https://supercarblondie.com/reason-why-airplanes-disappear-from-radar/
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Problems only arise if both radar and the transponder stop working.
While this situation is rare, it does happen, and the most infamous case is Malaysia Airlines Flight MH370, which went missing in 2014.
The Boeing 777 carrying 239 people disappeared from radar screens and simultaneously stopped sending transponder data while en route from Kuala Lumpur to Beijing..."