In the five weeks since Capt. Harvey Gibson's Trans World Airlines Boeing 727 dropped more than five miles in 20 to 30 seconds and scared the daylights out of all 89 people on board, the U.S. aviation establishment has mounted an extraordinary effort to find out what happened.

The investigation into what was [WORD ILLEGIBLE] called a supersonic barrel roll" is the most intense for a nonfatal accident that senior, aviation officials can remember. It is unravelling like a mystery novel, with random clues blind veils and false leads.

Investigators have not cracked the case. They are concentrating on possible mechanical failures. TWA's training and crew performance. Their investigation has been hampered by a [WORD ILLEGIBLE] in a tape recording.

"If something is mechanically wrong with that plane, the potential is somewhat enormous," said Dean Kampshrop, who is leading the investigation for the National Transportation Safety Board.

That is because the Boeing 727 is [WORDS ILLEGIBLE] of jet age aviation. There are more than 1,000 727s in the domestic fleet and that constitutes almost all of the commercial airlines flying in the United States.

Boeing has had a large team of people working on the problem, trying to figure out exactly what Gibson's plane did and what could possibly have caused it.

The safety board has been concentrating on mechanical questions refacing to the operation "slats," large plates on the front edges of the wing that are extended during takeoffs and landings to increase lift.

It is known that the No. 7 slat the third one out from the fuselage on the right wing came off Gibson's 727. It is not known if that was cause or effect, although TWA officials state with finality that that was the cause.

Then there is the question of crew performance. Did the crew do something that caused the incident? The question became prominent early in the investigation, when it was learned that the tape recording of crew conversations in the cockpit had been erased after the plane landed.

Gibson told investigators at a safety board inquiry that it was his routine practice to erase the cockpit voice recorder because I might say something unkind about some of the people in management and they might throw that tape out and sent it someplace.

He went to say that "Nobody wishes more than I do that we had this thing because what it would have done to make the two crew members that work with me . . . look real good because they did a real professional job.

Gibson was returned to flight status last week after being given four days of intensive review training at TWA's base in Kansas City, The other two crew members, first officer Jess Scott Kennedy and flight engineer Gary Nelson Banks, have also returned to duty afetr receiving about one day of review training.

Whydid Gibson receive so much more retraining time than the other crew members?

R. J. Kennedy, TWA's staff vice president for flight operations said that. "At the risk of appearing the I'm blowing smoke, the reason is that Gibson's the man who was in charge and who was responsible. The flight engineer and the first officer had no control.

Is TWA satisfied that the incident was not provoked?

"We're convinced it was not." Kennedy said "If we had any inclination or suspicion they wouldn't be out there."

Several Federal Aviation Administration officials, interviewed on a background basis, they continue to believe but have no proof, that the erasure of the cockpit voice recording was done intentionally, not by habit, because it would have told investigators something the crew did not want them to know.

After talking to Gibson and the other crew members the FAA swooped down on TWA's training base in Kansas City to take a look at everything pilots are taught particularly about high-altitude flying. According to sources, the FAA will make some administrative recommendations about thetraining program but has found no significant problem.

Up there at 39,000 feet, where the incident began there is little room for mistakes. The air is so thin that it is possible for a plane to be approching the speed of sound at the same time it is going too slowly to remain in the sky. In other words, given a plane mai to remain in the sky. In other words, given a plane too heavy for its altitude, a low-speed stall can occur at the sound barrier. That point is known as the "coffin corner" by aviation insiders.

Early speculation, particulary by FAA official, centered on the theory that Gibson hady by FAA official, centered on the theory that Gibson hadsomehow discovered the coffin corner for himself. Shortly before the incident began, he had asked air traffic control for permission to climb from 35,000feet to 39,000 feet. He testified later that he sought the higher altitude to avoid a head-wind problem.

Both Gibson and flight engineer Banks said they had discussions about whether it was okay, given their weight to go to that high altitude and decided that it was The Boeing 727-100 is certified by the FAA for altitude up to 41,000 feet.

TWA said that its calculations show that Gibson was flying about 30 knots short of the speed of sound and 70 knots faster than stall speed at the time of the incident.

Boeing, according to knowledgeable industry sources, has decided that the plane was well within its performance capabilities at the time of the incident, and that the "coffin corner" was not a problem.

Just what did the plane do? At request of the safety board, Boeing has been running a series of testsand simulations at its laboratories in Seattle.

Early reports had the plane suddenly start rolling to the right and then go through a "barrel roll" a complete rollover. Boeing's tests based on data from the flight data recorder or "blatests based on data from the flight data recorder or "black box" which was taken from the aircraft, show that the plane probably rolled to the verticle, but did not go all the way over then went to dive.

Gibson recovered control of the plane at about 10,000 feet after he had extended the landing gear. The plane apparently exceeded the speed of sound during its descent.

Banks, the flight engineer, sat immediately behind the captain and the first officer. "I thought we were going in," he said. "I didn't think there was any particular question about that. But I remember that, when the gear went down, I really heard anything that loud explode in the plane and it woke me right up out of worrying about going in.

Back in the cabin, Chell Roberts and his wife, Louise, were returning to Salt Lake City after a trip to Spain. Because of the United Airlines strike, they found themselves on TWA Flight 841 from New York to Minneapolis.

"You know how turbulence feels Roberts said. "It felt like that at first . Then I was pushed down into my seat. The noise started to increase, and you could tell we were going faster and faster. It was really hard to breathe. I tried to reach up and adjust the air, but I couldn't reach it, my armwas too heavy. About halfway through the fall, people began to start screaming. I told my wife I loved her and that we were going in . . . Then came the emergency landing in Detroit. Warning lights in the cockpit told the crew that the landing gear was not down and locked, so they flew by the tower once, heard good news, then landed safely.

On May 3, a TWA 727-100 took off from LaGuardia Airport in New York City for St. Louis. The No. 2 slat on the left wing failed to retract with other slats until the captain put all four of them out and brought them back again. It was a simple little thing, something the passengers probably never noticed.

The slat did it again on takeoff from St. Louis for Wichita. A complete check was run in Wichita, and the plane was flown to Phoenix , then to Los Angeles, then to San Francisco. In San Francisco the acutator , a hydraulic-driven pistol that moves the slats in and out, was replaced and shipped to Kansas City for Study.

The actuator is somewhat different the one on Harvey Gibson's No. 7 but the coincidence interests the safety board.

The slat from Harvey Gibson's 727 was found on a Michigan farm and brought to the board, where technicians have been studying every inch of it. Only about one-third of the actuator has been recovered. The rest is probably buried in a field. There are some signs that a bolt failed, but again, it is not known if that was cause or effect.

The Boeing 727 has a splendid safety record, and is generally regarded as one of the finest airplanes ever built. But the 727-100 series is getting old. The last one left Boeing in 1971.

"Any accident or incident which seems to happen for unexplained reasons, especially to an aging airplane, is bound to attract more scrutiny," a Boeing official said, "I don't know how many people we've got assigned to it, but it's been substantial."

At the safety board, Kampschror gets a lot of telephone calls many of them from 727 captains. "They want to know what's going on, if we've found anything yet," Kampschror said.