A solar-electric Zephyr approaches AALTO’s facility in Kenya for a landing after a 13-day stratospheric flight, the first with this particular prototype. Credit: AALTO
Losing a prototype is never the desired outcome of a flight test — much less the loss of two within three years.
That’s the position in which Airbus subsidiary AALTO found itself on April 28, after the company’s latest Zephyr 8 had to be ditched into the Indian Ocean for reasons the company is still investigating. The ultralight solar-electric aircraft had just completed its 67th day aloft in the stratosphere, surpassing the record set by the previous prototype in 2022.
On the face of it, the outcome appears to cast doubt on the feasibility of the long-duration stratospheric flights targeted by AALTO and other developers of HAPS, variously spelled out as high-altitude platform station, or high-altitude pseudo satellite.
But far from it, according to my interview with AALTO CEO Hughes Boulnois and Chief Technology Officer Pierre-Antoine Aubourg. The company has yet to achieve its target of 200 days of continuous flight needed for mobile connectivity and Earth observation services, among other business cases. Despite that, Boulnois and Aubourg are confident that AALTO has learned enough about its featherweight flyer’s handling characteristics and remote piloting requirements to press ahead with the next step toward beginning commercial operations in 2026.
“We have recently signed new contracts with our customers to fly to Japan, from our AALTOport in Kenya, to deliver critical stratospheric demonstrations there later this year,” Boulnois says. “We are also looking to start building an additional AALTOport in the Asia-Pacific region that would help us intensify our commercialization effort there in 2026.”
While AALTO has yet to complete an end-to-end long-duration flight, the executives said that the 67-day flight demonstrated substantial improvements in performance, thanks to new flight control laws written into the avionics software after the 2022 flight. That aircraft crashed after 64 days, hitting severe turbulence in a storm as it descended from the stratosphere to the much denser troposphere.
One of the major challenges for HAPS like Zephyr — which has a wingspan of 25 meters but a mass of only 70 kilograms — is that these designs fly well in the rarified air of the stratosphere, above the weather, but not in the unforgiving turbulence of the troposphere below. But there’s a twist to this, too: At the equator, the southern and northern hemisphere trade winds combine to form high-altitude storms in a region known as the ITCZ, or Intertropical Convergence Zone. Here, the resulting turbulence even rattles the stratosphere’s low-density air.
“The stratosphere is impacted by the ITCZ, and it creates dynamic weather and a turbulent environment there. And that is the enemy of HAPS, where, critically, we are trying to survive day and night,” says Aubourg.
But the new control laws proved a capable match for the ITCZ, he says, by giving the aircraft automated, active control of surfaces including its elevator and rudder. Also key are Zephyr’s two motor-driven propellors, which the new flight laws can command independently. This differential thrust allows the aircraft to dynamically compensate for some types of airframe-stressing turbulence, Aubourg says.
After taking off from the Kenya AALTOport in February, Zephyr crossed the ICTZ twice during its two-month sojourn across the Indian Ocean. “And being able to cross the ITCZ is a target for us commercially as we need to operate our HAPS anywhere in the world — so it was critical for us to prove the capability to cross it. The latest flight demonstrated our ability to survive in these two adverse environments,” Aubourg says.
To that end, AALTO is preparing for a series of demonstration flights for Japanese telecommunications company NTT DoCoMo, which last year invested $100 million in AALTO. New Zephyr prototypes are to fly from Kenya to Japan and back, carrying direct-to-smartphone and Earth observation payloads that turn the aircraft into “nonterrestrial networks,” or cell towers. NTT envisions these being particularly useful in disaster relief situations when ground networks are downed by tsunamis, earthquakes or typhoons.
In the meantime, the investigation into last month’s controlled descent continues. Among those who will be particularly interested in the results is Paul Stevens, CEO of Voltitude, a Farnborough startup that is developing a raft of flight envelope expansion technologies for HAPS aircraft. He told me by email that despite its unfortunate ending, AALTO’s latest flight was “an amazing achievement” that demonstrates “the viability of fixed-wing, solar-powered stratospheric flight.”
But he also acknowledged that the technology still has a ways to go: “The final technological hurdle for HAPS is enhancing their availability and reliability, especially during take-off and recovery. Navigating through the troposphere poses significant hazards and represents the next challenge the industry must solve to ensure safer and more frequent transitions to and from the stratosphere.”
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