What is the purpose of cross-bleed valves and when are they opened?

• A. Cross-bleed valves allow air from one bleed source to feed another engine start or pack when needed.
• B. Cross-bleed valves regulate fuel temperature.
• C. Cross-bleed valves vent air to atmosphere.
• D. Cross-bleed valves are opened only on the ground to start the APU.

Answer: (D)

Cross-bleed valves control how bleed air is shared between pneumatic systems. Their job is to connect a bleed source (like an operating engine or the APU) to another system that needs pneumatic power, such as starting another engine or feeding the aircraft’s packs.

They’re opened when there is a demand to use bleed air from one source to another—most commonly to start a second engine using bleed air from the first engine or from the APU, or to supply the packs. This isn’t limited to ground operations or to starting the APU; the valve may be opened during flight as part of a start sequence or pack operation.