During a typical engine start on a 737-800/900 using APU power, which sequence is correct?

• A. Engine ignition starts before bleed air is established.
• B. Bleed air is not used during start.
• C. APU provides bleed air and electrical power; engine ignition starts after bleed air established; monitor N2/EGT during start.
• D. Electrical power is supplied by main generators first.

Answer: (C)

When starting a 737-800/900 on the ground with APU power, the sequence hinges on using the APU to supply both bleed air for the engine start and electrical power for the airplane. The engine’s pneumatic starter needs bleed air to spin the compressor, so you bring the bleed air online first. Only after bleed air is established do you command the engine ignition, ensuring there is a stable air supply for a reliable light-off when fuel is introduced. As the start progresses, you watch N2 rise to confirm the compressor is accelerating normally and monitor EGT to verify proper combustion and detect any abnormal conditions. The APU also provides electrical power to essential systems during the start, so you don’t rely on the main generators until the engines are running. This combination—APU bleed air established, ignition after bleed air, monitor N2 and EGT, with APU electrical power keeping systems powered—is the correct sequence.