TTAWA, Dec. 13 - Paul M. Tellier, president and chief executive of Bombardier, the troubled Canadian aircraft maker, was unexpectedly ousted from his post Monday and quickly replaced by a three-person office of the president dominated by members of the Bombardier family.

Laurent Beaudoin, a son-in-law of the late founder, Joseph-Armand Bombardier, and the company's chairman, resumed the position of chief executive, a post he held from 1979 to 1999. He was joined by his son Pierre, president and chief operating officer of Bombardier Aerospace, and André Navarri, head of Bombardier's transportation equipment unit. Both will add the title of executive vice president.

The announcement sent Bombardier shares on the Toronto Stock Exchange down by as much as 27 percent. The shares were down 17 percent at the closing.

"Paul Tellier is one of the most respected executives in the country, so for him to go is a negative," said Karl J. Moore, a management professor at McGill University in Montreal who closely follows Bombardier. "But Laurent Beaudoin is the guy who built the company up into a global enterprise."

During a brief conference call, Mr. Beaudoin said the board of directors made their decision after learning that Mr. Tellier hoped to leave the company when his employment contract expired in a year's time. Monday marked the second anniversary of Bombardier's announcement that Mr. Tellier was joining the company from Canadian National Railway.

"The committee concluded that it would not be advisable to transfer the critical responsibilities of our long-term strategy to someone who has disclosed the desire to leave the corporation in the near term," said Mr. Beaudoin, who declined to answer questions. "We hired Mr. Tellier as an agent of change and he has delivered. I am pleased we are parting on good terms."

The management shift comes at a difficult time for Bombardier. Three of its jets have crashed in just over a month. Bombardier's debt rating was recently lowered to junk status. And earlier this month the company announced a meager $10 million in third-quarter earnings compared with $133 million a year earlier. It also said it would cut an additional 2,200 jobs, having already cut 5,400 in the rail division alone.

Two outside directors of Bombardier also left its board Monday but the company, which is controlled by the Bombardier family, offered no explanation.

In a statement issued by Bombardier, Mr. Tellier expressed no regrets.

"I leave with the satisfaction of having done what needed to be done as a first step before the corporation could focus on developing new avenues of value creation," he said.

Mr. Tellier's premature exit, however, will likely take some of the shine off his reputation as one of Canada's top managers. In the Canadian government, where Mr. Tellier rose to become the country's top civil servant, he introduced sweeping changes to the structure of the public service despite resistance from some unions and politicians. At Canadian National Railway, Mr. Tellier transformed a poorly performing government-owned company to a private line that is among North America's leading, and most profitable, operators.

But troubles at Bombardier have been mounting for some time. At one time, Bombardier's steady rail operations could have been counted on to offset the notoriously cyclical airplane business, but no longer. While Bombardier's corporate jet business has recently picked up, its more important sales of larger commercial aircraft have been falling behind those of competitors.

At the same time, traditional rail markets in Europe are stagnant and some promising new markets have failed to deliver. Last month, Florida voters rejected a proposed Tampa-to-Orlando high-speed rail system that would largely have been a Bombardier creation.

Some of Bombardier's problems were out of Mr. Tellier's hands. Because aircraft are priced in American dollars, the fall in the dollar has effectively increased Bombardier's operating costs at its main manufacturing plants in Canada and Northern Ireland. That alone may reduce the company's profits by $250 million his year and $50 million more next year.

But the company must now grapple with several major issues that arguably are of its own creation. In particular, it appears the company badly misread the regional airliner market, previously one of Bombardier's most significant businesses.

Regional carriers once preferred 50-seat aircraft, like Bombardier's CRJ200. But changes in passenger loads, leasing terms and rising fuel prices increasingly make planes of that size uneconomical. Raymond Jaworowski, senior aerospace analyst for Forecast International, an aerospace and military systems analysis firm based in Newtown, Conn., said he expected 3,728 regional jets with 70 to 120 seats to be sold over the next 10 years as carriers move up in size, with fading growth in smaller craft. But Bombardier's largest aircraft at the moment holds only about 90 seats. The CRJ900, a stretched version of a previously stretched model, cannot be expanded further. Bombardier's Brazilian rival, Embraer, however, already has two models on the market that can seat from 94 to 118 passengers. Both have sold well.

Mr. Tellier had set into motion preliminary moves to develop an entirely new aircraft, the CSeries, that can carry up to 150 passengers. Before that could happen, however, the plan would need board approval and the company would have to persuade governments (Bombardier has aircraft facilities in Canada, Northern Ireland and the United States) as well as suppliers to pick up two-thirds of the $2 billion development costs.

Mr. Jaworowski said the company was on the right track. If approved, the new line of airplanes would allow Bombardier to compete against Embraer as well as expand its market by taking on the smallest jetliners made by Boeing and Airbus. "It's definitely a clever strategy," Mr. Jaworowski said.

The downside, said Dr. Moore, is that even if the project is approved by Bombardier's directors next month as expected, the first CSeries airplanes will not be on the market for about five years or longer.

"They're going to have to be very careful with their cash," he said.

While Bombardier has a cash reserve of about $2 billion and untapped credit of $3 billion, it has no other obvious sources of significant cash. Earlier this year, Bombardier sold its original business - the division that makes Ski-Doo snowmobiles, personal watercraft, all-terrain vehicles and motorboat engines - to members of the Bombardier family, Bain Capital and the Caisse de dépôt et placement du Québec, the province's pension fund.

Despite many promises and the odd experiment, including a recent locomotive that packs a jet engine, Bombardier has found relatively few benefits to making both airplanes and trains. The year before Mr. Tellier arrived, Bombardier became the world's second-largest maker of train cars when it bought Adtranz in Germany. But Adtranz, which was widely seen as overstaffed, remains a headache. Of the 2,200 job cuts recently announced, 900 will come from the former Adtranz.