Discharge Indicator
The discharge indicator is installed in the fuselage skin between FR78 and FR79. The overboard discharge pipe connects the discharge indicator to the pressure relief device on the fire extinguisher bottle. A red disc is installed in the discharge indicator housing. You can see the red disc if the pressure relief device on the fire extinguisher bottle is in the correct condition. If the pressure relief device operates, the extinguisher agent flows through the overboard discharge pipe to the discharge indicator. The extinguisher agent pressure pushes the red disc out of the discharge indicator housing. This shows that the pressure relief device has operated and that the bottle is no longer serviceable.
The red disc is only ejected if the pressure in the bottle is too high. If the cartridge is fired and the agent is released into the APU compartment the red disc stays in the discharge indicator housing.