HIJACKED T.W.A. PLANE FLIES TOWARD EUROPE
TWA 727 commandeered on NY-to-Chicago flight makes refueling stop at Montreal Sept 10 and flies to Gander, Newfoundland, Sept 11 amid repts that hijackers are headed for Europe; hijackers release 33 passengers at Gander but continue to hold 59 others and crew members while awaiting longer-range plane for projected trans-Atlantic flight; FAA describes hijacking as 1st major domestic air piracy since introduction of extensive airport security measures nearly 4 yrs ago; TWA spokesman says it is unclear how--or even whether--security system at La Guardia Airport had been breached; hijackers are believed to possess some explosives, but it is unclear how they are armed; they apparently are seeking to publicize campaign for Croatian separation from Yugoslav federation (M)