Airplanes rely on engines to generate propulsion. While wings and other flight control surfaces generate lift, engines are responsible for generating propulsion. Jet engines, for instance, suck in air through the front so that it can be burned with fuel. The byproduct gases are released out of the back, which propels airplanes forward. Most commercial airplanes, however, aren’t powered by a single engine; they are powered by four engines. Why do airplanes have four engines exactly?

Increases Passenger Capacity

With four engines, airplanes can carry more passengers. Most wide-body airplanes have four engines for this reason. When compared to two-engine wide-body airplanes, those with four engines can carry more passengers. The Airbus A380, for example, has a passenger capacity of 853. Like most other wide-body airplanes, the Airbus A380 is powered by four engines. This allows it to carry more passengers than its two-engine and single-engine counterparts.

Increases Thrust

Another reason airplanes have four engines is to increase thrust. Thrust is the force that engines use to propel airplanes through the air. For airplanes, it’s needed to overcome drag. All objects create drag as they move through the air — and airplanes are no exception. Fortunately, engines can overcome drag by the use of thrust. Jet engines create thrust by ejecting byproduct gases out of the back. As these byproduct gases are ejected, they’ll create thrust, which in turn generates propulsion. And the more engines an airplane has, the more thrust it will create.

Previously Required for Transatlantic Flights

In the past, airplanes were required to have four engines for transatlantic flights. Airplanes with fewer engines lacked the thrust needed to achieve these long-distance flights. Therefore, requirements were set for the use of four-engine airplanes. Even today, many airplanes that perform transatlantic flights have four engines.

Safety

There are also safety reasons why airplanes have four engines. Flying, of course, is statistically a much safer method of transportation than driving. Nonetheless, airplanes can still experience mechanical failure that can lead to accidents. Airplanes with four engines aren’t immune to mechanical failure, but they are less likely to experience an accident than those with fewer engines. If one of the engines fails, they’ll still have three other engines operational.

The number of engines airplanes have can vary. Some airplanes only have a single engine. Other airplanes have two engines. With that said, a four-engine configuration is particularly common. Airplanes often have two engines on each wing. It allows for a higher passenger capacity, increases thrust, improves safety and was previously required for transatlantic flights.