An alert from British Airways has revealed an industry practice that consciously emphasizes aircraft, increasing greenhouse gas emissions.
"Fuel refueling" sees aircraft filled with extra fuel, usually to avoid paying higher prices for refueling at their destination airports.
It can mean extra annual emissions equivalent to a metropolis.
BA said it was common to carry extra fuel for "operational, safety and pricing reasons". [1[ads1]9659003] BBC Panorama has discovered the airline's aircraft generated an additional 18,000 tonnes of carbon dioxide last year through fuel refueling.
Cost savings made on a single flight can be as small as just over 10 pounds – although savings can run to hundreds of pounds.
Scientists have estimated that one in five of all European flights involves an element of fuel refueling.
The practice of European routes can lead to additional annual greenhouse gas emissions similar to those produced by a city of 100,000 people.
Critics say that the widespread use of the practice undermines the aviation industry's claims that it is committed to reducing carbon emissions.
John Sauven, Greenpeace UK chief executive, told the BBC this was a "classic example of a company putting profits in front of the planet."
He added: "This is why we cannot afford another decade to believe the company's environmental cleanup and wait for voluntary carbon reductions to appear.
" We need tough rules to limit aviation emissions, because as long as there is money in pollution, they will pollute as much as they can. "
International Airlines Group (IAG), the company that owns BA, says it wants to be the world's leading airline group in the field of sustainability.
BA boasts that it even prints the magazine in the aircraft on lighter paper to save weight.  Nevertheless, BBC Panorama has seen dozens of internal BA documents showing that up to six tonnes of extra fuel have been put on the fly in this way.
Airlines can save money on the fact that the price of aviation fuel differs between European destinations. .
Make a profit
BA insiders say that – like many airlines operating short routes in Europe – the company has computer software that calculates whether it can save on fuel tanking costs.
The software will calculate if there is a cost savings to be made. If so, crews charge extra fuel.
An example of documents seen by Panorama shows that a new BA flight to Italy carried almost three tons of extra fuel.
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The extra weight meant that the aircraft released more than 600 kg extra carbon dioxide – the same emissions a person is responsible for a return trip to New York.
The cost savings for that trip were less than £ 40, but the documents Panorama has seen show that it can be even lower than that.
IAG made an annual profit of € 2.9bn (£ 2.6bn) in 2018, around 80 per cent of which came from BA.
A BA insider described the practice as "hypocritical".
"For such a large company to try to save so small amounts while emitting so much extra CO2 seems irresponsible in today's climate," he said.
BA said it was common practice for the aviation industry to make it carry extra fuel on some flights.
The airline said for BA this mainly applies to short-haul destinations "where there are significant price differences between European airports."
It said that the additional emissions from the airline represented about two pr. per cent of the total additional emissions generated by all airlines that fuel Europe, based on Eurocontrol research.
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BA pointed out that since 2012, all flights in Europe have been covered by the EU's Emission Trading System.  It added that by 2020, the company will offset all CO2 emissions from its UK domestic flights.
Eurocontrol, the body that coordinates air traffic control for Europe, has estimated that tankers in Europe resulted in 286,000 tonnes of additional fuel being burned each year and the release of an additional 901,000 tonnes of carbon dioxide.
It calculates that the practice saved airlines a total of € 265m (£ 228m) a year.  Eurocontrol described the practice as "questionable" at a time when aviation is being challenged for its contribution to climate change.
But the BA whistleblower said, "I've been a BA employee for a long time.
" I'm very proud to be a part of BA, but in all honesty it makes me sad and disappointed. " Panorama: Can Flying Go Green? S on BBC1 at 20.30 GMT on November 11.