SES

A sustainer engine start, or SES, is an air-start procedure that uses one operational engine on a multi-engine aircraft to supply compressed air to start another engine. On a twin-engine jet, if the ground air-start cart fails or is unavailable, the flight crew can run one engine at high power, divert some of its compressor discharge air through crossbleed ducting, and direct that pressurized air into the starter motor and fuel control system of the inert engine. Within seconds, the second engine begins to turn and ignite.

The procedure works because modern jet engines produce substantial excess compressor air at idle and climb settings. This air is hot, at 200 degrees Celsius or higher, and pressurized at 40 to 50 psi in some aircraft. The bleed system directs this flow to the starter turbine, which spins the main rotor, and also supplies pneumatic pressure to open fuel control valves and prime fuel nozzles. The crossbleed valve, a solenoid-operated check valve in the bleed duct between engines, opens only on demand.

When and Why SES Matters

SES is most useful on the flight line, during ferry flights, or when a serviceable air-start cart is not at hand. Airlines and military operators train pilots to perform SES as a standard procedure; it appears in the aircraft quick-reference handbook and engine start checklist. The procedure does impose a small penalty: the running engine operates at higher power for 30 to 60 seconds, burning extra fuel and creating higher turbine temperatures. Maintenance must monitor engine parameters and ensure crossbleed ducting, shutoff valves, and pressure relief systems are serviceable.

Not all twin-engine jets are equipped with SES capability. The crossbleed system adds weight, plumbing, and valve components; some regional aircraft or older models lack it entirely. Large four-engine aircraft like the Boeing 747 or Airbus A380 rarely need SES because ground-cart failures are rare and engine-to-engine bleed routing becomes complex. Turboprops, which use different starter and pneumatic architectures, employ equivalent procedures under different names.

Maintenance technicians must understand crossbleed system layout, relief valve settings, and the location of manual shutoff cocks. A stuck or leaking crossbleed valve can cause uncommanded air flow into an engine during ground operation, creating hazardous conditions. Periodic functional tests, usually performed during scheduled maintenance, confirm that the system opens, seals, and relieves pressure correctly. SES is not a workaround or emergency fix; it is an engineered system built into the aircraft's pneumatic architecture and mandated by the certification standard.