TOKYO (AP) _ A Thai airliner crippled by what police believe was a grenade blast, came close to crashing because of damage to the hydraulic systems needed to control the plane, a preliminary report said Tuesday.

''It's too early to tell if it was sheer luck or not,'' Deputy Investigator Hiroshi Fujiwara of Japan's Transport Ministry told a news conference. ''The spacing between the hydraulic pipes must have had some effect,'' he said.

One of the three hydraulic systems stayed intact, enabling Capt. Ampole Ploymekha to make an emergency dive and land the Airbus A-300 safely at Osaka airport in western Japan, 42 minutes after the blast occurred.

Sixty-two of the 247 people aboard Flight 620, bound for Osaka from Bangkok via Manila, were injured in the Oct. 26 accident.

After the explosion and a sudden loss of cabin pressure, Ampole dived about 24,800 feet, but most passengers escaped injury because they were wearing seat belts.

In August 1985, a Japan Air Lines Boeing 747 lost all four of its hydraulic systems when escaping cabin pressure damaged the tail. It crashed on to a central Japan mountaintop, killing all but four of the 524 people aboard.

The Airbus blast in a rear toilet ripped the starboard side of the rear bulkhead that pressurizes the main cabin, Fujiwara said. But escaping pressure apparently rushed out through an emergency pressure relief hatch, he said.

The explosion also cut two aluminum pipes in separate hydraulic systems. Pieces from the ruptured bulkhead severed the pipe on the starboard side, while metal fragments believed to be grenade shrapnel sliced one on the port, Fujiwara said. Pipes of the third system, running under the floor in the center of the aircraft, escaped damage.

''At this point, we cannot say if this was a result of additional structural cover over the central pipe or if it was only luck,'' he said.

Fujiwara said that unlike the JAL accident, the Airbus blast did not damage the vertical tail fin where the hydraulic system pipes converge.

He said the French government, which directs the European consortium Airbus Industrie, will analyze the flight data recorder. Possible clues from the cockpit voice recorder were erased because the machine, which uses a 30-minute endless tape, was not switched off after the landing.

Osaka police are questioning a hospitalized passenger who entered the toilet moments before the explosion. The suspect, identified by Japanese newspapers as Seiki Nakagawa, an alleged member of Japan's largest underworld organization, reportedly told police he went to Manila to recruit women to work in nightclubs.

A Japanese police detective is in Manila tracing Nakagawa's activities, hoping to find out how he acquired the grenade, Japanese newspapers said.