Thousands of planes from Airbus's widely-used A320 family have been ordered for repairs following a software issue.
The aircraft manufacturer is carrying out software updates for 6,000 of its jets - around half the global fleet - threatening travel disruption for airline passengers.
The UK's Civil Aviation Authority said it expects some disruptions to airlines and flights, with easyJet and Wizz Air saying they will take some planes briefly out of service to do the repairs.
But why have airlines been told to carry out a software update for the planes, and how is solar radiation involved?
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What triggered the repair order?
It is understood the incident that triggered the unexpected repair order involved a JetBlue flight from Cancun, Mexico, to Newark, New Jersey, on 30 October.
The flight suffered a control problem and a sudden uncommanded drop in altitude, basically a sharp loss of height, which left 15 passengers with injuries and forced the flight to make an emergency landing in Tampa, Florida.
After investigating the incident, Airbus said "intense solar radiation" may corrupt data critical to the functioning of flight controls.
The issue is known as bit flip, where solar radiation can strike a computer's memory, changing its data from a 0 to a 1 and vice versa - a risk which also affects spacecraft.
Read more: Which airlines are affected by Airbus disruption?
'Very concerning' - but 'very low likelihood' of such an event
The situation was "very concerning", travel expert Simon Calder told Sky News presenter Gillian Joseph.
However, he said there was a "very low likelihood" of such an event happening, adding: "In aviation, nothing is taken for granted."
He said: "Aviation remains extraordinarily safe. And that is partly because as soon as a possible threat is identified, then action is taken immediately."
What is the fix?
The fix involves reverting to earlier software, but must be carried out before the planes can fly again, according to a bulletin to airlines.
Airbus said for most of the affected aircraft, the required update would only take between two to three hours.
However, some jets may need to have their hardware replaced to adopt the required software - a process which would take a longer time.
The Airbus bulletin traced the problem to a flight system called ELAC (Elevator and Aileron Computer), which sends commands from the pilot's side-stick to elevators at the rear of the plane, Reuters reported.
Those elevators control the aircraft's pitch or nose angle, determining which way it is flying.
The A320 was first launched in 1984 and is the main competitor to the Boeing 737 MAX, which was grounded worldwide between March 2019 and December 2020, as well as during January 2024, after fatal crashes in 2018 and 2019 caused by faulty flight-control software.
(c) Sky News 2025: Why Airbus plane's sudden drop in altitude led to thousands needing software updates
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