What is the maximum continuous operation temperature above ISA for the APU?

What is the maximum continuous operation temperature above ISA for the APU?

• A. -54°C
• B. ISA +25 (50°C)
• C. ISA +35 (52°C)
• D. 52°C

Answer: (C)

APU continuous operation is defined relative to ISA, not a fixed ambient value. The APU may run continuously as long as the ambient temperature is within ISA plus a specified delta, which for this unit is ISA plus 35 degrees. In manuals this is shown as ISA+35°C, and the numeric embodiment of that limit is about 52°C. So the correct concept is that the maximum continuous ambient temperature equals ISA plus 35°C, around 52°C in practice. Other options don’t match this envelope: a smaller delta like ISA+25°C would restrict operation earlier than allowed, a negative value isn’t the continuous-operating limit, and a fixed 52°C doesn’t convey the relative ISA relationship that defines the limit.

APU continuous operation is defined relative to ISA, not a fixed ambient value. The APU may run continuously as long as the ambient temperature is within ISA plus a specified delta, which for this unit is ISA plus 35 degrees. In manuals this is shown as ISA+35°C, and the numeric embodiment of that limit is about 52°C. So the correct concept is that the maximum continuous ambient temperature equals ISA plus 35°C, around 52°C in practice.

Other options don’t match this envelope: a smaller delta like ISA+25°C would restrict operation earlier than allowed, a negative value isn’t the continuous-operating limit, and a fixed 52°C doesn’t convey the relative ISA relationship that defines the limit.