What other function can an APU provide while the engines are not running?

An Auxiliary Power Unit (APU) serves several critical functions, especially when the main engines are not operating. One of its primary roles is to provide an emergency source of electrical power. This capability is crucial for maintaining essential systems and operations on the aircraft when the engines are shut down, such as powering cockpit instruments, cabin lighting, and other vital components.

Having an APU that can supply electrical power enhances safety and operational efficiency, allowing ground operations or emergency situations to continue without reliance on external power sources. This redundancy is particularly important in unforeseen circumstances, ensuring that critical electrical systems remain operational even if the primary sources of power (the engines) are unavailable.

The other options listed do not accurately describe functions typically provided by an APU. In-flight navigation support involves systems connected to the main aircraft power sources, while auxiliary thrust for takeoff and de-icing wings are not within the typical capabilities of an APU.