Among the flights with air carrier "Southwest Airlines Co.: WN", provide the tail number of flights with an actual elapsed time lower than the 80% of the average actual elapsed time of listed flights.
Southwest Airlines Co.: WN refers to Description = 'Southwest Airlines Co.: WN'; tail number refers to TAIL_NUM; actual elapsed time lower than the 80% of the average actual elapsed time refers to ACTUAL_ELAPSED_TIME < (MULTIPLY AVG(ACTUAL_ELAPSED_TIME), 0.8);
SELECT T2.TAIL_NUM FROM `Air Carriers` AS T1 INNER JOIN Airlines AS T2 ON T1.Code = T2.OP_CARRIER_AIRLINE_ID WHERE T1.Description = 'Southwest Airlines Co.: WN' AND T2.ACTUAL_ELAPSED_TIME < ( SELECT AVG(ACTUAL_ELAPSED_TIME) * 0.8 FROM Airlines )