A jet engine is the engine that pushes an aircraft forward by throwing hot gas backwards at high speed. It pulls air in at the front and compresses it. It burns fuel in the compressed air, and the hot gas that results spins a turbine and then leaves at the back. In most airliners, the turbine also spins a large fan at the front, which produces most of the push.
Only a handful of companies make jet engines for airliners. An engine is very expensive to design and certify under safety certification rules. So there are few makers, and an airline that buys an aircraft chooses from the few engine types that the aircraft maker offers. A single aircraft type, such as the A320, is often offered with a choice of two engine types.
An engine maker earns much of its money after the sale. An engine is used for decades and it needs inspection, repair and spare parts again and again. This is the idea of aftermarket services. Some airlines pay the maker for each hour the engine flies, instead of paying for each repair. Then the maker has a reason to keep the engine healthy.
An engine that fails unexpectedly is very costly for an airline. The aircraft stays on the ground, which airlines call AOG, short for aircraft on ground. The passengers must be moved to another flight, and the engine has to be removed and replaced. So airlines want to repair an engine before it fails.
Take, for example, an airline in India with an engine on an A320. On every flight, sensors record the temperature of the exhaust gas and the vibration of the engine. Say the exhaust temperature rises by 8 °C over three months. Asset performance management software compares the engine with how it normally behaves, and raises an alert. The airline's engineers then book an inspection at its next planned stop. This is predictive maintenance: the engine is repaired before it fails. The airline records the inspection in aviation maintenance software.
The engine is installed into the aircraft and tested along with it. Its control software must follow the same strict safety standards as the rest of the aircraft.