The ruling, by the US International Trade Commission (ITC), means that a tax of nearly 300% will not now be imposed on orders of C-Series planes by American carriers.
Boeing, the American giant that is the world's largest aerospace company, accused Bombardier, a smaller Canadian rival, previous year of accepting illegal government subsidies from Canada and Britain for its C Series programme, allowing it to sell the jets cheaply in the United States.
The decision reverses the determination made by the U.S. Department of Commerce that Bombardier should pay tariffs of almost 300% on sales of its 100- to 150-seat CS 100 and CS 300 jets.
A USA agency voted 4-0 to reject proposed duties on Bombardier's CSeries, dismissing Boeing's claim that it had been harmed by the sale of jets at low prices to Delta Air Lines. British Prime Minister Theresa May tweeted that she welcomed the ruling. Bombardier's C Series wing factory is located in Northern Ireland. "It was workers who relentlessly travelled to Brussels, Montreal and Washington to make the case, workers who took the campaign to every council in Northern Ireland and who mobilised in their thousands". Boeing also alleges that Bombardier is propped up by unfair government subsidies that allow it to undercut foreign competitors.
The ITC's decision caps an eight-month trade dispute that kicked off when Boeing filed a petition with U.S. authorities in April 2017.
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Delta in a statement said it is "pleased by the ITC's ruling rejecting Boeing's anticompetitive attempt to deny USA airlines and the US traveling public access to the state-of-the-art 110-seat CS100 aircraft when Boeing offers no viable alternative". Most observers, including Bombardier and the Canadian government, had expected Bombardier to lose its case.
By voting in Bombardier's favor, the commission has certified that it does not believe Boeing's business has been materially harmed by Bombardier's trade practices, a key qualifier for imposing tariffs. The C Series is the most innovative and efficient new aircraft in a generation.
Bombardier on Friday called the decision a "victory" for "the rule of law". In its statement, Bombardier underlined that the aircraft's construction "development and production represent thousands of jobs in the United States, Canada, and the United Kingdom". "The parties have not broken ground on anything in Mobile related to the joint venture, let alone a full C Series production facility".
The Democratic Unionist MP for Belfast East, Gavin Robinson, said he was "absolutely delighted".
Bombardier is one of Canada's largest employers with more than 24,000 workers across Canada, with an estimated 40,000 spin-off jobs at its suppliers.
"I have worked with the company and union representatives over the past year to resist the economic threat posed to jobs at the Bombardier Belfast operation, and by extension to our local economy". "With the cultural context that's prevailing in the United States right now, with the protectionism and the Trump administration that's in place, I was convinced, and a lot of people were also convinced, that they'd support the decision and go forward with the Boeing complaint".