how to use WHERE IN mysql stored procedure

You can use the string concatenation and the PREPARE statement to run dynamically built queries.

somestring must be constructed in a valid SQL format like '1','2','3'

DELIMITER $$
DROP PROCEDURE IF EXISTS `abc`.`table1`$$
CREATE PROCEDURE  `abc`.`test`
(IN somestring VARCHAR(255))
BEGIN
    @s=CONCAT("
    SELECT * FROM abc.table1 
    WHERE flight_type IN (",somestring,");")
    PREPARE stmt FROM @s;
    EXECUTE @s;
END $$
DELIMITER ;

You can use FIND_IN_SET() if somestring is formatted a,b,c,d:

SELECT *
FROM abc.table1
WHERE FIND_IN_SET(flight_type, somestring)