Why Aren't Most Planes Equipped with Four Engines?

In the world of commercial aviation, you might wonder why most planes are not made with four engines, especially considering the A320 series and other models. The standard configuration of two engines is not just a matter of tradition; it is driven by a complex interplay of financial, technological, and safety considerations. Let's delve into the reasoning behind this design choice.

Cost and Profit Margins

The primary reason for the prevalence of two-engine aircraft is the cost efficiency and financial implications. Commercial jets are purchased by airlines, whose primary objective is to maximize profits. The cost of acquiring a two-engine plane is significantly less than that of a four-engine plane. This price difference is not just in the initial purchase but also extends to fuel, maintenance, and operational costs. In terms of fuel, a two-engine aircraft will use half as much fuel compared to a four-engine aircraft, making it a more economical choice in the long run.

Technological Advances and Reliability

Technological advancements have played a significant role in the evolution of aircraft design. Improvements in engine technology and materials have enabled manufacturers to rely on two engines to transport the same number of passengers over the same distance. Previously, four-engine designs were necessary for larger aircraft such as the Boeing 707, Douglas DC-8, Boeing 747, and Airbus A340. Modern systems and continuous improvements have made two-engine jets not only feasible but also highly reliable.

Reliability has improved to such an extent that the probability of both engines failing at the same time is now almost unthinkable, except in extremely rare circumstances. In the past, transoceanic flights required three or four engines for redundancy. However, as reliability has proven more consistent, airlines have increasingly switched to two-engine configurations, which offer a cost-effective solution without compromising safety.

Emergency Situations and Flight Safety

Some might argue that having spare power would be advantageous. However, the potential for catastrophic failure extends beyond just engine loss; multiple engine failures can occur in extremely rare circumstances, such as those caused by volcanic ash clouds. In such situations, the number of engines does not provide a significant safety advantage. Modern twin-engine jets are designed to glide for a considerable distance even in the absence of engine power. In fact, there have been documented instances where twin-engine jets managed to land safely even with no engine thrust.

Examples like the British Airways 747 incident, where all engines shut down due to a volcanic eruption, illustrate that the number of engines is not a preventive measure for catastrophic failure. Consequently, the design focus for future aircraft continues to emphasize reliability, safety, and cost efficiency. As such, the demands of governing bodies like the FAA ensure that new engines undergo rigorous testing to establish their reliability. This stringent testing helps predict and mitigate risks, making single-engine operations a feasible possibility in the future, should regulations change.

Therefore, the choice to equip most planes with two engines is driven by the need for financial prudence, technological advancements, and proven safety records. While the idea of having four engines might seem more secure, the practical realities of cost and performance make two-engine designs a preferred choice in the current aviation landscape.