Following the discovery of a defective engine part, Cape Air grounded its entire fleet of Cessna 402 planes this week, disrupting travel plans for thousands of passengers and costing the respected commuter airline hundreds of thousands of dollars in lost revenue.

"This is the most painful event that Cape Air has faced," declared communications director Michelle Haynes. "This is a very busy time of year for us. We have a limited time to realize our profit."

The decision to ground the planes came late Tuesday evening and took effect Wednesday. The grounding affected all Cape Air service in New England including Boston, Hyannis, Martha's Vineyard, Nantucket, New Bedford, Providence and Provincetown. It also resulted in cancellations in company service in Florida and the Caribbean.

Cape Air president Dan Wolf voluntarily called for the shutdown, after mechanics found an abnormally fast wear pattern on the crankshaft counterweight of the planes' engines. The examination came after three engine failures on Cape Air flights, including one on Tuesday afternoon from Nantucket to Logan International Airport in Boston. The other two happened over Memorial Day weekend. All three flights landed safely and no one was injured.

The Cape Cod Times reported yesterday that the excessive wear on the malfunctioning planes appeared to be random.

Mechanics worked through the night on Tuesday at the airline's Hyannis headquarters adjacent to the Barnstable Municipal Airport to replace the defective parts in each of the company's 49 twin-engine propeller planes. On Wednesday, Cape Air, which expected to fly 2,000 to 3,000 passengers that day, operated with less than six planes, Ms. Haynes said. By yesterday afternoon, 15 planes were in use. The airline expected to add ten more planes today. Service will be restored as planes are determined to be safe for flying; the airline expects to be fully operational by Monday at the latest.

While most flights were cancelled Wednesday, with no flights going to or from Nantucket, the Vineyard was one of few locations where some service remained. Ms. Haynes said as it happened, the Vineyard already had a plane that was outfitted with the new part. On Wednesday, only one flight on the Boston-Vineyard route was cancelled, and one plane left the Martha's Vineyard Airport for Providence at 3:25 p.m. Yesterday every flight between the Vineyard and Boston ran on schedule and Cape Air reported three scheduled Providence flights. "If there's a success in this story, it's going to be the Vineyard," Ms. Haynes said.

The grounding made the nightly news on Tuesday, which is how some people planning to fly on Cape Air first heard that their flights had been cancelled. Ellen Kaplan, an Island attorney, was watching the 11 p.m. news when she heard of the cancellations. She was booked on the 7 a.m. flight to Boston to argue a case in front of the Massachusetts Appeals Court. Luckily, she said, she keeps a car in Woods Hole, so she woke up early and took the 6 a.m. ferry. "It worked out, but I'm really exhausted," Ms. Kaplan said Wednesday afternoon. As it turned out, her flight was the only flight to Boston that was cancelled that day.

But it was a different story elsewhere. On Wednesday Cape Air was expected to carry only 250 of the 1,200 people booked to fly on New England routes. Extra staff were available to help provide displaced passengers with alternative transportation including limousine service, ferries, buses and other airlines.

Colin Ewing, Cape Air service manager at the Martha's Vineyard Airport, said the company had accommodated many travelers, including a couple that arrived at the airport to board a flight to Nashville, Tenn., via Providence, R.I. When they discovered their flight had been cancelled, Mr. Ewing put them in a taxi to the New England Fast Ferry, arranged for a limousine to take them from New Bedford to T.F. Green airport in Providence and booked them on a later Southwest flight to Nashville.

"People have been very understanding," Ms. Haynes said. "They understand that it's all about the safety."

Outside of New England, finding alternative accommodations was not as easy. "You cannot take a ferry to the British Virgin Islands," said Ms. Haynes. In Florida and in the Caribbean, the company is working with other airlines to reroute customers and has increased its on-site staff. Passengers holding tickets for cancelled flights can reuse the tickets on other flights within a year or receive a full refund.

Cape Air notified the Federal Aviation Administration of the shutdown on Tuesday. "There was a question and it's done," Ms. Haynes said. "There wasn't a moment's hesitation," she added. She estimated that all told the grounding affected thousands of people and 300 to 400 flights.

"The consequences are tough," she said. "People have families and places to go to." The company will also take a financial hit at a time when many customers had planned to use the airlines to start their summer vacations. Ms. Haynes could not say how much the repairs would cost.

The grounding caused no turmoil at the Vineyard airport, manager Sean Flynn said yesterday. "We have received no complaints," he said, "which is to the credit of Cape Air." Steamship Authority general manager Wayne Lamson said the ferries had not seen a noticeable increase in traffic.

Cape Air has operated without a fatal accident throughout its 18 year history, although a previous scare involving cracks in the wing parts of two aircraft occurred in February 2005. The company operates 130,000 flights annually and replaces the engines in its planes every 24 to 26 months. This spring, the company launched a marketing partnership with JetBlue Airways Corp., which allows passengers to book a single ticket to travel between the two airlines.

"We seem to be back on track as best as we can be," said Ms. Haynes when reached by telephone yesterday. "But it is such an inconvenience. We really have some work to do to restore confidence in the airline."