At 6 a.m. on April 1st, a pale grey morning with a hint of spring in the air, the doors of Canada's mammoth $20 million overhaul and maintenance base at Dorval, Que., clattered open and a sleek, red-trimmed jetliner rolled soundlessly onto the tarmac. With veteran flyer, Captain George Lothian, at the complicated network of electronic controls, the big plane taxied to the domestic terminal of Montreal's International Airport to board passengers. At 7.15 a.m. it moved over to a new, 10,000 foot runway and opened out its 4 powerful motors. In 22 seconds, to the applause of a sprinkling of spectators who had gathered to witness its inaugural flight, the slender, missile-shaped aircraft lifted effortlessly into the brightening sky, climbed rapidly, headed westward. Six hours and 20 minutes later, after a 54-minute station stop at Toronto, the mighty airship set its 127 passengers gracefully down at Vancouver's scenic Island Airport. Commercial aviation in Canada had entered the jet age. Flying time for the 2498-mile journey had been cut in half and a Confederation promise to knit Canada's farflung provinces into a closer unity had become a reality beyond the dreams of the country's Founding Fathers. Taking advantage of the 3-hour time differential in travelling to the West Coast, it is now possible for a Montrealer to leave Dorval airport at 10 a.m., be on time for a 12.30 luncheon engagement in Vancouver. Without any doubt, during the next decade -- when Canada will have a fleet of jet-liners plying the skies from British Columbia to Newfoundland, and throughout the air lanes of the world -- Canadians will come to know their own country and understand their neighbours as never before in their history. Other countries flying the big, 550-mile-an-hour DC-8 jets are the United States, Japan, Holland, Greece, Italy, Brazil, the Philippines, Sweden, Norway, Denmark. Switzerland, France and Great Britain.