In the literature, numerous types of hydrogen-related damage mechanisms have been identified. One of these is damage caused by hydrogen embrittlement. Hydrogen embrittlement is a damage mechanism in which atomic hydrogen enters the metal lattice and diffuses within the material, leading to a reduction in the material’s toughness and ductility. As a result of this process, the material may exhibit sudden and brittle fracture even under stress levels that it would normally be able to withstand.
Hydrogen can cause embrittlement in many metallic materials, and such damage often results in catastrophic failures that occur suddenly and without prior warning. The phenomenon of hydrogen embrittlement was first reported in 1874 by William H. Johnson. Johnson immersed iron specimens for several minutes in concentrated hydrochloric acid or dilute sulfuric acid and subsequently observed a significant reduction in the ductility of the material. This experiment is considered the starting point of studies on hydrogen embrittlement in metals.
When evaluated in terms of materials used in aviation and space technologies, hydrogen embrittlement constitutes a significant cause of failure, particularly in high-strength steel components. For example:
10 October 1992 – CH-47 Chinook Helicopter (No. 89-00173, U.S. Army):
During a flight training exercise in Alaska, partial loss of control occurred, followed by an accident. Investigations revealed that a failure caused by hydrogen embrittlement in the actuator system was the root cause of the accident.8 September 2002 – Piper PA-32R-301T:
While cruising at approximately 3,500 feet, the aircraft experienced a failure due to the fracture of the crankshaft gear retaining bolt. The aircraft made a forced landing in a wooded area, resulting in two fatalities and two serious injuries. Investigations determined that the zinc-plated bolt had fractured and exhibited intergranular separation.June 2002 – Another Piper PA-32R-301:
After 448 hours of operation, a similar bolt failure occurred. Investigations conducted by the FAA and Lycoming concluded that the failure was caused by hydrogen embrittlement. Following this event, the FAA issued an Emergency Airworthiness Directive in October 2002 mandating the replacement of the affected bolts.22 September 2010 – Bell 412 Helicopter:
After an uneventful flight, a rotor speed decay occurred during the landing approach, and the helicopter performed an autorotation landing on water. Six occupants sustained minor injuries. Metallurgical examinations revealed that hydrogen had penetrated the material and caused a fatigue crack in the drive gear.
Following similar incidents, aviation authorities have implemented preventive measures. For instance, on 12 October 2011, the Australian Civil Aviation Safety Authority (CASA) issued an Airworthiness Bulletin warning all aircraft owners, operators, and maintenance personnel about failures observed in MS21042 and NAS1291 series nuts. These failures were characterized as hydrogen-related delayed cracking (hydrogen embrittlement), occurring under sustained tensile stress due to the presence of hydrogen in the steel as a result of improper heat treatment during manufacturing or improper replating processes.
Hydrogen is expected to be used more extensively in aviation in the future. Hydrogen-powered aircraft are anticipated to play a role in achieving the aviation sector’s net-zero carbon emissions target by 2050. Accordingly, aviation authorities are continuing their preparations for the development and certification of hydrogen-powered aircraft.
In this context:
December 2024 – FAA: The Hydrogen-Fueled Aircraft Safety and Certification Roadmap was published.
December 2024 – EASA: A workshop on the certification of hydrogen-powered aircraft was organized.
2025 – IATA: "A Concept Of Operations Of Battery And Hydrogen-Powered Aircraft At Aerodromes" was published.
In the FAA roadmap, it is emphasized that existing airworthiness standards do not adequately address fuel cells or hydrogen-powered propulsion systems and that new standards must be developed. Within this framework, the FAA has defined short-term objectives up to 2028 and medium-term objectives covering the 2028–2032 period.
Short-term objectives include hazard identification and the definition of risk mitigation strategies. One of the identified hazards is hydrogen embrittlement. The safe use of hydrogen in aviation is a critical issue from both materials engineering and airworthiness standards perspectives. As increased hydrogen utilization is anticipated in the future, it is expected that the necessary technical and legal infrastructure will be established accordingly.
In addition to hydrogen embrittlement, other hydrogen-related potential hazards identified in the FAA roadmap will be addressed in future articles.
With love, respect, and kind regards,
References
Civil Aviation Safety Authority (CASA). (2021). Airworthiness Bulletin 14-002: Cracked MS21042 and NAS1291 Series Nuts – Hydrogen Embrittlement. (Accessed On : 01.12.2025) https://www.casa.gov.au/sites/default/files/2021-09/airworthiness-bulletin-14-002-cracked-ms-21042-nas-1291-series-nuts-hydrogen-embrittlement.pdf
Federal Aviation Administration (FAA). Hydrogen-Fueled Aircraft Safety and Certification Roadmap. (Accessed On : 05.12.2025) https://www.faa.gov/aircraft/air_cert/step/disciplines/propulsion_systems/hydrogen-fueled_aircraft_roadmap
International Air Transport Association (IATA). (2025). Concept of Operations of Battery and Hydrogen-Powered Aircraft at Aerodromes. (Accessed On : 05.12.2025) https://www.iata.org/globalassets/iata/publications/sustainability/concept-of-operations-of-battery-and-hydrogen-powered-aircraft-at-aerodromes.pdf
NASA Technical Report. (2016, April ). Hydrogen Embrittlement. (Accessed On : 01.12.2025) https://ntrs.nasa.gov/api/citations/20160005654/downloads/20160005654.pdf
National Transportation Safety Board (NTSB). (2002). Aviation Investigation Final Report, Kaza No. IAD02FA091.(Accessed On : 07.12.2025) https://www.ntsb.gov
National Transportation Safety Board (NTSB). (2010). (Accessed On : 07.12.2025) Aviation Investigation Final Report, Accident No. ERA10TA493. https://www.ntsb.gov
Skybrary. Hydrogen-Fueled Aircraft Certification. (Accessed On : 01.12.2025) https://skybrary.aero/articles/hydrogen-fueled-aircraft-certification
Techspray. How to Mitigate the Risk of Aircraft Damage from Chemical Agents. (Accessed On : 07.12.2025) https://www.techspray.com/how-to-mitigate-the-risk-of-aircraft-damage-from-chemical-agents?srsltid=AfmBOorw8_Lb2gr-VSCVmz-b_uNQJwR5zvYqbv8qCctC4tKmWvFYd9q3