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If, as most aircraft experts believe, the Air-India Boeing 747 that crashed in the Atlantic Ocean yesterday was downed by a bomb or other sabotage device, it would apparently be the first time a jumbo jet crashed for that reason.

While 747's and other jumbos have been damaged in the air or destroyed on the ground by saboteurs, safety specialists could not recall any previous jumbo-jet crash resulting from sabotage. By contrast, the records show several cases of conventional-sized jets and propeller airliners having been blown up in the air.

Jumbos, which include such aircraft as the Lockheed L-1011, McDonnell Douglas DC-10 and Airbus A-300, are harder to destroy, technical experts said, because the larger interior allows for greater dissipation of explosive force before it can reach and severely damage vital structures and sensitive flight controls.

The experts emphasized that all theories about the downing of the Air-India plane were still speculation. And they said it was conceivable, though highly unlikely, that the crash was caused by a mechanical or structural failure. They were confident that investigators could make a determination through a sophisticated examination of debris recovered from the crash site off the coast of Ireland.

One Huge Blast Likely

One aspect of the Air-India case the experts found noteworthy was that the plane was evidently destroyed in one huge explosion, which was suggested by the fact that the aircraft was a normal blip on a radar scope one second and was gone the next. On other occasions when planes have been fatally crippled in the air, the radar blip has remained on the scope for some minutes as the aircraft fell out of control. This was true, for instance, of the Korean Air Lines Boeing 747 that was shot down by a missile fired by a Soviet fighter in September 1983.

The conclusion of industry experts was that if a bomb caused the Air-India crash, the device was more powerful than anything known to have been used in previous aircraft sabotage.

Thomas Ashwood, first vice president of the Air Line Pilots Association and head of an antiterrorism committee for the International Air Line Pilots group, said in a telephone interview: ''A 747 wouldn't break up and disappear because of an engine disintegration. It might crash, but that would be gradual. The rapidity with which the Air-India plane was gone suggests it would have been an explosion of great magnitude.''

Based on the available information, most experts discounted the likelihood of any direct connection between the crash yesterday and the Middle East terrorist crisis that began with the hijacking of a Trans World Airlines Boeing 727 on June 14. They said the only connection between the Middle East hijackers and the possible saboteurs of the Air-India plane might be the obvious fact that acts of terrorism had a way of encouraging similar actions.

Sikh-Hindu Feud Raised

It was in that context that airline industry officials and others said they tended to believe that if a bomb was planted on the Air-India flight it might be linked to a feud between Indian Sikhs and Hindus. One well-informed security official said several threats of violence had been made to Air-India in recent weeks.

The incident revived security proposals that always accompany such crashes and that call for X-ray and other screening of all luggage and cargo that goes into airliners.

But airline officials said it was impractical to carry out such precautions daily because the time consumed would require a severe cutback in the number of scheduled flights. Security experts said El Al Israel Airlines had been routinely X-raying cargo items on passenger craft for years. However, they pointed out that El Al operated a relativly small number of flights and had special reason to fear acts of sabotage.

Many other world airlines increase X-raying and other security measures on special occasions, such as when planes are operating in areas of suddenly increased tension or when direct threats are made. And it is taken for granted that, after the Air-India crash yesterday, this will be done on a much greater scale, at least for a while.