Gas turbine aircraft engines such as turbojets, turboshafts and turbofans often use air/pneumatic starting, with the use of bleed air from built-in auxiliary power units (APUs) or external air compressors now seen as a common starting method. Often only one engine needs be started using the APU (or remote compressor).

Starting a modern jet engine is a multi-step process that involves using compressed air rather than a simple ignition key. First, pilots activate the Auxiliary Power Unit (APU), a small engine in the tail that provides electricity and air pressure while the main engines are off. To start a main engine, the pilot opens a valve to send high-pressure air from the APU to an air turbine starter, which physically spins the engine's core up to a specific RPM (usually around 20-25%). Once the engine is spinning fast enough to compress its own air, the pilot introduces fuel and activates the igniters. This creates a self-sustaining combustion reaction. The pilot then monitors the Exhaust Gas Temperature (EGT) and oil pressure to ensure a "clean start." Once the first engine is running, its own air can be used to start the second engine (a "cross-bleed start"). This complex sequence ensures the massive turbines reach operational speed safely without overheating or "stalling" during the initial ignition phase.