74 Saigon Planes Fly 2,000 to Thailand
Amer and Thai officials in Bangkok rept 2,000 S Vietnamese pilots and people, in 74 S Vietnamese planes, flee to Thailand and land without warning at U Taphao air base in south; say they request asylum; planes reptdly are Amer-supplied F-5 jet fighters, C-47 transports and C-130 cargo planes; Thai spokesman says Amer authorities at air base have been asked to turn over craft to Thai Govt, which will return them to new Govt in S Vietnam; says pilots and passengers 'must leave Thailand'; Amer Embassy officials in Bangkok decline comment; development embarrasses Thailand, which wants to disengage itself from US and have better relations with Communist neighbors (M)