An airplane engine starter is an electromechanical mechanism capable of developing large amounts of mechanical energy that can be applied to an engine, and its designs range from solenoid-actuated high-torque motors to components containing automatic engaging and disengaging mechanisms operated by an adjustable torque-overload-release clutch.
The air turbine starter, a unit that supplies the cranking power needed to start the engines on every aircraft that flies, receives its high-volume, low-pressure air from a ground support equipment known as the Air Start Unit, ASU, or Air Start Cart, which is specifically designed to provide high-volume, low-pressure air.
Because the starter must deliver reliable torque while guaranteeing safe engagement and disengagement, both the airborne starter and the ground-based Air Start Unit incorporate built-in safeguards that protect the engine and the personnel during the start sequence.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the name for an airplane engine starter?
An airplane engine starter is simply termed the starter, yet it is known by many trade names that reveal its type or power source. Self-contained starting systems carried on the aircraft are called Auxiliary Power Unit (APU), Jet Fuel Starter (JFS), Gas Turbine Compressor (GTC) or, when the unit is rolled to the aircraft, Air Start Unit (ASU). Inside the airframe the most prevalent forms are the air-turbine starter (ATS), the electric series-wound starter motor, the inertia starter and the starter/generator. Because each design converts stored energy into mechanical torque that turns the engine, all are correctly classed as electromechanical starters.
How does an airplane engine start?
An airplane gas turbine engine starts when the right components are rotating, fuel is introduced and ignition produces sparks in the hot section. Air/pneumatic starting uses bleed air from the onboard auxiliary power unit, a ground power unit or cross-bleed air from an already running engine. The APU itself is a smaller turbine engine started using a battery which operates a gearbox whose output shaft turns the high-pressure compressor to reach a self-sustaining speed that leads to a successful start.
Compressed air passes through the turbine wheel, accelerates the high-pressure compressor and supplies sufficient air to support combustion. Fuel is turned on after ignition is turned on and combustion occurs in the combustion section. Once the crew has verified stable rotation and the required flow of fuel, ignition is turned off and the engine runs under its own power. After the first engine is started using APU bleed air, the crew opens the cross-bleed valve so that bleed air from the running engine starts the remaining engines, completing the aircraft engine start sequence.
How does an airplane engine starter work?
An airplane engine starter works when the pilot turns the key or flips the switch in the cockpit. A solenoid then sends power from the battery to the starter motor. The motor, typically a series-wound unit that develops high starting torque, rotates a pinion gear. This small gear engages with the teeth on the flywheel, causing the flywheel to rotate. In piston engines the flywheel is attached to the propeller hub whereas in turbine engines the same principle applies through a reduction gear train. As the starter motor turns the flywheel, the flywheel's motion is transferred to the crankshaft, beginning the compression stroke and drawing the fuel-air mixture into the cylinders. While the engine is spinning, the starter provides current and the magnetos - mounted on the engine accessory case - generate a strong current that travels to the spark plugs, igniting the mixture.
Once the engine reaches a predetermined speed, its internal generator can sustain operation and the starter automatically disengages. An overrunning clutch in the starter adapter allows the pinion gear to withdraw from the flywheel, preventing the engine from driving the starter. Modern starters use permanent magnets for lighter weight and greater efficiency, but all types retain a torque overload release clutch to prevent damage if the engine backfires or overspeeds. Whether powered by the aircraft battery or an external ground unit, the starter's whole purpose is to spin the flywheel long enough for ignition to occur and after that, the engine runs on its own and the starter motor cuts out.
How does an engine crank work in aviation?
An engine crank works in aviation when at start-up, battery power supplied by the battery energises the starter motor. The starter solenoid and starter adapter extend the starter jaw so that the starter gear pinion jaw engages the propeller hub ring gear teeth. Once the teeth are meshed, the pinion turns the engine crankshaft, and the crankshaft rotates the propeller hub. While the crankshaft rotates, the crankpin pulls the connecting rods. The connecting rods force the pistons downward, and the pistons compress the fuel-air mixture already drawn in during the preceding intake stroke. When the mixture ignites at top dead centre, expanding gases furnish the first useful power stroke, the crankshaft absorbs power from all the cylinders and transfers it to the propeller, and flywheel attached to the crankshaft flange stores rotational energy to smooth the engine's rotation between evenly spaced power strokes. As rpm climbs above self-sustaining speed, the starter pinion disengages, the crankshaft continues to rotate with its heavy flywheel, and the engine runs on its own until the pilot selects cut-off.
Throughout the sequence, oil pressure results from the clearance between the rod journal bearings and the main journal bearings in the engine crankcase. Oil passes through oil galleries to the crankshaft saddles, lubricates the main journal bearings and the rod journal bearings, and forms an oil film on every bearing surface so that the crankshaft journals ride on a film of oil. Counterweights balance the weight of the pistons and rods, reducing vibration, while magnetos mounted on the accessory case and driven by the crankshaft create the high-voltage spark that sustains combustion. Thus the crankshaft converts the up-and-down motion of the pistons into rotational motion, steadily rotates the propeller, and delivers smooth power to the propeller for the entire flight.
What is an aircraft engine air starter?
An aircraft engine air starter is a pneumatic mechanical device that uses compressed air to crank and start engines. It is used on large engines where an electric starter is too big, heavy or expensive. The air starter delivers a forceful burst of pressurized air directly to the aircraft's pneumatic engine starting system through hoses attached to the aircraft's under belly. This highly compressed air is used to drive a fluid motor inside the starter. The force of the expanding air spins the turbine wheel of the starter and mechanically connected through reduction gears to the highest-pressure compressor of the jet engine, turning the engine and giving it the initial rotation it needs to reach ignition speed.
