo and military flights are the focus. Reliable recently signed a $17 million contract with the U.S. Air Force that involves testing autonomous cargo flights. Flying-taxi maker Joby Aviation also recently tested its own pilotless Cessna for the Air Force over the Pacific Ocean.

Reliable Robotics is among the startups automating flight by doing away with the need for human pilots on cargo and military aircraft.

Boeing and Airbus, the world’s leading makers of large commercial jets, have separately worked on introducing more automation in their aircraft, and both say the aim is to improve safety. The two companies have also examined fully autonomous flight, but say they aren’t working on commercial-jet designs that don’t include pilots in cockpits.

Truly autonomous flight with human passengers will face a host of technical and regulatory hurdles, and—perhaps the most difficult of all—a tough sell to travelers.

Pilots, the faces and voices of safety for the flying public, reassure during startling turbulence or other in-flight scares. Their presence conveys someone is there to save the day.

“I don’t think we’re going to go full autonomy passenger operations immediately," says Robert Rose, Reliable’s chief executive. “And the reason for that has more to do with passenger acceptance than it does the technology."

Advances in technology over past decades have already reduced the number of people needed on the flight deck. Aviation industry officials credit automation with steady improvements in safety, including new cockpit features that have reduced pilots’ workloads and helped them avoid potentially catastrophic slip-ups. Commercial aircraft can already essentially fly themselves while on autopilot in normal conditions and practically land automatically, though pilots must plug inputs into the automation, closely monitor cockpit systems and stand by to take over flying manually if needed.

Earlier in the jet age, more pilots were on the flight deck, along with navigators and flight engineers. In recent decades regulators have generally required two pilots in the cockpits of commercial flights, one to fly and the other to monitor the operation. The Air Line Pilots Association labor union that represents aviators at U.S. and Canadian carriers argues that safety depends on having a pair of well-trained, well-rested pilots ready to intervene with cockpit automation and jointly resolve emergencies like engine failures.

An Airbus project has centered on potentially reducing the number of pilots needed in the cockpit to one during certain phases of flight. It is called extended minimum crew operations, or eMCO. Airbus says its aim is to reduce fatigue by building cockpit features that would allow pilots to rest while one of them flies during certain phases of cruise, the high-altitude portion of flight.

The European plane maker has been working on modifying its A350, a large wide-body jet used on long-haul international flights, with the expectation cargo carriers might use such a system first. The systems would involve “smart automation" features that kick in, for instance, when a plane depressurizes—causing it to descend to 10,000 feet, sending an emergency transmission to air-traffic control and avoiding other aircraft on the way to lower altitude. Airbus said the smart feature is already in service on the A350.

On long-haul flights, the wide-body planes such as the A350 might carry three or four pilots. The eMCO system would require only one in the cockpit during certain periods, while the others can rest in crew-only areas. Airbus’s design and testing has involved alerting resting pilots when the pilot in the cockpit needs help. “It’s going to support the crews, not replace them," says Airbus chief commercial test pilot Malcolm Ridley.

The European Union Aviation Safety Agency also now requires at least two pilots in cockpits of commercial planes. It said a study couldn’t demonstrate that the Airbus system in development was currently as safe as that. “Any consideration of this will depend on the new technologies first proving their safety benefit," said a spokeswoman for the agency.

Boeing long built aircraft with pilots at the center of their design philosophy, de-emphasizing automation. The thinking shifted after a flawed flight-control system overpowered pilots and sent two of Boeing’s 737 MAX jets into fatal nosedives in fall 2018 and again in spring 2019. The accidents also exposed Boeing’s faulty assumptions that pilots could handle such emergencies, leading the company to focus on developing more automation, improved so that it wouldn’t rely on pilots responding perfectly when things go wrong.

A Wisk autonomous electric air taxi, which is being tested with ground controllers instead of a pilot on board.

Boeing’s pilotless flying taxi called Wisk has been a test bed for automation. The plan is to have no pilots on board, but instead use ground controllers to oversee the electric vehicles. Boeing executives have signaled they are focused on designing the company’s commercial jetliners with two pilots in the cockpit while using Wisk’s advances in automation to reduce their workload, so they can make critical decisions faster. “These types of technologies are going to be an incredible enhancement," Jim Webb, Boeing’s chief commercial pilot, said in September at a pilot union conference in Chicago.

Greater autonomy in aviation will become less novel over time—just as with driverless taxis, predicts Eric Allison, chief product officer at flying-taxi startup Joby. “The way these technologies are getting woven in is certainly going to make it much more mundane to interact with, effectively, robots on a regular basis—whether they are robot cars or [robots for home chores] or robot airplanes," he said.

Reliable Robotics’ new autopilot likely at first will involve “safety pilots" behind the controls ahead of further regulatory approvals, says CEO Rose, who previously worked on autonomous technologies at SpaceX and Tesla. Ground controllers will be capable of acting as remote pilots able to intervene, but the system itself will need to be able to resolve in-flight emergencies on its own. Rose says his goal is to win Federal Aviation Administration certification for using fully remote pilots with the company’s planes in 2028.

The FAA declined to comment about Reliable’s specific work but noted the agency laid out a pathway for certifying remotely piloted and autonomous aircraft in 2020.

In addition to software, the company has been working on an onboard radar system so its autopilot can see and avoid other aircraft as pilots do. Collision-avoidance systems on commercial jets typically use a mix of transponders and other sensors, but not radar. They have also been developing stronger actuators—machines that pull and push and take the place of pilots’ muscles to move planes’ rudders and other flight-control surfaces in conditions where humans would otherwise have to intervene.

Reliable Robotics is working on a new radar to help its autopilot see and avoid other aircraft.

Jason Ambrosi, president of the Air Line Pilots Association, predicts autonomous flight won’t be ready any time soon and says he doesn’t think Reliable’s eventual system would apply to larger aircraft. “It’s a robot flying a Cessna Caravan in the desert remotely," Ambrosi says. “It’s a lot different than an A350 or a 777 leaving out of JFK over to Europe."

Rose said Reliable’s system will be designed to operate in busy airport environments, starting with Albuquerque, N.M.

“We’re talking about a certified system that is certified for use in and around populated areas," he says. “We do anticipate many of the lessons learned there to transfer to other operations around the country and much of the world."

Write to Andrew Tangel at andrew.tangel@wsj.com