by Charles Miller

"What caused that?" This is a question it seems like I hear almost every day of my life. Whenever there is a problem with a computer or the internet, or almost anything, people want to know "What caused that?" Not often, but sometimes what caused something to go wrong with an electronic device can only be assumed caused by a "SEU."

A Single Event Upset (SEU) is a type of soft error, called "soft" because the hardware was not damaged. "Single Event" means the error is not reproducible, the cause cannot be found, and that exact error might never occur again. "Upset" I will explain in the next paragraph.

On October 30, 2025, a JetBlue Airways Airbus A320 flight from Cancún to Newark experienced an in-flight incident. After an emergency landing in Tampa the plane was taken out of service and its systems were subjected to exhaustive testing for days to prove there was absolutely nothing wrong with them. The pilots, crew, and passengers who had experienced a terrifying roller coaster ride would probably not agree. You might even say they were "upset."

This incident is still under investigation, but a similar event in 2008 involving a Quantas Airlines Airbus A330 was linked to intense solar radiation affecting the aircraft's flight control systems. Our planet Earth is constantly bombarded by cosmic rays. When one of these particles pierces a transistor or other semiconductor it has the capacity to sometimes strike in just the right place to cause a "bit flip" while causing no physical damage whatsoever.

A commercial airline pilot with whom I consulted in writing this column pointed out that Airbus heavily relies on electronics, whereas Boeing aircraft are much less digital in their controls. Computerized avionics are more efficient and incredibly reliable albeit not perfect.

All modern electronics use binary code consisting of ones and zeros. A "bit flip" is changing one byte of binary language from a one to a zero or vice versa. And flipping just one bit can result in a big change. This is only a hypothetical not an account of what happened to those planes, but: If a plane is cruising along at 537 miles per hour, the speedometer would report to the computers in binary that the speed was "1000011001" which is 537 expressed as a binary number. If a bit flip were to occur and if it flipped the third digit from 0 to 1 then the binary number would be "1010011001" which is decimal number 665. If the second digit in 1000011001 was flipped then the speed would be 1100011001 which is 793. In this hypothetical, the plane's computer would probably slam on the brakes because 665 or 793 MPH would be dangerously fast for a passenger airliner. And slamming on the brakes is basically what happened to those Quantas and JetBlue flights. Remember, this illustration is only a hypothetical intended to help everyone better understand binary numbers.

The point I wanted to make is that a cosmic ray that causes a bit flip can cause a big change in the number. Flipping just one bit can result in a change of 4 or 8 or 256 or 1,024 or 2,048 or 4096 or more in the decimal number, depending on which bit is flipped.

While absolutely no problem could be found in the systems of the JetBlue airliner, investigators did discover the Airbus A320 software lacked some radiation-mitigation algorithms used in some other different plane models. The manufacturer Airbus SE expressed concern that "intense solar radiation" could corrupt data, and affect flight-control functions; and took immediate action. Some 6,000 Airbus model A320s were taken out of service to install software upgrades to guard against a repeat of the JetBlue incident.

The computer chip maker Intel estimates that in every computer, smartphone, and tablet there could be one bit flip every few months; but that is not knowable because these SEU soft errors leave no clues. Single Event Upset events caused by cosmic radiation are less frequent at ground level but do still occur. They are more frequent at high altitude, where commercial airlines fly, and even more frequent in space. Thankfully, the alert JetBlue flight crew took only eight seconds to disconnect their misbehaving autopilot and successfully return to normal flight.

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Charles Miller is a freelance computer consultant with decades of IT experience and a Texan with a lifetime love for Mexico. The opinions expressed are his own. He may be contacted at 415-101-8528 or email FAQ8 (at) SMAguru.com.

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