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DLR Study Reveals New Drivers of Contrail Formation Beyond Soot

DLR research finds volatile compounds and oil vapors contribute to contrail ice crystals despite soot reduction by lean-burn engines using low-sulfur fuels.

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Aviation’s climate impact is heavily influenced by contrail cirrus clouds, which form when hot engine exhaust meets cold, humid air at cruising altitudes. For years, the prevailing scientific consensus held that soot particles were the primary drivers of ice crystal formation in these contrails. However, a new study published in the scientific journal Nature challenges this long-held understanding, revealing that reducing soot does not automatically equate to fewer contrail ice crystals.

According to an official press release from the German Aerospace Center (DLR), recent measurement flights demonstrate that volatile organic compounds and lubricating oil vapors play a crucial role in contrail formation, particularly when aircraft utilize extremely low-sulfur fuels and modern lean-burn engines. The findings stem from the NEOFUELS/VOLCAN project, a collaborative research initiative involving DLR, Airbus, CFM International, and academic partners.

The research highlights a critical gap in current climate models, which may underestimate the environmental impact of contrails by failing to account for ice formation on liquid volatile particles. As the aviation industry pushes toward climate-compatible flight, we expect these insights to shape future engine designs, fuel compositions, and oil venting architectures.

Chasing Emissions at Cruising Altitude

To investigate the emissions of modern lean-burn engines, researchers conducted a series of complex flight tests in the spring of 2023. The NEOFUELS/VOLCAN campaign marked the first time emissions and resulting contrails from a lean-burn engine were measured in flight.

High-Speed Chase Maneuvers

The DLR utilized its Falcon 20E research aircraft to trail an Airbus A321neo equipped with CFM LEAP-1A engines. Over the course of 15 flights, the Falcon 20E performed high-speed chase maneuvers at an altitude of 10 kilometers above the Mediterranean and the Atlantic. The research aircraft sampled the exhaust plume at distances ranging from 40 to 250 meters and intercepted fully developed contrails several kilometers downstream.

By modifying engine control settings, CFM International enabled the researchers to compare emissions under both lean-burn and rich-burn operations. The engines were also tested using fuels with varying levels of sulfur and aromatics, providing a comprehensive dataset on how different variables affect contrail properties.

Beyond Soot: The Role of Volatile Particles

The flight measurements yielded unexpected results regarding the relationship between soot and contrails. While lean-burn operations successfully reduced soot emissions by three orders of magnitude compared to rich-burn conditions, the number of contrail ice crystals remained high.

A Shift in Scientific Understanding

The data indicated that the concentration of ice crystals far exceeded the number of measured soot particles. Instead, researchers observed a massive formation of liquid volatile particles in the cooling exhaust plume.

“The defining moment came when the initial data revealed no soot, but plenty of contrail ice crystals,” said Christiane Voigt, scientific lead of the project at DLR and Johannes Gutenberg University Mainz (JGU), in the DLR press release. “It immediately became clear that advancing our understanding of contrail formation will be essential for shaping the technological future of aviation.”

The study found that when using ultra-low-sulfur fuels, volatile organic compounds and lubrication oil vapors become increasingly significant in the formation of new particles. While lower sulfur content in fuels did reduce the number of contrail ice crystals, the presence of these other volatile elements means that soot reduction alone is insufficient to mitigate contrail-related climate impacts.

Updating Climate Models and Mitigation Strategies

The findings from the NEOFUELS/VOLCAN project extend the classical theory of contrail formation. Because most current climate models do not incorporate ice formation on liquid particles, they likely underestimate the true climate impact of aviation contrails.

Engineering Levers for Climate-Compatible Flight

To address these newly identified drivers of contrail formation, future mitigation strategies will need to look beyond current emission standards, which primarily regulate gases and non-volatile particles. The DLR notes that while current fuel sulfur content is capped at 0.3 percent by mass, with typical levels around 0.046 percent, further reductions may be necessary.

