Back in January, furious over what he said was Canada's refusal to grant certification to new models of US-made Gulfstream jets, the president threatened to decertify Bombardier planes in the United States and impose a 50 per cent tariff on Canadian aircraft.Those threats apparently were never carried out, but Trump said Canada quickly certified Gulfstream planes the following month because of his intervention.Trump's social media post Monday signalled continued acrimony over Bombardier, amid a broiling bilateral tariff fight.Even if Trump sought to follow through on his latest threat, it remained unclear how it would happen. The Federal Aviation Administration, not the White House, is the US agency that certifies or decertifies aircraft in the country.Bombardier models remain US-certified, and the company is currently exempt from US tariffs.Last month, as trade negotiations collapsed, Trump slapped 50 per cent tariffs on some US$20 billion worth of Canadian products ranging from hockey sticks to cement.Canada announced it is imposing retaliatory tariffs on a similar amount of US products beginning early Tuesday.The duties of 15 per cent, 25 per cent and 50 per cent will apply to imports from the United States covering steel and aluminium products as well as dairy goods like cheese.Negotiations between the two sides broke down late last month after days of meetings in Washington, with Canadian Prime Minister Mark Carney saying he decided to suspend the trade talks.At the time, Carney said the Trump administration's terms were ultimately unacceptable, and that US negotiators had introduced restrictions on Canadian trade deals with other countries at the eleventh hour.US officials also made unacceptable "threats" to the French language and "Quebec culture", he added, referencing the French-speaking province in eastern Canada.Ottawa and Washington have not resumed negotiations since, and have continued trading barbs.