115. In addition to the main starting air compressor, another air compressor, driven by a separate power source, is installed to __________.

A secondary air compressor with a separate power source is installed to provide a backup source of starting air, ensuring engine starting capability even if the primary compressor or its power source fails; this redundancy is standard practice and distinguishes it from air used for scavenging, supercharging, or reversing, which utilize separate systems.