Air-conditioning test sets (testers) are used to test ACSs for proper operation and to troubleshoot system malfunctions. Some newer aircraft being introduced into the fleet have an aircraft installed tester. This built-in tester (BIT) performs many tests of the system in flight. The following is an example of the F-18 automatic test sequence.

The ACS BIT starts when electric power is applied to the ACS temperature/ flow controller. In-flight BIT is a complete sequence of tests on the ACS temperature/ flow controller and 10 electrically interfacing components. Nine seconds are required for a complete BIT sequence, and the BIT sequence is repeated every 90 seconds. If the same test indicates a failure on two consecutive BIT sequences, the BIT processor removes a ground from the signal data converter, and a maintenance code is recorded. After a maintenance code is recorded, BIT is inhibited until electrical power is removed and reapplied. When the aircraft is on the ground, BIT operation is the same as in-flight BIT except that only part of the BIT sequence is performed.

BIT tests the flow modulating system pressure regulator valve to produce the following indications: BIT tests for OPEN CIRCUITS in the valve torque motor and position feedback transducer during any mode of operation, and will produce a failure signal if an open circuit is detected.

BIT tests for VALVE STUCK OPEN condi-tion by comparing a high avionics airflow condition with a high torque motor current (closing signal to valve). When both of these indications exist, a failure signal is produced.

BIT tests for VALVE STUCK CLOSED when airflow to the cabin and avionics are low. The BIT failure signal is inhibited if the valve position transducer indicates the valve is full open.