Charlotte, N.C. ? Flying too close to the known limits for a plane’s weight and balance can have catastrophic consequences for a commuter aircraft — as the crash that killed 21 people in Charlotte earlier this month may very well have shown.

Investigators have yet to establish the cause of the crash of the US Airways Express turboprop, which went down at the airport Jan. 8 after taking off at an extremely steep angle.

But they are focusing on the possibility that heavy takeoff weight and improper weight distribution combined with a malfunctioning elevator, the tail assembly that controls the plane’s pitch, to cause the accident.

The tragedy has focused attention on how the industry calculates the weight of its passengers and cargo. And it has raised questions about whether that method is realistic in this land of expanding waistlines.

“I think it’s one of the things that may make commuter flying riskier, especially when you’re flying with a loaded airplane — the possibility that it could be out of weight or out of (balance) because of variations in the average passenger weight and the distribution of weight,” said Jim Burnett, a former chairman of the National Transportation Safety Board.

Among the weight and balance concerns:

l The plane was full, with 16 men, two women and one child among the 19 passengers. Air Midwest, the airline that operated the flight, assumes — with Federal Aviation Administration approval — that passengers flying in winter average 175 pounds each, including clothing and carry-ons.

But given the super-sizing of American waistlines (adult men averaged 180.7 pounds in 1994, the most recent year in which statistics from the Centers for Disease Control and Prevention are available) and the increased size and weight of carry-ons, that standard could have been exceeded on the flight.

l Investigators have said the plane’s captain and a member of the ground crew debated before takeoff whether the flight was overloaded.

John Goglia, the NTSB member who headed the crash scene investigation, said a ground crew member believed the plane was limited to 26 bags. Goglia said Capt. Katie Leslie decided that all 31 checked bags could remain on board. He said pilots and others interviewed during the investigation thought the plane “looked heavy” as it prepared for takeoff.

Emergency workers are shown in this Jan. 8, 2003, file photo, near the charred US Airways hangar after a commuter plane crashed into it at Charlotte/Douglas International Airport in Charlotte, N.C. Investigators have yet to establish the cause of the crash that killed 21 people, but they are focusing on the possibility that heavy takeoff weight and improper weight distribution contributed to the crash.

Air Midwest assumes — again, with FAA approval — that each piece of checked baggage weighs 25 pounds on average. But some in the industry believe that estimate is too low.

In the Southeast, for example, many travelers bring along their golf clubs, which can weigh well over the estimate. Also, commuter airlines often deliver passengers to big-city airports to catch international flights, for which travelers are likely to pack heavy. (That was not the case with the Charlotte flight, which was headed for Greer, S.C.) It is also believed that many passengers are packing more in their checked luggage these days because carry-ons are so closely screened.

The maximum takeoff weight for the Beech 1900 that crashed in Charlotte is just over 17,000 pounds. The NTSB has said that, according to the plane’s documentation at least, it was within 100 pounds of that weight.

As for its weight distribution, Goglia has said that on paper, at least, the plane was within 1 percent of the rearward limit for its center of gravity. (The more luggage that is put in the back of the plane, the farther to the rear moves the plane’s center of gravity. Flight rules specify the farthest allowable point.)

Given those conditions, said Paul Czysz, a professor emeritus of aviation and engineering at St. Louis University, a miscalculation could have easily made the plane unbalanced.

Mary Schiavo, a former inspector general for the U.S. Transportation Department, said margins of error are thin in commuter planes like the 19-passenger Beech 1900.

“The small plane, being so light, you don’t have a lot of leeway,” said Schiavo, now a Los Angeles-based lawyer who litigates air disaster cases.