Why Do Airliners Have Multiple Engines?

Airliners are designed with multiple engines for a variety of reasons, including redundancy, performance, and safety. The decision on the number of engines is often driven by the specific requirements of the aircraft and the regulatory standards set by organizations such as the Federal Aviation Administration (FAA).

The Importance of Redundancy

A common misconception is that airliners need two engines to maintain balance during flight. However, the primary reason for having multiple engines is redundancy. If one engine fails, a twin-engine aircraft can still operate with reduced performance, whereas an aircraft with only one engine would need to make a forced landing.

Early airliners, such as the DC-3, were equipped with two engines for redundancy. As aircraft capabilities expanded, especially with the advent of intercontinental flights, the need for additional engines led to the development of four-engine aircraft. The Boeing 747, for example, is a classic four-engine jetliner known for its reliability and performance.

Design Considerations and Aircraft Diversity

The number of engines in a commercial aircraft is carefully selected to provide the best efficiency and performance for the specific mission. Aircraft like the _PC-12, Boeing 737, and MD-11 use two, three, or even four engines depending on their design and intended use. For instance, a short-haul aircraft might benefit from two engines to reduce fuel consumption, while a long-haul aircraft might require more engines for added safety and redundancy.

The Rise of Jet Engines and ETOPS

When jet engines first emerged, they were less powerful and less reliable than their piston engine counterparts. Consequently, four-engine aircraft were the norm in the early days of commercial air travel, with the ability to maintain flight even if an engine failed. However, as jet engines advanced, their power and reliability improved dramatically.

Over time, engineers and manufacturers recognized that two engines could be sufficient for most commercial flights, provided that the aircraft could safely return to an airport within a certain timeframe. This led to the advent of ETOPS (Extended Twin-engine Operations), a certification that allows twin-engine aircraft to fly longer distances.

The FAA introduced the 60-minute rule, which mandated that twin-engine aircraft must be within 60 minutes flying time of an airport. This rule effectively led to the development of tri-jets with a central engine to minimize the risk of asymmetric thrust. While airlines initially resisted this compromise, the safety benefits were undeniable.

As technology continued to improve, the 60-minute rule was eventually extended to 120 minutes, and later to 240 minutes. This allowed for further advancements in aircraft design and operation, leading to the phased retirement of the 240-minute ETOPS certification.

Why Not Just One or Two Engines?

One might wonder why airlines don't use just one engine or two to reduce costs. The answer lies in the practical considerations of flight safety. A single-engine aircraft is fundamentally less safe, as a single point of failure can lead to a sudden and uncontrollable descent. Dually, while two engines can reduce the risk, the 60-minute rule ensures that there is always a nearby airport for emergency landings.

Moreover, certain aircraft, like the Douglas DC-10 and Lockheed L-1011, were designed with three engines to provide a balance between redundancy and fuel efficiency. The central placement of the third engine minimized the risk of asymmetric thrust, while maintaining the necessary level of safety.

Therefore, while it might seem counterintuitive at first glance, the use of multiple engines in airliners is a carefully considered decision that prioritizes safety, performance, and reliability in the ever-changing landscape of air travel.