What prevents the engines and APU from supplying bleed air at the same time? This also prevents the APU from supplying air to anti-ice systems.

• A. APU LCV Interlock
• B. Bleed Air SOV
• C. Engine Bleed Valve
• D. Fuel Control Unit

Answer: (A)

The key idea is that bleed air sources are kept mutually exclusive by an interlock. On this airplane, the APU Load Control Valve (LCV) interlock prevents the APU bleed path from opening if any engine bleed valve is already open, and it also stops the APU from supplying air to anti-ice when engine bleed is in use. This ensures only one bleed-air source is active at a time and prevents bleed air from the APU feeding anti-ice when engines are already providing bleed air.

The other components exist as part of the bleed-air hardware, but they don’t by themselves enforce this one-at-a-time condition. A bleed air shutoff valve can isolate a source, and engine bleed valves control bleed from the engines, but the interlock is what guarantees that the two sources can’t operate simultaneously. The fuel control unit isn’t involved in this particular protection.