Cosmic Rays, Bit Flips, and the Day 6,000 Jets Were Grounded

 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 exhit unu

sual paths of failing uder hardware system interaction with unforeseen software practices.

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