American Airlines flight AA109 was back in service just 24 hours after a mystery illness left several crew and passengers feeling sick and faint.
The aircraft was on route to Los Angeles from London Heathrow when two travellers and seven members of the cabin staff became ill.
Passengers were asked if there was a doctor on board before the captain made the call to turn the flight around above Iceland at around 2.50pm on Wednesday.
Paramedics and fire crews surrounded the plane as it landed five hours after taking off - but all those affected 'felt fine' and were discharged.
American Airlines changed the air conditioning filters, then took the plane for an hour long test flight on Thursday lunchtime.
They didn't find anything wrong and the plane went back into service that afternoon.
A spokesman said: "It's very difficult to pin point exactly what it was.
"As soon as they landed everybody felt fine
"We checked the air conditioning and as a precaution changed the air filters.
"We did a test flight on Thursday before we put it back in service.
"It flew for about an hour round the south of the UK to see if situation could be replicated."
He added the airline ran 7,000 flights a day, and while he could only recall one similar incident the past few years, the diversion was "rare but routine at the same time".
One theory is the sickness could have been caused by chemicals in cleaning products used on the plane.
The spokesman said the decision to return to London rather than land at Reykjavik was due to the fact the illness appeared to be serious, but not life-threatening, he added.
Better facilities and connections at Heathrow meant the passengers could be transported to their final destinations more easily, and the plane could be checked more quickly.
Lee Gunn, a British passenger on the flight, told the Mirror Online: "About 2.5 hours into the flight just as we were passing Iceland we had a Tannoy announcement asking for any doctors, nurses or medical professionals on board to report to the boarding doors to assist with unwell passengers.
"The lights then came on in the cabin and there was lots of commotion.
"I'm a bit of a plane geek myself so thought 'here we go, Reykjavik here we come' but it was announced about 20 minutes after we were going to divert back to LHR.
"It was also reported that 7 of the crew - 13 on board in total I believe - had fallen ill, along with 'many' passengers."
The flight was met by Paramedics as it touched down at Heathrow just moments ago - five hours after it first took off.
It is thought the passengers luggage may have been retained for checks by the carrier.
A statement from American Airlines said: "American Airlines Flight AA 109, a Boeing 777-300, operating to Los Angeles International Airport (LAX) from London Heathrow, is returning to Heathrow due to a medical emergency.
"The aircraft departed London Heathrow at 1205 local time and is expected to land at Heathrow at 1700 local time.
"We apologise to our customers for the inconvenience to their travel plans."
A second American Airlines plane was forced to make an emergency landing just a day later when passengers and crew passed out mid-flight.
The flight, which was flying from Brazil to the United States, was travelling at a height of more than 30,000ft when three members of staff became ill.
According to reports, Flight 904 which left Rio de Janeiro for Miami around 11pm yesterday, but was forced to change direction and land in Brasilia, Brazil.
A statement from the airline said: "One passenger and three flight attendants complained of lightheadedness.
"The aircraft landed safely at BSB at 12:37 a.m. and was met by paramedics who evaluated the passenger and crew members.
"None requested further medical attention.
"Our maintenance team is currently inspecting the aircraft and performing a thorough maintenance check.
"Out of an abundance of caution, the flight was cancelled last night and our passengers have been re-accommodated on other flights."
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