When starting a turbine engine, a specific sequence must happen in the right order: an external power source spins the engine, fuel is introduced, igniters fire, and the engine accelerates until it can sustain itself without help. The entire process typically takes 30 to 60 seconds, but each phase has to hit precise parameters or the start must be aborted. Whether you’re learning about jet engines, studying for a pilot certificate, or working on the ramp, understanding this sequence matters because a bad start can destroy an engine in seconds.
The Basic Start Sequence
A turbine engine cannot start on its own. Unlike a piston engine where a brief crank can fire the cylinders, a turbine needs to be spun to a significant speed before combustion is even possible. The starter, which can be pneumatic (air-driven), electric, or hydraulic depending on the aircraft, rotates the compressor and turbine assembly through a gearbox. This gets air flowing through the engine at a rate high enough to support stable combustion.
Once the engine reaches a specific rotational speed, typically around 20 to 25 percent of its maximum, fuel is introduced and the igniters are activated. These igniters work like heavy-duty spark plugs, producing a high-energy arc that lights the fuel-air mixture in the combustion chamber. Once the flame is established and the exhaust gases begin driving the turbine, the engine starts accelerating on its own power in addition to the starter’s input.
Self-Sustaining Speed and Starter Cutout
The critical milestone during any turbine start is reaching self-sustaining speed. This is the point where the energy produced by combustion is enough to keep the engine running without any help from the starter. Research on micro gas turbines has shown that this threshold is precise: in one 30-kilowatt system, the engine could only sustain itself above a specific RPM, and the starter motor was not disconnected until the engine reliably exceeded that speed. In large aircraft engines, self-sustaining speed is commonly in the range of 45 to 60 percent of maximum RPM, depending on the engine model.
The starter continues assisting through this acceleration phase to prevent the engine from stalling or hanging at a low speed. Once the engine passes its self-sustaining threshold and continues accelerating toward idle, the starter disengages. At idle, most turbine engines settle around 55 to 70 percent RPM. From there, the engine is ready to be advanced to higher power settings.
What Can Go Wrong: Hot Starts, Hung Starts, and No Starts
Three problems can occur during a turbine start, and each one demands a different response.
A hot start happens when exhaust gas temperature (often called EGT or ITT depending on the engine) climbs past the maximum limit during the start sequence. This usually means too much fuel is burning relative to the amount of air flowing through the engine. Hot starts are the most damaging type of start failure because the extreme heat can warp turbine blades and damage internal components. If EGT rises toward the red line, the start must be aborted by cutting off fuel immediately.
A hung start occurs when the engine lights off but fails to accelerate to idle speed. It stabilizes at some low RPM and just stays there. This often signals a weak starter, insufficient airflow, or a problem with fuel scheduling. The engine is running but not producing enough energy to accelerate itself further.
A no start is straightforward: the igniters fire, fuel flows, but combustion never establishes. Possible causes include fouled igniters, insufficient fuel pressure, or not enough airflow to support combustion.
How Bleed Valves Protect the Compressor
During the start sequence, the compressor is spinning well below its design speed. At low RPM, the angle at which air meets the compressor blades is far from optimal, creating conditions ripe for a compressor stall or surge. A compressor stall is not a stoppage; it is a disruption of smooth airflow through the compressor stages that can cause loud bangs, vibration, and rapid temperature spikes.
To prevent this, many turbine engines use variable bleed valves that open during low-speed operation. These valves release excess air pressure from between compressor stages, keeping the airflow stable as the engine accelerates. On the CFM56 engine family commonly found on narrow-body airliners, the variable bleed valve system is specifically designed to prevent engine surge by dumping air that would otherwise create a pressure buildup the compressor cannot handle at low speeds. As the engine reaches higher RPM and airflow stabilizes, these valves gradually close so that all the compressed air feeds into the combustion chamber.
Some engines also use variable stator vanes, which are adjustable guide vanes in the compressor that change their angle based on engine speed. At low RPM, these vanes adjust to direct air onto the compressor blades at a more favorable angle, reducing the risk of stall during the vulnerable start and acceleration phases.
Cold Weather Considerations
Cold temperatures affect turbine starts differently than piston engines, but they still matter. Oil viscosity increases dramatically in cold conditions, which means the starter has to work harder to rotate the engine and oil may not flow adequately to bearings during the first moments of operation. In extremely cold environments, pre-heating the engine or at least the oil system may be necessary before attempting a start.
While the specific temperature thresholds vary by engine type and manufacturer, the general principle holds: the colder the conditions, the longer you should allow for oil to circulate and temperatures to stabilize before advancing power. Some operators in cold climates use ground power carts to motor the engine, spinning it without introducing fuel, to circulate oil before committing to a full start sequence.
The Role of the Pilot During Start
In most modern aircraft, the start sequence is semi-automatic. The pilot positions the start switch and moves the fuel lever or condition lever at the appropriate time, while a computer or fuel control unit manages fuel scheduling, igniter timing, and starter cutout. That said, the pilot’s job during a start is to monitor three things closely: RPM (to verify the engine is accelerating normally), exhaust gas temperature (to catch a hot start before it causes damage), and oil pressure (to confirm lubrication is established).
Each aircraft type has published limits for all three parameters during the start sequence, including a maximum allowable time for the start. If the engine hasn’t reached idle within that time window, or if any parameter exceeds its limit, the start is aborted. After an aborted start, most procedures require a cool-down period and a “dry motor” run, where the starter spins the engine without fuel to purge any unburned fuel from the combustion chamber before a second attempt. Attempting a restart with pooled fuel inside the engine is a reliable way to cause a hot start or even a fire.
Ground Versus In-Flight Restarts
Everything described above applies to ground starts, where the engine is stationary and depends entirely on the starter for initial rotation. In flight, the situation is different. Air is already flowing through the engine due to the aircraft’s forward speed, which provides a natural “windmilling” effect that keeps the compressor turning. This makes in-flight restarts generally easier in terms of airflow, though the engine still needs igniter activation and fuel to relight.
Some in-flight restarts can be accomplished without a starter at all, relying on windmill speed alone, provided the aircraft is within the correct airspeed and altitude envelope specified by the manufacturer. Others require the starter to supplement the windmill RPM before fuel is introduced. Flight manuals specify the conditions for each type of relight, including minimum airspeeds and maximum altitudes where a successful restart is possible.