What condition will result in an APU bottle low indication?

An APU bottle low indication is typically a warning that alerts the crew to a potentially critical situation involving the Auxiliary Power Unit (APU). The correct choice is associated with an APU fire on the ground. When there is a fire detected in the APU while the aircraft is on the ground, it can activate the fire suppression system. This system utilizes a fire extinguishing agent stored in a bottle. If the extinguishing agent is low, it is a critical safety issue; hence, the bottle low indication is a necessary alert to the crew to ensure they are aware of the situation and can take appropriate safety measures.

In other operational circumstances, such as normal operation of the APU or low fuel levels, the APU may function without triggering a low indication. Additionally, detecting a fault in the APU often leads to alerts regarding operational status, rather than specific warnings about low extinguishing agent levels in the fire suppression system. Thus, the alert system is designed to prioritize situations, like an APU fire, where the safety of the aircraft and its occupants is at risk.