The jet engine start valve is the unit that opens to admit the air supplied by the air starter unit into the air turbine starter. During start, the valve regulates pressure of the starter operating air and normally shuts off automatically when the engine accelerates beyond starter speed. Some start valves have an override that requires a small tool, so a licensed aircraft engineer can manually crack the valve for maintenance or in an emergency.
An aircraft engine air starter pneumatic ground support equipment, alternatively called an air start unit aircraft (ASU), is a self-contained piece of ground handling equipment that supplies the necessary quantity of air at specified pressure through one, two or three hoses attached to the aircraft's under belly. Applicable to wide-body passenger aircraft, business jets, military helicopters and turboprop transports, the unit stores or produces compressed air by powering a compressor with a diesel engine, an electric motor, via a gas turbine, or from large pre-filled air reservoirs, and it can deliver continuous flow or be of bottle type. When the aircraft's own APU is inoperable or if the airplane is already running, a cross-bleed start is conducted, bleed air from an already-running engine is directed through hoses to start the remaining engine, pulled from the APU or from a ground-operated air cart supplying pneumatic power to keep aircraft operations efficient and minimize downtime.
What is a jet engine electric starter?
A jet engine electric starter is an electromechanical mechanism that gives the initial energy required to spin the compressor. It consists of an electric motor - often a 12 or 24-volt series - wound motor - mounted on a right-angle drive adapter. Reduction gears, an automatic engaging and disengaging mechanism, and an adjustable torque overload release clutch are housed within the same assembly so the starter can develop the high starting torque needed to overcome compressor inertia yet protect itself against overload.
Power is drawn either from an onboard battery or from a ground electrical supply and operation is initiated by a switch or key in the cockpit. Once energized, the motor turns the shaft inside the starter adapter, providing instant and continual cranking until the engine reaches self-sustaining speed. The motor then disengages when engine speed exceeds starter speed.
Electric starters are common in smaller jets, whereas larger jet engines require a dedicated source of compressed air. Modern direct-current starters are compact and reliable, but their convenience and stronger starting current have been gained at the expense of extra weight and increased complexity.
What is recommended before starting the engine in aviation?
Before starting the engine in aviation, the pilot must guarantee the ramp area surrounding the airplane is clear of persons, equipment, and other hazards, including the propeller swing path and fire hazards. A propeller area check is mandatory, followed by shouting ‘Clear’ or ‘PROP CLEAR’ before switching on the starter. The pilot reviews the before-start checklist, which includes verifying that the master switch, fuel valve, magnetos, mixture (rich for normal starts, idle cutoff for hot starts), throttle, carburetor heat (cold unless hot start), circuit breakers, radios, transponder, and brakes are set correctly. Seats must be adjusted and locked, seat belts and shoulder harnesses fastened, and flight controls confirmed free and moving correctly. The engine area is inspected for fuel leakage, and oil pressure and temperature are expected to be in the green after start. For cold starts, priming is required using the primer (not the throttle), and the fuel pump must be switched on temporarily. Hot starts involve opening the throttle, switching the fuel pump on, advancing the mixture to full rich to purge vapor, then retarding to idle cutoff before employing the starter. Preheating is recommended when temperatures are substantially below freezing.
What are the limitations of engines starting in aviation?
The limitations of engine starting in aviation include that all starting systems have operating time limits because of the high energy used during cranking or rotation of the engine. Starter limitations for small aircraft limit cranking periods to 30 seconds with a two-minute rest between cranking periods. Limitations for the engine are boundaries which, if exceeded, will cause irreversible internal damage to an engine. The starter motors on small aircraft have operational limits with cool down times that should be observed. If the starter has not started after three, 10-second periods of operation with a pause between each, a five-minute cooling off period is required. A wet start can happen when the engine receives fuel but fails to ignite.
The starter must continue to assist the engine above the self-accelerating speed to avoid a delay in the starting cycle that results in a hot or hung false start. A hot start occurs when the engine's internal temperature exceeds safe limits whereas a hung start lacks enough energy to speed up the compressor and leaves the RPM much lower than required. The starter must not be employed when N2>20 %; above this speed the drive gear is automatically disengaged and any re-engagement constitutes a fresh cycle. Adherence to these starter limits protects the starter and ensures reliable, damage-free engine starts.
How did old airplanes start up their engines?
Early piston engines were started by hand, through the oldest and simplest method of swinging the propeller. The pilot grasped a blade and gave it a sharp turn and the impulse set the magnetos spinning so they provided current to the spark plugs, which then ignited the fuel in the cylinders. If the engine was cold, a primer injected fuel right at the intake ports to create a combustible mixture before the blades moved.
Larger radial engines, like those on the B-17, used a hand-cranked inertial starter. A manual crank input shaft on the left side of the cowl let the operator wind up a heavy flywheel attached to the propeller. When the stored kinetic energy was released, the flywheel turned the crankshaft and rolled the engine over. Between the First and Second World Wars geared hand-starting, electrical, and cartridge-operated systems were developed for still larger powerplants, yet many trainers like the BT-13 kept both the manual crank and an electric starter so the pilot could choose either method. Whatever the technique, the sequence on multi-engine aircraft was methodical: on the C-54 the crew started engines 3, 4, 2, 1, and at shutdown they reversed the order - 1, 4 - so the airplane always remained balanced and under control.
