scope:

Description of Driving Event:

Three Pad B ECS chillers condition air for the orbiter and payload changeout room. The ECS chiller GN2 control system, which regulates the amount of cooling provided by each chiller, is controlled by HIM 161 and HIM 213. The auto/direct mode select control allows chiller GN2 control via the auto or direct mode. Normally, HIM 161 controls chiller temperature/humidity in the automatic mode. If this mode should fail, chiller control is maintained by switching to the direct mode. If the HIM 161 direct mode fails, there is a potential for partial/total loss of chiller GN2 control and subsequent orbiter/payload damage.