Charles Alcock is managing editor of the AIN Media Group, having rejoined the company in 2019. He has been an aviation journalist since 1986, and previously worked for AIN from 1990 to 2017, most recently as editor-in-chief. His main focus is on covering new aviation technology and business models, as well as international news.
Having been raised in the London area, Alcock studied at the University of Manchester in the UK and earned an honors degree in Politics and Modern History. He speaks English and French, and some German.

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Helios Horizon is looking to take electric aviation quite literally to the next level as it steps up its efforts to prove the efficacy of operating at altitudes as high as 44,000 feet. The U.S.-based start-up is preparing to resume flight testing in the fall with a Pipistrel Taurus light aircraft converted to use its battery electric propulsion system in a bid to reach the higher flight levels it says will deliver longer range.
Flights conducted near Bishop, California, in April and May saw the aircraft expand its envelope from 17,500 feet to 24,000 feet, using no more than 60% of its battery capacity. The Helios Horizon team, which includes veterans of green aviation projects such as Airbus’s Perlan, Solar Stratos, and Solar Impulse, is aiming to progressively fly higher and reach stratospheric altitudes by early 2026.
According to Helios Horizon founder and chief test pilot Miguel Iturmendi, the main technical challenge is the thermal control of the propulsion system. “It is important to never let the heat build-up to start with because it is hard to dissipate this at altitude due to the lack of oxygen,” he told AIN.
In Iturmendi’s view, temperature control remains a common problem for electric aircraft developers. He maintained this is especially challenging for eVTOL vehicles during the critical transition phase of flight and has not yet been adequately addressed by the advanced air mobility sector.
So far private donors and foundations have funded efforts to achieve a leap forward in the range of light aircraft. Helios Horizon is now in talks with potential corporate sponsors for further flight testing but has no specific plans to bring the technology to market.
“I’m not an entrepreneur, and this is really just a technology demonstration to show what is possible in terms of range and endurance,” Iturmendi said. “Probably around 90% of people don’t even think electric aviation is possible.”
<h2>Longer Range Light Aircraft On the Horizon</h2>
The converted Taurus aircraft, which is in the 500 kilograms (1,100 pounds) weight class, is already flying around four times higher than the standard model. “We could soon have a light sports aircraft that can fly for four or five hours at maybe 90 to 100 knots, so around the same or better than a combustion engine but without the emissions and with fewer overhauls needed,” Iturmendi said.
Anticipating further improvements in battery technology to an energy density of around 400 watt-hours per kilogram, Helios Horizon believes light aircraft could fly as long as six to seven hours, with a range of up to around 700 nm. For the next round of flights, it is modifying the airframe and extending the wing for the next round of flights.
Iturmendi pointed out that the purpose-built aircraft used for the Solar Stratos and Solar Impulse aircraft never flew above 15,000 feet and 30,000 feet, respectively. For the Perlan 2 project, he flew at 65,000 feet. He has also worked as a test pilot for the Honda Jet, Aurora Odysseus, the Naval Research Laboratory, and Universal Hydrogen. In 2019, he was inducted into the Living Legends of Aviation and holds four world records for altitude and speed.
Helios Horizon makes its own batteries using materials provided by suppliers in China, South Korea, Germany, and the U.S. The current configuration consists of three sets of lithium-ion polymer pouches each with a nominal rating per cell of 3.72 volts and 55 amps. The company is planning to increase each cell to 60 amps.
For the stratospheric flights in the unpressurized Taurus, Iturmendi has designed and made his own partial pressure suit, which has been tested by the University of North Dakota. Since the aircraft has a motor driver, he explained, running out of battery power is not a concern as the pilot can windmill the propeller and charge the battery while descending to land.
“As the entire aerospace industry struggles with its goals to decarbonize aviation, I hope Helios Horizon sends a clear signal that zero-emission flight doesn’t have to be an obstacle to achievement,” Iturmendi said. “If our small team can develop an electric airplane that flies twice as high as anything comparable in size the industry has developed, what does that say about what the major aircraft manufacturers should be capable of?”

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