APU ground start is prohibited with ambient temperature

• A. -40 C
• B. -50 C
• C. -30 C
• D. -20 C

Answer: (A)

Starting an APU on the ground relies on reliable fuel flow, electrical power, and lubrication to bring the unit to life. In extremely cold ambient conditions, several factors converge to make a safe, reliable start uncertain. Jet fuel can begin to gel or wax out near this temperature, which risks fuel starvation to the APU. At the same time, battery capacity and starter performance drop in the cold, and lubricants become more viscous, placing extra stress on the starter and associated hardware. To prevent a failed start or potential damage, the system designates a prohibition at ambient temperatures around -40 C. That’s why the threshold given is -40 C—the point at which ground starts are not allowed. Warmer temperatures (like -30 C or -20 C) are within the permissible range, while colder temperatures (such as -50 C) fall outside the intended limit in practice.