The present invention relates to a tester, its use and a method for testing signal lines of a flight control system for a trimmable horizontal stabilizer (THS) motor of an aircraft. The tester comprises at least one test-relay (52, 54) to be connected with a relay socket of the flight control system, when the signal lines of the flight control system are to be tested, and at least one indicator (60, 70, 80, 90) being electrically connected with the at least one test-relay (52, 54) for indicating whether a voltage being applied to the test-relay (52, 54) is equal to or larger than a predetermined voltage. The method according to the invention comprises the steps of connecting at least one test-relay (52, 54) of a tester (1), in place of the original relay, with the relay socket of the flight control system, applying a voltage to the at least one test-relay (52, 54) and determining whether a voltage being applied to the at least one test-relay (52, 54) is equal to or larger than a predetermined voltage.