Perhaps the timing is a bit perverse -- right on the eve of the holiday travel season -- but "The Tragedy of Flight 103: The Inside Story," a 90-minute docudrama premiering tomorrow night at 9 on HBO, is a dynamite shocker any way you look at it.

The film works better as "docu" than as "drama," with some of the scenes and much of the acting stiff or even amateurish. But its account of events leading up to the terrorist bombing of a Pan Am 747 over Lockerbie, Scotland, two years ago -- a disaster that took 270 lives -- is filled with telling and damning details.

Most of them serve to amplify the conclusion of a presidential commission that the calamity "may well have been preventable."

Co-produced by England's Granada Television, the film ticks off one blunder or miscalculation after another. It opens with a fake-out: A man bribes a ticket agent into letting his unchecked bag onto an airplane. The man turns out to be an Israeli security expert staging a demonstration of how easy it would be to smuggle an explosive device onto a Pan Am jet, years before the tragedy actually occurred.

"Itis only the will of providence that has saved your airlines from a terrorist attack," the security man tells Pan Am executives. They dismiss the demonstration as a stunt and reject offers of Israeli help.

In 1986, 2 1/2 years before the crash, Pan Am, faced with declining bookings and financial woes, announced with considerable fanfare its new Alert security system designed to thwart terrorist sabotage. According to the film, produced and directed by Leslie Woodhead, the Alert system was a sham, one that suffered from Pan Am's reluctance to finance and staff it properly.

Bomb-sniffing dogs present when the system was unveiled to the press were in fact just stray German shepherds purchased at a kennel the day before, according to the film. Stringent searches that might have uncovered the plastic explosives hidden in a portable radio -- believed to have caused the explosion -- were not used because it was feared they would irk passengers.

Efforts by a security expert to make the Alert program meaningful were thwarted by Pan Am Chairman C. Edward Acker, the film says, and by fellow Pan Am executives who used the Alert gimmick for public relations rather than to increase airline safety.

The film ends with the radar blip that had been Flight 103 breaking up and disappearing from an air traffic controller's screen, the first indication that the plane and all its passengers and crew had ceased to exist.

Because its charges are so exhaustive and specific, "Flight 103" is the kind of film that would never be produced or shown by one of the major commercial networks. HBO is not subject to pressure from advertisers because it has no advertisers.

Understandably, Pan Am has denounced the film, with one company official calling it "a cheap shot" that shows "reckless disregard for the facts." Another Pan Am spokesman complained that the company was given no input into the production. HBO has responded by saying that Pan Am attorneys and a company representative met with a researcher from Granada in May "and were afforded an opportunity to comment on a number of topics that Pan Am now challenges."

"Granada has provided HBO with convincing substantiation for the version of events depicted in the docudrama," an HBO spokesman said yesterday.

While the dramatic structure of the film is ragged, that almost enhances rather than hobbles its credibility. Woodhead and writer Michael Eaton avoid the slicked-back simplistics that usually mark docudramas based on such catastrophes. The old "Bridge of San Luis Rey" approach is rejected; we don't meet any of the passengers or crew of the doomed flight or hear any of their life stories.

Instead, the action takes place backstage at airports, in Pan Am boardrooms and in Mideast and European countries where pro-Palestinian terrorists conspire to commit mass murder.

Ned Beatty plays Acker, announcing to his staff, "I want to convince our passengers they're perfectly safe on Pan Am." Also portrayed is Chief Operating Officer Martin Shugrue (Harry Ditson), who is now an Eastern Airlines executive starring in Eastern's commercials. Both Acker and Shugrue left Pan Am before the Lockerbie disaster.

As Fred Ford, earnest supervisor of the Alert system, Peter Boyle plays essentially the same part he played in ABC's movie "Challenger," about another airborne catastrophe -- and both roles seem variations on the Mr. Fixit that George Kennedy used to play in the "Airport" pictures. Ford is forever running around shouting warnings that are ignored.

He is frustrated by Pan Am's refusal to back up its promises with actions -- despite levying a $5 "security fee" on each passenger's ticket. When Ford spies a group of symptomatically lackadaisical security personnel at an X-ray machine, he growls to a colleague, "If those guys were to get work flipping burgers under the golden arches, it would be a step up the career ladder."

Others in the cast include Vincent Gardenia as a Pan Am vice president, Gorbachev look-alike Timothy West as Col. Wilfred "Jumbo" Wood, who succeeds Ford as security chief, and in the weirdest bit of casting, ponytailed Michael Wincott as the Eurotrashy head of security at Frankfurt Airport.

It's hard to tell whether Wincott's mumbled, oily performance is meant as character assassination or is just bad acting.

Where it falters as teleplay, "Flight 103" succeeds as indictment. A worried security agent says, "I just hope to God our lords and masters know what they're doing," and what the film maintains, alarmingly and convincingly, is that you shouldn't count on it.