The Arrow Air Inc. DC8 that crashed on takeoff from Gander, Newfoundland, on Dec. 12 was at least 12,000 pounds heavier than the cockpit crew had calculated, U.S. aviation safety officials said yesterday, a dangerous situation that needed little else to become calamitous.

According to sources, investigators have been concentrating on two possible additional problems -- ice on the wings or inadvertent deployment of controls called ground spoilers, which extend from the top of the wing to aid in braking. If they deploy in flight, the plane would be forced to the ground.

All 256 people on board -- including 248 U.S. Army soldiers returning home from peace-keeping duty in the Middle East -- were killed.

It was below freezing with light rain and snow at the time of takeoff. The Arrow crew did not seek de-icing treatment for the plane's wings. Ice on the wings of aircraft significantly degrades performance.

Not all of the controls that extend the ground spoilers have been recovered from the frozen crash site, and the McDonnell Douglas DC8's history includes three instances of inadvertent ground-spoiler deployment. Investigators plan to return to the Gander crash site next week to thaw, retrieve and study other parts of the wreckage.

The weight miscalculation was reported in a recommendation from the National Transportation Safety Board to the Federal Aviation Administration that airlines not be permitted to use the formula that Arrow used. The FAA responded immediately, saying it hopes to act promptly to "implement the practices recommended by the board."

According to the board's letter, also sent to the Defense Department, the Arrow crew calculated the weight of each passenger at 170 pounds, including carry-on luggage. Canadian investigators have checked the weight of uniforms, civilian clothes and other personal items and determined that the average was 220 pounds.

"This excess . . . does not consider possible errors involving the weight of baggage carried in the cargo holds," the NTSB said. Thus, the experts do not know if the plane was illegally overweight at takeoff.

The Arrow flight crashed after a refueling stop. It had made two previous takeoffs with essentially the same load, in Cairo and Cologne, West Germany. It was probably heavier during both takeoffs than at Gander because more fuel was needed than was required for the final leg from Gander to Fort Campbell, Ky.

A weight and balance calculation is used by a flight crew to "trim" an aircraft, or put it in balance. When trim is established, the plane will behave predictably as it lifts off the ground. If trim is miscalculated, the plane might lift off at an angle higher or lower than is anticipated.

There are any number of scenarios about how a plane with ice-laden wings and miscalculated trim might crash, the sources said.

The NTSB letter said the investigation "has not established how Arrow obtained approval from the FAA" for its weight calculations "without including a method" for calculating "actual passenger weights that deviate from the standard average weights."

The board also recommended to the Defense Department that it develop a standard procedure for determining actual weights of passengers, baggage and cargo on military charter flights.

Arrow, based in Miami, declared bankruptcy Tuesday.