Baltimore-Washington International Airport sidelined 18 shuttle buses powered by compressed natural gas after a bus erupted into flames in a parking lot last month, destroying eight cars. It was the second such incident since August.

No one was hurt in either fire. But in both cases, the flames destroyed the buses and left a trail of damage in the parking lot. A total of 19 parked cars and trucks were left with burned metal, smoke-blackened interiors, shattered windows and, in one case, a flat tire.

Both fires appear to have started in the engines of buses used to carry passengers between the satellite parking lots and the airport's main terminal, said Nick Schaus, deputy administrator at BWI. Federal investigators later determined that natural gas released during the August fire contributed to its severity, officials said yesterday.

Hudson Bus Lines, which owns and operates the buses under a contract with BWI, has made repairs and installed fire suppression systems in the vehicles. Four of the buses will start service at the airport today, according to Schaus.

Hudson Bus officials could not be reached for comment yesterday.

The 45-passenger buses, which have been operating at BWI for seven years, are part of the airport's efforts to improve air quality.

Because they use compressed natural gas, "they're much cleaner burning buses," said Schaus.

Compressed natural gas is the most common type of alternative fuel used in transit in this country and is championed by environmentalists pushing for emissions reductions. The buses cost more to buy, maintain and operate than diesel buses.

Schaus said that airport officials are confident about the safety of the buses and that federal officials have given their okay to putting them back into service at the airport. "We've taken all the prudent measures we possibly can."

But questions remain about their safety. Federal officials said there was a fire in the same model of bus in Boise, Idaho, in 1996, and investigators are still looking into the engine design as a result of that incident.

After the August blaze and again last month, BWI police left notes on the charred cars for the unsuspecting owners, all of whom were traveling and could not be reached by authorities on the days of the fires.

On Dec. 22, the fire started as a shuttle bus was traveling in the blue satellite parking lot. A passenger had just been dropped off about 6:30 a.m. when he spotted flames coming from the engine in the rear of the bus, Schaus said.

The passenger alerted the bus driver, who radioed for help. He and a remaining passenger then left the bus.

By the time the fire was extinguished, the bus was a total loss, three cars were destroyed and the others suffered blown windows, melted door frames, bubbled paint and a flat tire, officials said.

CAPTION: Firefighters try to extinguish a blaze that destroyed a shuttle bus in a satellite parking lot at Baltimore-Washington International Airport in August. Two fires involving the shuttle buses damaged 19 cars but caused no injuries.