When low pressure air is supplied, what needs to be off?

• A. Packs
• B. Pneumatic systems
• C. Hydraulic pumps
• D. Electrical power

Answer: (A)

When you introduce a low‑pressure air source into the aircraft’s pneumatic system, you want to isolate the part of the system that actually conditions and uses bleed air for cabin air—the packs. Shutting them off prevents the external air from feeding into the packs or backflowing into the engine bleed lines, which could disrupt pressure regulation, cause unintended airflow, or damage components. The packs are the component that would most be affected by an external air supply, so turning them off keeps the system stable and avoids interactions with the ground-air source. The other items listed aren’t directly involved in this isolation step during the external-air test or operation, so they don’t require switching off in this context.