Federal Aviation Administration officials said yesterday they are trying to find the airplane with the leaky lavatory that is responsible for the 25 pound green blob of frozen waste that landed in Ripley, Tenn., Sunday.

What some residents of the small community thought was an unidentified flying object is evidence that FAA has not completely solved the leaky lavatory problem, despite taking the firm action of revising its handbook in 1974 after an engine was ripped off a jetliner because of a similar incident.

"This is more excitement than we've had in this little town in a long time," said Debbie Crowell, a dispatcher in the sheriff's office of Tennessee's Lauderdale County. The blob, she said, "has been taken care of. It had a nice little odor to it. We sent a three ounce sample over to the University of Tennessee Laboratory."

The blob landed outdoors, near an unused school building. It was first placed in a deep freeze in the sheriff's office until the phenomenon was explained by the FAA's regional office in Memphis.

"This is not supposed to happen," said the FAA's Fred Farrar. "But it's nothing new. We've had 'em crash into people's kitchens."

If there is leak to the outside in airplane lavatory plumbing, the liquid will freeze in the subzero temperatures of high altitude flying.If the leak continues, the blob grows in size until it either falls off of its own weight, is shaken off by the plane's vibration, or comes unglued as the plane descends into warmer temperatures.

Lavatories are flushed with a bluish green chemical into a holding tank. Both the flushing tank and the holding tank have outside valves so they can be filled and drained by ground crews. It is these valves that have been known to leak.

The worst year for falling blobs was 1974, when 14 incidents were reported in the United States. On several occasions, blobs penetrated the roofs of residences.No known personal injuries have resulted, Farrar said.

On April 30, 1974, a National Airlines 727 enroute from El Paso to Houston lost one of its three engines - literally. The engine dropped off the plane 60 miles east of E1 Paso; the plane made a routine landing.

National Transportation Safety Board investigators determined that a leaky valve had allowed fluid to trickle from the front lavatory along the outside of the plane to the air intake of the jet engine on the right rear. Ice began to build up and was ingested into the engine's fans. The engine stopped so suddenly that the change in forces ripped it from the plane. Investigators found that human feces had hit the fan.

The safety board, an independent agency that makes recommendations on safety matters but has no regulatory authority, suggested that the FAA require modifications on the drain valves and other lavatory plumbing "to prevent leakage in flight."

The FAA revised its handbook for ground crews on handling lavatory fluids, Farrar said. In April 1976, a year that saw four falling blobs, the FAA asked for comment on a proposal that the valves be modified.

Modifications have never been required. "Because of the great reduction in the number of incidents, our people wonder whether [modification] is really necessary," Farrar said. There was only one incidentin 1977, he said.