Defense Secretary Richard B. Cheney declared an "airlift emergency" yesterday, ordering 20 commercial airlines to provide as many as 181 aircraft to help ferry U.S. war supplies and equipment to the Middle East.

The Military Airlift Command implemented the second phase of the Civil Reserve Air Fleet (CRAF) program. For most of the buildup, CRAF Level 1 was in effect, allowing use of up to 38 planes. Airlift command officials said 17 planes would be pressed into service and that it would work with the airlines to make sure there were no schedule disruptions.

Domestic airlines, which have hauled 65 percent of the troops and 22 percent of the cargo destined for the U.S. buildup in Saudi Arabia, said they would be able to provide as many planes as necessary for war transport while keeping up a full domestic schedule.

"We have unbelievable lift capacity," Edward A. Merlis, vice president for policy and planning of the Air Transport Association, said.

Rail and truck lines also said they would have little trouble keeping ammunition and other military supplies moving to ports, even in a protracted Persian Gulf conflict with U.S. warplanes delivering maximum loads of munitions every day.

The American Public Transit Association said it is concerned that big city public transit systems would be stretched beyond their capacity if the price of gasoline rises dramatically. The Washington-based association estimated that if crude oil jumps to $75 a barrel, pushing gasoline to $2.54 a gallon, public transit ridership would increase 28 percent as people left their cars at home.

"The transit industry will not be able to absorb this huge growth in such a short time," the association said in recommending that transit systems develop contingency plans, including possible temporary use of school buses.

In the early hours of the conflict, however, the price of oil fell

Unlike World War II, intercoastal shipping is unlikely to be affected. During that war, railroads were suddenly hit with extra oil shipments because intercoastal tankers proved too vulnerable to Nazi U-boat attack.

"I don't suppose we're looking for the Iraqi navy to bombard our shores," one Coast Guard officer said.

Sabotage is a more immediate worry than transportation capacity, although sources said there have been no specific threats to domestic airlines, railroads, transit systems or ports.

Airport security has been stepped up, and the FBI has been meeting for weeks with freight transportation companies to intensify precautions. Railroad bridges and tunnels in isolated areas, as well as subway tunnels, would be particularly vulnerable to sabotage. The Coast Guard has stepped up surveillance in ports.

Sources in the railroad industry said shipments of ammunition have been placed at undisclosed locations for rapid movement to ports, and that more than 900 cars of ammunition are sitting at one East Coast port awaiting space on ships.

"We are prepared to offer more {freight} cars, whatever it takes," said Aden C. Adams, vice president for merchandise sales and marketing at CSX, which has more military installations and more ports on its lines in the East and Midwest than any other railroad.

CSX has moved more than 100 special military trains for the supply of gulf operations, he said, including a surge in ammunition shipments in the last three weeks.

"We have an adequate car supply; no strain at all," said John Bromley, a spokesman for the Union Pacific Railroad, the second-largest shipper of military goods.

Thomas Donohue, president of the American Trucking Associations, said the industry has an excess capacity of equipment of about 15 percent, and, in coordination with the Pentagon, he could quickly put as many trucks as needed on the road. they have experienced almost no shortages. "The only thing people are reporting in tight supply is flatbed trucks," said one industry representative. "You can get them, but you have to look for them. Outside that, no one is seeing any problems."

Flatbed trucks would be used to haul heavy military equipment.END NOTES