Which option correctly states the APU start temperature range?

Which option correctly states the APU start temperature range?

• A. -54°C to ISA +35
• B. -60°C to ISA +25
• C. -50°C to ISA +30
• D. -40°C to ISA +20

Answer: (A)

The question is testing the APU’s permissible ambient start envelope—the temperature range within which the APU is allowed to start reliably. The correct range is -54°C as the minimum ambient temperature and ISA +35°C as the maximum. The -54°C limit protects the APU’s fuel and lubricants and ensures the starter can operate in extreme cold; going colder can cause fuel gelling, lubrication issues, or insufficient starter power. The upper limit, ISA +35°C, keeps starts within the unit’s thermal and mechanical design limits; at temperatures hotter than this (above 35 degrees above International Standard Atmosphere), performance and reliability degrade, and a start may not be safe or guaranteed. ISA refers to the International Standard Atmosphere, so ISA +35°C accounts for temperature variation with altitude, not just a fixed ambient temperature. Other ranges don’t match the aircraft’s defined APU start envelope, which is why this one is the correct specification.

The question is testing the APU’s permissible ambient start envelope—the temperature range within which the APU is allowed to start reliably.

The correct range is -54°C as the minimum ambient temperature and ISA +35°C as the maximum. The -54°C limit protects the APU’s fuel and lubricants and ensures the starter can operate in extreme cold; going colder can cause fuel gelling, lubrication issues, or insufficient starter power. The upper limit, ISA +35°C, keeps starts within the unit’s thermal and mechanical design limits; at temperatures hotter than this (above 35 degrees above International Standard Atmosphere), performance and reliability degrade, and a start may not be safe or guaranteed.

ISA refers to the International Standard Atmosphere, so ISA +35°C accounts for temperature variation with altitude, not just a fixed ambient temperature. Other ranges don’t match the aircraft’s defined APU start envelope, which is why this one is the correct specification.