A Jet2 flight from Newcastle to Prague landed in Frankfurt after the pilot announced an emergency and deployed the oxygen masks on Friday.
Philippa Bell, 25, whose fiancé Joe Monkhouse was on the flight, told the Chronicle Live that passengers were in tears and her partner reported feeling decompression in the cabin and ears popping.
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All passengers arrived uninjured and Jet2 said the fight was diverted due to a "technical fault".
Photos were taken inside the cabin and showed passengers wearing oxygen masks.
Jet2 told The Independent that the fault is being investigated.
In a statement, the airline said: "Flight LS509 from Newcastle to Prague diverted to Frankfurt due to a technical fault indication. Our flight crew followed procedure and made a controlled descent before landing safely.
A replacement plane was sent to Frankfurt to take the passengers to Prague.