But could a mere subatomic particle from outer space realistically cause a modern airliner’s control system to fail catastrophically? In mid-Oct. 2025, the answer came with a stark Cruifen value. An Airbus A320 operating a JetBlue carrier’s Cancun-to-Newark service was midway through its cruise, precariously plummeting through altitude with control, when passengers were thrown into cabin fixtures. At least three required emergency care for head lacerations, with 15 more hospitalized. Just weeks before, this incident would trigger the largest-ever airspace stand-down, as over 6,000 A320‑family aircraft would be ordered down for mandatory inspections.
The series of events was tracked back to a phenomenon that was well documented in high-altitude flight, although relatively rare: avionics “bit flips” caused by cosmic rays. At cruise altitudes, the intensity of neutron radiation is some 300 times higher than at sea level, than near mean sea level, due to galactic and solar cosmic rays interacting with atmospheric nuclei to form cascades of secondary particles, such as high-energy neutrons, that can penetrate aircraft surfaces. Impacting semiconductor devices within the flight computer, such as sensitive areas of a computer chip, can change a binary digit in memory or in a computer program’s logic from a “1” to a “0.” Such events, called single-event upsets (SEU) or single bit flips, do not cause physical harm but may yield unexpected errors.
The A320 family utilizes triple redundant fly-by-wire computers controlling the elevators, ailerons, and other control surfaces. In the October incident, several inputs were entered with a revised software load, the most recent being ELAC B, referred to as “L104,” without engaging the cross-check protections that would normally occur. Airbus reported that intense solar activity on this specific day produced a scenario that “could compromise critical control information” so that the airplane experienced an uncommanded pitch-down, which was manually controlled. Europe, as well as the United States, immediately issued emergency airworthiness directives mandating that operators remove L104 in favor of the previous “L103+” standard.
Such issues, however, are not unknown in the field of aerospace engineers. Indeed, the vulnerability of avionics equipment to single-event effects was identified as a concern in the late 1980s with the focus on scaling down the size of transistors, as well as incorporating boron-containing dielectrics within memory components, which heightened susceptibility to thermal neutrons. Where more contemporary measures might prevent such issues as ECC memory, watchdog timers for reset-hung processors, or so-called radiation-hardened components, NASA’s radiation hardness assurance measures, for example, provide a system-based synergy that includes component testing, circuit-level design, as well as system modeling. However, as illustrated in the Airbus example, established systems can still exhibit unusual paths of failing under hardware system interaction with unforeseen software practices.
The physics involved with the threat is uncompromising. Primacy cosmic rays consist of approximately 89 % protons, 10 % helium nuclei, and 1 % higher-mass ions, with origins in the Sun as well as other distant astrophysical objects. Upon interacting with atmospheric molecules, they will initiate subsequent showers of subatomic particles, such as fast and thermal neutrons. Thermal neutrons pose particularly threatening hazards within microelectronic components that contain boron-10, as they can be captured with the emission of an alpha particle as well as a lithium ion, providing sufficient charge for a bit transition. Aircraft body materials, fuel, cargo, as well as passengers, may “thermalize” higher-energy neutrons, producing a higher flux within the avionics bay areas.
Airbus has been exploring this environment. In 2019, its avionics subsidiary collaborated with the laboratory,
specifically with the Institut Laue-Langevin, in testing specially designed helium-3 proportional counters on their test planes, determining neutron spectra at altitudes of up to 12km. These measurements validated calculations, although simulating neutron intensity levels inside aircraft remains problematic, dependent on latitude, altitude, as well as the levels of solar activity. Neutron intensity is highest on polar routes, where the background magnetic field deflects fewer incident charged primaries, as well as intermittent events of enhanced radiation intensity within minutes.
Upsets caused by cosmic rays usually remain transient, although the effects depend on their location within the control logic. In non-critical systems, a flipped bit may trigger a nuisance error, a reset, or other issues. In control chains relevant to aircraft flight, even transient corruption may generate unusual control signals for the actuators. Although older, still-famous is the case of the Qantas Flight 72 incident in 2008, with the involved Airbus A330, where a likely SEU caused uncommanded pitch-down movements due to an air data inertial reference unit. Nowadays, the required assessment, if feasible, of susceptibility to SEE with subsequent mitigation for avionics operating up to 60,000 ft, as defined by relevant avionics regulations such as the IEC 62396, is ensured.
This was shown in November 2025, with most planes back in service within hours of the rollback. However, a few were required to undertake hardware fixes, causing delays. Although this caused a lot of disquiet as it happened over the Thanksgiving holiday weekend, safety was, as always, the priority. However, this incident is a reminder that, despite all the other dangers involved in air travel, the one thing that always gets through is the universe.
