American Airlines unveiled an unusual new fare plan yesterday that will allow travelers to buy long-term travel passes ranging from a limited pass for $8,000 to a lifetime, unlimited-mileage pass for $250,000.

Called the AAirpass, the new ticketing plan allows frequent travelers to buy passes for different extensive amounts of travel:

* Five-year, 10-year and 15-year fixed-term packages provided up to 25,000 miles of coach travel each year to travelers af any age. The five-year pass is $19,900, the 10-year pass is $39,500 and the 15-year pass is $58,900. Additional blocks of 5,000 miles also are available to travelers whose needs would exceed 25,000 miles a year.

* A regular lifetime pass, at an introductory cost of $66,000, provides up to 25,000 coach miles a year for life to travelers at least 52 years of age. The pass may be pur-chased in advance at any age.

* A five-year pass for travelers 65 years of age and older allowing 12,500 miles of coach travel year year costs $8,000. This pass may be purchased in advance by a taxpayer who will be 65 by the end of next year.

* A lifetime pass available to travelers 62 years of age or older for $15,500 buys up to 12,500 coach miles a year for life. This pass also can be purchased in advance by anyone who will turn 62 by the end of next year.

* A pass good for a lifetime of unlimited travel in first class or coach costs $250,000. There is no age requirement.