Additionally, optimizing lubrication oil venting systems could provide engine developers with a new engineering lever to minimize volatile particles and, consequently, the climate impact of contrails.

AirPro News analysis

At AirPro News, we note that the aviation industry has heavily invested in lean-burn engine technology as a primary means to reduce soot and nitrogen oxide emissions. However, this Nature study underscores the complexity of atmospheric chemistry and the unintended consequences of optimizing for a single emission metric. If volatile organic compounds and lubricating oils are significant contributors to contrail cirrus clouds, engine manufacturers may need to redesign oil venting architectures, a component previously overlooked in climate mitigation discussions. Furthermore, we believe this could accelerate regulatory pressure to mandate ultra-low-sulfur sustainable aviation fuels (SAF) globally, as traditional jet fuel may no longer align with the industry’s net-zero climate targets once these updated contrail models are adopted by policymakers.

Frequently Asked Questions

What are contrails and why do they matter?

Contrails are line-shaped ice clouds that form behind aircraft at cruising altitudes when hot engine exhaust mixes with cold, humid air. They are a major contributor to aviation’s overall climate impact because they can trap heat in the Earth’s atmosphere.

Did lean-burn engines reduce contrail formation?

While lean-burn engines reduced soot emissions by three orders of magnitude during the tests, the number of contrail ice crystals remained high. This indicates that other factors, such as volatile organic compounds and oil vapors, drive contrail formation when soot levels are low.

How were the measurements taken?

The German Aerospace Center (DLR) used a Falcon 20E research aircraft to fly closely behind an Airbus A321neo. The Falcon sampled the exhaust plume and contrails at distances between 40 and 250 meters during 15 flights at an altitude of 10 kilometers.

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Photo Credit: DLR

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Honeywell and Shield AI Partner on Certifiable Autonomy Stack

Honeywell Aerospace and Shield AI signed an MoU at Farnborough 2026 to develop a certifiable autonomy software stack.

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Honeywell Aerospace and Shield AI signed a Memorandum of Understanding (MoU) on July 22, 2026, at the Farnborough International Airshow to develop a certifiable autonomy software stack for defense and commercial aircraft.

Announced in a joint press release, the collaboration targets a primary hurdle in uncrewed and autonomous aviation: regulatory Certification. By integrating Honeywell’s certified aerospace hardware with Shield AI’s mission autonomy Software, the companies intend to create a standardized foundation that eliminates the need to rebuild and recertify trust architectures for every new aircraft platform.

Integrating Anthem Avionics with Hivemind autonomy

The agreement centers on pairing the Honeywell Anthem avionics, navigation, and sensing portfolio with the Shield AI Hivemind Software Development Kit (SDK). Hivemind provides artificial intelligence-piloted flight capabilities, while Anthem supplies the design-assured hardware foundation required by aviation regulators.

This combined architecture is designed to support both defense and commercial applications, allowing operators to deploy autonomous systems across various uncrewed aircraft platforms without engineering bespoke hardware solutions for each vehicle.

“Honeywell Aerospace’s certified avionics, navigation, and sensing systems paired with Hivemind, which delivers mission autonomy for intelligent, collaborative operations, create a trusted foundation for AI-piloted flight,” said Gary Steele, Chief Executive Officer of Shield AI. “This collaboration reflects how the aerospace and defense ecosystem is evolving, bringing the best of autonomy and certified hardware to address transportation and national security challenges.”

Addressing the certification bottleneck

The aerospace industry faces persistent challenges in certifying artificial intelligence and machine learning systems. Regulators require strict design assurance before allowing autonomous systems to operate in shared airspace or complex military environments.

Honeywell Aerospace (Nasdaq: HONA) aims to bridge this gap by providing the certified hardware layer that regulators already recognize, enabling Shield AI’s software to operate within an approved safety envelope.

