It’s a hot summer’s day in Prague, but Jaromir Lang is unfazed when he arrives for his interview.
Affable with a wide grin, the engineer is a relaxed representative for the Czech Republic’s premier aerospace company Aero Vodochody.
The company is best known for the L-39 Albatros. Aero made more than 2,800 of the versatile and affordable L39 between 1968 and 1996, making it the most successful trainer jet ever. More than 30 air forces globally have used the Albatros for training and light ground attack missions.
Lang is the chief designer of the L-39NG Skyfox, the successor to the L-39 Albatros. The aircraft, which entered service in 2023 and was renamed Skyfox last year, is a crucial part of Aero’s future. Even more so because the company has recently been through some tough times.
Originally a Soviet state-owned and run enterprise, Aero employed around 4,000 employees at its peak. Since the end of the Cold War and amid declining sales, the company has changed hands several times and today employs around 1,700 people.
BACKGROUND AND EDUCATION
Growing up in the Czech Republic during the Soviet-era, Lang was interested in technology from a young age and attended the aviation faculty of the Czech Technical University (CTU) in Prague. He specialized in the area of aviation engine design.
The route into Aero from CTU is well trodden. Lang graduated and started at the company in 1986. “At that time the company produced one L-39 per day. For the Skyfox, we have a production rate of around one per month currently,” he says.
Lang spent 14 years in Aero’s flight test department before moving into the design office. Development of L-39NG started in 2014 with Lang as chief designer.
“The L-39 had become old. It is impossible to buy some parts because of sanctions against Russia. And the engine had become obsolete,” says Lang.
The AI-25 engine used on the L-39 was designed in the late 1960s and was based on the civilian engine used to power the Yak-40 business jets. “After 1990, we increasingly met with problems to keep the engine in operation – the supply of spare parts, repairing and so on,” Lang says.
“The L-39 had become old. It was not possible to buy some parts because of sanctions against Russia and the engine was obsolete”
“After some analysis, we finalized on two final candidates to replace the engine. The Williams International FJ44 and the Honeywell TFE731 turbofan. The advantage of the Williams was that it is small, which meant we could use it in a new generation aircraft and for retrofits on the existing L-39.”
Where the inlet diameter of the TFE731 is around 700mm the Williams is around 590mm. The TFE731 was thus too large to fit on the existing L-39, additionally Williams offered a good deal on providing ongoing support for the program.
The largest difference between the original L-39 and the L-39NG Skyfox is the longer service life that the NG update offers – up to 15,000 flight hours compared to the original’s 4,500 to 6,000 flight hours. Extending the aircraft’s longevity was one of the main aims of the program, confirms Lang, and was achieved using modern materials that meet Western standards.
Crucially, the design philosophy behind the L-39 did not change during development, preserving the distinctive sleek design created by the Czech engineer and aerodynamicist Jan Vlček. “The L-39 was designed using the philosophy of safety first, and we kept this for the L-39NG,” says Lang.
With the L-39 approaching the end of its service life and used by 30 air forces around the world, the other aim was to offer an aircraft that can easily replace the L-39.
“We were tasked to keep it as similar as possible to the L-39, to keep the flight qualities and performance almost identical, and we succeeded,” says Lang. “It’s thinner, a little shorter, but the wing profile is almost the same. Keeping the aerodynamic conception of the original L-39 was important.”
DEVELOPMENT CHALLENGES
Four aircraft were built for the test program. Two were used for flight testing, one for static testing, and another for fatigue testing. Before that a technology demonstrator was built, the L-39CW, a base model L-39 (C) rebuilt with the new Williams (W) engine. An adapter was developed to retrofit the new Williams engine to the L-39. The L-39CW was first flown in September 2015 and accumulated around 300 hours of flight time.
The L-39NG made its maiden flight in December 2018 and the baseline aircraft was first certified in September 2020. EU / NATO certification to EMAR 21 standards was granted in July 2022.
The flight test campaign started in March 2019 and covered flying qualities and performance testing. Ground vibration testing was conducted, flutter testing, systems testing, engine and engine integration testing and failure simulations, including in-flight engine relighting.
“There were several moments during development that were difficult,” says Lang. “The first was to accept the job, because I was well experienced in engines, but not so much in aircraft design!
“But we have a good design team at Aero Vodochody, and there is a lot of specialists around. It was teamwork that made the design successful.”
However, as the deadline for the first flight approached, the stress began to mount. Lang says, “The first flight was very intense. The investors announce a date for the maiden flight, and as that gets closer, you are met with more problems to solve.
“You start thinking about postponement, but the investors say no, it’s impossible. You need to prepare the aircraft for the maiden flight to be safe, to receive the flight permission from the aviation authorities.
“I can tell you, after the aircraft landed from the maiden flight, I was very relieved.”
PRODUCTION AND WEAPONIZATION
As of June 2025 L-39NG Skyfoxs had been delivered to the Vietnamese Air Force in August 2024 (six of 12), the Czech Air Force in January (two of four) and the Hungarian Air Force in May (three of 12).
Lang says, “I’m happy that the aircraft is in production. From the beginning of the design phase we had the intention to further modify it for light attack roles using light weapons. The aircraft has five attachment points for the pylons – four under-wing attachment points and one centerline pylon.”
An Eastern standard pylon was developed for the Vietnamese Air Force, which uses Russian armaments. Rocket firing tests were done at a test range in Hungary to assess how firing influences the engine’s operation as well as the accuracy of the weapons.
“Now we are working on the Western version. We have already developed the full set of Western pylons, and we are working on the integration of the stores management system to control the weapons. The weapons will be controlled by MIL-STD-1553 and 1760. We are planning for the first firing tests now,” says Lang.
“After the aircraft landed from the maiden flight, I was very relieved”
TEAM BUILDING
“Aero is a smaller company now, but it is a special one. We have good teams, in design development and flight test. We all sit in the same building close together. The people make the job enjoyable,” says Lang.
“But the best part of the job for me is still flying, especially during testing. I have accumulated 400 flight hours, mainly in engine testing. Out of those hours, there are a lot without the engine running, gliding,” he adds, grinning.
Aero nurtures its close relationship with the CTU in Prague to help ensure a pipeline of young talent into the company. After 36 years of service, Lang recognizes the importance of bringing young talent into the firm. “Out of the people that stay, which is around 60%, as they get more experience, they become better and better. It’s satisfying to see them progress,” he says.
“I’m lucky to have this job. Sometimes it’s very challenging, but generally it is a pleasure because I like the work so much. Moments like the maiden flight, the certification of the aircraft, selling to the first customers – make it all worthwhile,” he says.
