Under what conditions should bleed air isolation valves be used to mitigate a leak?

• A. Only when the APU is offline.
• B. To isolate the APU bleed air from the aircraft systems while troubleshooting.
• C. To permanently seal leaks without inspection.
• D. To increase bleed air flow to compensate.

Answer: (B)

Bleed air isolation valves are used to shut off the APU bleed air from the aircraft systems so you can troubleshoot a leak safely. When a leak is suspected, closing the valve isolates the bleed air source, preventing bleed air from feeding the rest of the system and making it possible to locate the leak without triggering other issues or damage. This is a diagnostic step to identify and pinpoint the leak, not a permanent fix. Once the source is found and repaired, normal operation resumes. It isn’t about permanently sealing leaks, nor about increasing bleed air flow, and it’s a tool used during troubleshooting rather than simply when the APU is offline.