“Autonomy software can fly an aircraft, but trust has to be engineered into the architecture, not bolted on after,” said Matt Milas, President of Defense & Space at Honeywell Aerospace. “Our role is to build the certified, design-assured foundation that lets a warfighter, a regulator and a fleet operator all trust the same platform. That’s the layer we bring to this collaboration, the one that turns intelligence like Hivemind into something that can be deployed safely and at scale anywhere in the world.”

AirPro News analysis

The partnership between Honeywell and Shield AI highlights a critical maturation point
Photo Credit: Honeywell Aerospace

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Vertical Aerospace Signs eVTOL MoU for Portugal Resort

Vertical Aerospace and VIC Properties sign an MoU to evaluate Valo eVTOL services at the Pinheirinho estate in Comporta, Portugal.

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UK-based manufacturer Vertical Aerospace and Portuguese real estate developer VIC Properties signed a Memorandum of Understanding (MoU) on July 27, 2026, to evaluate the integration of electric air mobility services at the 400-hectare Pinheirinho estate in Comporta, Portugal.

The agreement, announced in a joint press release, initiates site and infrastructure planning to establish the luxury resort destination as a test case for regional Electric Vertical Take-Off and Landing (eVTOL) transport. The partnership aims to connect major transit hubs, such as Lisbon, directly to the coastal estate, which will house the Six Senses Comporta resort.

Infrastructure and resort integration

VIC Properties, which manages €3 billion in assets and was established in 2018, is developing the Pinheirinho estate with a focus on luxury hospitality. The MoU outlines a framework to assess the physical and operational requirements for hosting Vertical Aerospace’s piloted, four-passenger Valo aircraft on the property.

João Cabaça, Co-founder and Chief Executive Officer of VIC Properties, stated that the estate is being developed to create a new benchmark for luxury hospitality on the Atlantic coast.

“A partnership with Vertical, a recognised pioneer in this emerging sector, is a natural extension of that ambition, as we truly believe that air mobility will shape the experience, the reach and connectivity of next-generation destinations such as Pinheirinho Comporta,” Cabaça said.

Vertical Aerospace expands operational footprint

The Portuguese agreement follows a series of regulatory and developmental milestones for Vertical Aerospace throughout July 2026. The manufacturer recently concluded its participation at the Farnborough International Airshow, where it conducted the first public demonstrations of the Valo eVTOL.

Vertical Aerospace currently holds approximately 1,500 pre-orders for the Valo aircraft across four continents. The company has secured commitments from commercial operators and lessors including American Airlines (AA), Japan Airlines (JL), GOL Linhas Aéreas (G3), Bristow Group, and Avolon.

Recent regulatory and funding milestones

Beyond the MoU with VIC Properties, Vertical Aerospace announced a strategic regulatory collaboration on July 22, 2026, with Saudi Arabia’s General Authority of Civil Aviation (GACA) to advance certification frameworks. The day prior, the UK government announced a £3.4 million backing for the ECLiPSE programme, led by Vertical Aerospace, to develop next-generation charging and thermal management technologies. The company also joined Honeywell Aerospace’s Project VERTI-GO, an EU-funded initiative focused on safely integrating eVTOL aircraft into European airspace.

Stuart Simpson, Chief Executive Officer of Vertical Aerospace, emphasized the role of the Valo in transforming regional transit for premium markets.

“The most exceptional destinations deserve an equally exceptional way to arrive. Vertical is creating a safer, cleaner, and quieter way to travel that is quicker, simpler and more enjoyable. This partnership is about understanding how a new category of air travel can integrate into the future of resort living and luxury hospitality,” Simpson noted.

AirPro News analysis

We view the partnership between Vertical Aerospace and VIC Properties as a strategic alignment of advanced air mobility with high-net-worth consumer markets. By targeting luxury resort destinations like the Pinheirinho estate, eVTOL manufacturers can establish early use cases where the premium cost of early-stage electric air travel aligns with customer expectations and willingness to pay.

Connecting a major international gateway like Lisbon directly to a coastal resort bypasses traditional ground infrastructure bottlenecks, providing a tangible value proposition for the Valo aircraft. Furthermore, Vertical Aerospace’s concurrent push for regulatory alignment in Europe and the Middle East suggests a deliberate strategy to secure operational footholds in regions heavily invested in luxury tourism and next-generation infrastructure.

