The first column is denoted by $0, the second with $1 and so on. So, the correct set of commands will be as below,
airportDetail = LOAD '/user/edureka_181724/hdpProject/airports_mod.dat' using PigStorage(',');
mappedAirportData = FOREACH airportDetail GENERATE (int)$0 AS airport_id,(chararray)$1 AS name,(chararray)$2 AS city,(chararray)$3 AS country,(chararray)$4 AS iata_code;
dump mappedAirportData;