An investigation has found a CityJet aircraft had part of its air system installed the wrong way round, which prevented it from pressurising correctly.
The Air Accident Investigation Unit (AAIU) was looking into the scheduled passenger flight to Newcastle from Paris in July 2016.
It found that during the climb out from Paris Charles De Gaulle Airport, the flight crew noticed that the cabin rate of climb and cabin altitude had increased rapidly, and that a white advisory light - indicating a pressurisation fault - was on.
This was followed by a caution light which indicated that pressurisation had failed.
This resulted in the flight crew declaring an emergency mayday to Air Traffic Control, deploying oxygen masks and performing an emergency descent.
The AAIU said: "During completion of the cabin high altitude checklist, they discovered that a ram air switch on the overhead panel, which controls the position of the ram air valve, was in the OPEN position instead of the normal SHUT position.
"The flight crew moved the switch to the SHUT position, which closed the ram air valve and restored normal system operation."
The pressurisation mode was then re-selected to AUTO and control of the pressurisation system was regained.
The flight crew briefed the cabin crew, carried out a check and decided to continue to Newcastle.
It was subsequently found that a non-return valve in the aircraft's ram air system had been installed the wrong way round.
This incorrectly installed valve, in combination with the ram air valve being in the open position on the flight, prevented the aircraft from pressurising correctly.
Among its recommendations, the AAIU said CityJet should "review its safety occurrence management processes to ensure that recommendations arising from its internal investigations are addressed in a timely manner".
It also said that BAE Systems, the makers of the aircraft in question, should advise operators of plans equipped with ram air systems to verify that the ram air non-return valve is correctly installed.
