Typically, control of the system is performed by a series of switches. The pilot will select the desired mode of operation with the switches available which will feed the selection to a timer box, controlling the pumps to provide the selected mode of operation. In modern system designs for FIKI aircraft a timer box provides three different types of operation for the main metering pump. These are Normal Mode, High Mode and Maximum Mode.
Normal Mode is considered the appropriate mode of operation to provide ice protection during typical icing encounters. Normal Mode is achieved by operating both pumps simultaneously for a time cycle of 30 seconds on and 90 seconds off. This timed mode of operation results in using 50% of the design flow rate. High Mode represents the design flow for the system and is achieved by running one pump continuously. Finally, Maximum Mode provides a flow rate for severe icing encounters. Upon selection of the momentary switch to Maximum Mode both pumps run for 2 minutes. During this on cycle the system is provided twice the design flow rate.
Many systems also contain a Backup Mode which bypasses the timer box. Backup runs one pump continuously and is identical to High Mode, but will use the opposite pump as High. Backup is designed to be electrically isolated from the remainder of the system to provide an independent means of pumping fluid in the event of a main system failure.