British Airways said there were two pilots and six cabin crew members on the flight. All passengers were evacuated safely by the crew and airport emergency services.
The plane had flown from London Heathrow.
"Pilots are highly trained and ready to handle emergencies of all kinds, but when one happens, the only thing that matters is whether the job gets done", Brian Strutton, BALPA general secretary, said in a statement.
The BA flight as passengers are evacuated.
Footage and photos shared online show the aircraft filling up with smoke.
The airline has explained that smoke was caused by "technical issues" without specifying. He said "She is OK now but apparently they flew for 10 minutes with cabin full of smoke, no info from crew and oxygen masks failed to release".
Mrs Fitzpatrick, who was on holiday with her husband Stephen, said fire crews were waiting on the runway when the plane landed and passengers were directed to the terminal.
Footage from those onboard shows the thick smoke obscuring vision so badly that it is hard to make out the seat in front. She said "Terrifying experience on flight to Valencia".
"Eventually they managed to open the emergency doors and told us to go down the chutes". This latest fume event only came to light because members of the media were on board the flight. It felt like a horror film. People were crying and hyperventilating.
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