Sources: Business Wire, Vertical Aerospace

Photo Credit: Vertical Aerospace

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Safran and AURA AERO Sign TG600 Deal for ERA Aircraft

Safran Helicopter Engines and AURA AERO signed a definitive TG600 turbogenerator agreement on July 27, 2026, for the hybrid-electric ERA.

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Safran Helicopter Engines and AURA AERO signed a definitive agreement on July 27, 2026, to develop and integrate TG600 turbogenerators into the prototype of the Electric Regional Aircraft (ERA). The contract secures the critical power-generation architecture for the 19-seat hybrid-electric aircraft, advancing the program toward its first flight and industrialization.

Announced in a joint press release, the agreement transitions a June 2023 Memorandum of Understanding into a binding development phase. The ERA will utilize a distributed hybrid-electric Propulsion system, relying on two Safran TG600 turbogenerators to supply power to eight ENGINeUS electric motors mounted along the wings.

Propulsion architecture and technical specifications

The integration of the propulsion system is a joint effort between Safran Helicopter Engines, Safran Electrical & Power, and AURA AERO. Each TG600 turbogenerator is capable of delivering 600 kilowatts of electrical power output. These units will generate the electricity required to drive the eight ENGINeUS motors during flight phases that demand sustained power, enabling the aircraft to achieve its maximum range of 900 nautical miles (1,500 kilometers).

Cédric Goubet, President of Safran Helicopter Engines, stated that the TG600 will be an essential element of the ERA’s hybrid propulsion system.

“We are particularly pleased to reach this important milestone in our Partnerships with Aura Aero and thus contribute to advancing the decarbonization of aviation alongside this up-and-coming French aircraft manufacturer. Our teams will now work with those of Aura Aero towards the first flight of a prototype.”

The turbogenerator agreement follows a related announcement on July 20, 2026, in which Safran Electrical & Power and AURA AERO expanded their strategic partnership. That agreement focused on the serial production of the fully electric INTEGRAL E trainer aircraft and confirmed the selection of the ENGINeUS motors for the ERA. Jérémy Caussade, President and Co-founder of AURA AERO, noted that strengthening the partnership with Safran Electrical & Power consolidates the technological and industrial roadmap for both aircraft programs.

Market momentum for the ERA and TG600

The definitive agreement with Safran provides industrial backing for the ERA as AURA AERO works to convert its initial market interest into a firm backlog. In March 2026, French private airline Pan Européenne Air Service placed the first firm Orders for the ERA. Prior to this firm order, AURA AERO had secured approximately 700 Letters of Intent (LOIs) for the aircraft, representing an estimated total value of $12 billion.

The TG600 turbogenerator is also gaining traction beyond the ERA program. On July 15, 2026, Safran Helicopter Engines secured a life-of-program production agreement with United States-based Manufacturers Electra. Under that Contracts, Safran will supply the TG600 for Electra’s EL9 Ultra Short hybrid-electric aircraft, demonstrating broader industry adoption of the 600 kW power generation architecture for next-generation regional platforms.

AirPro News analysis

We view the July 27 agreement as a critical de-risking milestone for AURA AERO. Transitioning from a Memorandum of Understanding to a definitive development contract with a Tier 1 aerospace supplier like Safran provides the ERA program with necessary industrial credibility. The sequential announcements throughout July 2026, including the INTEGRAL E production agreement and the Electra TG600 contract, indicate that Safran is aggressively positioning its turbogenerator and electric motor portfolios as the default propulsion solutions for the emerging sub-20-seat hybrid-electric market. Securing firm hardware commitments is essential for AURA AERO to convert its substantial $12 billion backlog of Letters of Intent into firm orders, following the initial commitment from Pan Européenne Air Service.

Sources: AURA AERO

Photo Credit: AURA AERO

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