Which component drives the engine start using bleed air in a cross-bleed start?

• A. The hydraulic pump
• B. The air turbine starter
• C. The electric motor starter
• D. The fuel pump

Answer: (B)

Cross-bleed starting relies on a source of high-pressure bleed air to spin the engine through a pneumatic starter. The device that converts that compressed air into rotation is the air turbine starter, which is connected to the engine’s accessory gearbox. When bleed air from a running engine or the APU passes through the start valve, it drives the turbine inside the ATS, turning the engine’s compressor and gear train. Once the engine reaches the proper speed, fuel is introduced and ignition is established, and the engine becomes self-sustaining. The other options don’t initiate rotation in this method: the hydraulic pump powers hydraulics, not the engine start; an electric motor starter isn’t used for a cross-bleed start on this aircraft; and the fuel pump delivers fuel after rotation has begun.