When an American Eagle commuter plane crashed into a grove of pine trees near the Raleigh-Durham Airport in North Carolina 13 days ago, the weather was bad, the pilot had an uneven training record and the airline was struggling to recover from bankruptcy.

In an industry that has seen explosive growth and change in the years since deregulation, those factors figure prominently in a recent rise in commuter airline accidents.

The Raleigh crash, which killed 12, was the sixth commuter accident in four months and the third to involve companies providing connecting service for a major airline. In December, two passengers were seriously injured when another American Eagle plane crashed near Dulles International Airport, and in January, a Continental Express plane crashed into a mountain near Durango, Colo., killing nine people.

Overall, the commuter accident rate rose in 1987 after a four-year decline and is three times higher than the rate for major airlines, according to government figures.

"We're obviously concerned any time we see something that may indicate a real rise as opposed to a temporary one," said National Transportation Safety Board (NTSB) member John K. Lauber. "It's always hard to sort this out. But in the commuter accidents we've investigated last year, there are some common themes that come through. Pilot training, experience, experience level in the cockpit, operating procedures, standardization in the cockpit."

In addition, weather, as a rule, plays a larger role in accidents on commuter airlines than it does with the major carriers.

"Our airplanes tend to fly at lower altitudes, so we don't get to avoid the weather," said John Frederickson, executive director of the Regional Airline Association. "Plus, we tend to operate into smaller airports that don't have the sophisticated navigational landing aids that the other big airports do."

Commuter airline pilots are generally less experienced than pilots at major airlines. Much commuter pilot training occurs in the airplane, which limits the type of emergency procedures that can be safely practiced. By contrast, the major airlines use multimillion-dollar simulators, which allow pilots to practice emergency procedures too dangerous to practice in flight.

In addition, pilot training programs are greatly affected by the high turnover rate among commuter pilots, which averages about 60 percent annually among the 160 commuter airlines operating.

Of the 5,300 pilots flying for commuter airlines, 4,010 were hired last year, according to Future Aviation Professionals of America (FAPA), an Atlanta-based organization that monitors pilot hiring. Of those, a third moved from one commuter airline to another, while others moved from private corporate aircraft, air taxis or other general aviation backgrounds.

"We've never seen this hiring volume before anywhere," said Kit Darby, a FAPA vice president. "All this recent boom in hiring has been for growth. Clearly, there's strain in training and integration of people within the operation. It takes a little while to settle down, and when you have a lot of turnover at smaller companies, people may just be getting to the point of settling down and they're leaving."

At AVAir Inc., the airline involved in the Dulles accident and the Raleigh crash, nearly a third of its 85 pilots leave every year, said company spokesman Steve Meeham.

Both pilots in the Raleigh crash were killed. Federal investigators say they think that the copilot, Kathy Digan, was flying the plane when the crash occurred. Digan, 28, had worked for AVAir for nine months and accumulated 2,450 hours of flying time, including 450 hours in the Swearingen Metroliner, the plane she flew the night of the crash.

Digan's experience level was good, by industry averages. But last summer, it took Digan three tries to pass a check flight to test her piloting skills. In September, she was criticized in a memo by a captain who flew with her, and investigators say they are looking closely at AVAir's training program to see whether it was a factor in the crash.

"Experience is very difficult to measure," Lauber said. "The way we do it now is in terms of hours, but it's not a good measure. Take someone who has 1,000 hours as a flight instructor in Arizona or Florida or California, where the weather is good. That 1,000 hours is by no means the same 1,000 hours experience that another pilot has flying night cargo or canceled-check runs back in the Northeast."

The relationship between an airline's balance sheets and safety is even more difficult to pinpoint.

"I'm not aware of any research on the industry that develops a strong link," said Clinton Oster, who as director of the Transportation Center at Indiana University has researched the commuter airline industry extensively. "Even though it seems intuitively plausible, when you go out and look at it in a hard way, it's difficult to prove. That may be just because it's difficult to prove. It may also be because it isn't there."

But investigators probing the most recent accidents have noted subtle links. The Ryan Air Service plane involved in the November crash near Homer, Alaska, which killed 18, was heavily loaded with cargo and was tail-heavy. Ryan, which voluntarily agreed last month to shut down operations temporarily after an inspection by the Federal Aviation Administration, has a history of flying planes overweight. Likewise, the plane involved in the Durango crash was flying a special approach route into the Durango Airport, approved by the FAA, designed to save time. Investigators say the route was safe, but required special attention from pilots.

Even more subtle are the changes in management at small airlines struggling to stay afloat financially that indirectly affect the continuity and quality of pilot training and company operating procedures. The NTSB found this to be the case in 1985 when it investigated separate crashes involving Bar Harbor Airlines and Henson Airlines. Ultimately, the board cited failings in pilot training and operating procedures as factors contributing to each crash.

AVAir, which filed for reorganization in bankruptcy court Jan. 15, was given a cash transfusion of $750,000 by American Airlines' parent company, AMR Corp., and began flying again Feb. 3.

AMR owns a portion of Air Midwest that operates American Eagle flights at Nashville, and has long-term marketing agreements with nine other commuter airlines. Likewise, United, Continental, Eastern, Delta, Northwest, USAir, Pan Am and TWA have agreements with commuter airlines to "feed" their hub operations at major airports.

Steve McGregor, a spokesman for American, said the Nashville subsidiary intends to buy AVAir and operate American Eagle flights at Raleigh. American, McGregor said, "has put a great deal of effort into linking American Eagle with American Airlines." But those efforts, he said, involve flight schedules, ticket sales and uniform styles, not pilot training programs or operating procedures.

Lauber said the proliferation of agreements between the major carriers and the commuter airlines raises another issue -- especially since accident investigators are turning up the same causes in commuter accidents they uncovered when the major airlines were growing up.

"In some cases, we're reinventing the wheel here," Lauber said. "We're learning lessons that were learned a long time ago and in many cases the hard way. And, there's a significant question of what that relationship between the majors and the commuters ought to be. Should it be strictly a marketing agreement or ought there be safety and operation agreements as well?

"When I buy a ticket from a major carrier," he said, "I have a right to expect the same level of safety regardless of the fact that when I show up at the gate, I'm flying on a commuter bearing the paint scheme of that larger airline."