Why Did McDonnell Douglas Build The DC-10 With 3 Engines?

If you have heard of the DC-10 Twin, then you might ask a deceptively simple question: why did McDonnell Douglas create the DC-10 with three engines, rather than a more efficient two-engine layout? At first glance, adding a third engine seems unnecessarily complex, particularly as modern long-haul airliners now thrive with only two. Yet this solution was very difficult to implement back in the 1960s for several reasons. It reflected the technological limitations, regulatory climate, and market demands of that era. Understanding why the DC-10 was conceived as a trijet helps illuminate the transition between the early jet age of four-engine giants and today’s era of fuel-efficient twinjets.

In our article, we will explore that history in detail. We will examine the circumstances that led McDonnell Douglas to adopt a three-engine design, the proposed but unrealized DC-10 “Twin,” and the crucial role of ETOPS (Extended-range Twin-Engine Operations Performance Standards) in shaping global aviation. By revisiting the regulatory landscape, airline strategies, and the technical ambitions of the DC-10, we can see how McDonnell Douglas responded to its time, and why the trijet eventually gave way to the twinjet revolution.

What Is The Short Answer?

Photo: Kambui I Wikimedia Commons

The short answer is that McDonnell Douglas built the DC-10 with three engines because airline regulations in the 1960s and 1970s did not permit twin-engine widebodies to fly long-distance overwater routes. At the time, a rule known as the “60-minute rule” restricted twinjets from operating more than an hour’s flying time away from a diversion airport. This made them unsuitable for transoceanic and many remote transcontinental services. By fitting three engines, the DC-10 avoided these restrictions and became immediately viable for major international routes, without the need for complex exemptions or unproven regulatory changes.

There were also performance reasons. Jet engine reliability was steadily improving, but not yet to the point where airlines or regulators trusted long-haul operations with only two engines. Having three engines meant airlines could advertise strong safety margins and avoid restrictions. Furthermore, the third engine, mounted in the tail, provided additional thrust for high-altitude takeoffs at airports like Denver International Airport and Mexico City International Airport, where thinner air challenged aircraft performance. For airlines, that meant versatility and route flexibility, both key selling points in a competitive marketplace.

The DC-10 Twin, which removed the third engine, was studied by McDonnell Douglas in the early 1970s. This variant would have been lighter, more fuel-efficient, and closer in concept to Airbus’ brand-new A300. However, it could not escape the long-haul limitations imposed on two-engine designs. As a result, McDonnell Douglas shelved the idea, leaving the DC-10 as a trijet throughout its career; meanwhile, the Airbus A300 became a popular choice for traveling across Eurasia throughout the 1970s and 1980s.

Other Factors That Influenced The DC-10 Engine Layout

Photo: Kplastique | Wikimedia Commons

There are several interconnected factors that shaped the DC-10’s three-engine design: regulatory frameworks, market competition, and technical feasibility. The FAA’s “60-minute rule” applied to all twin-engine jets at the time, regardless of size or reliability. Airlines sought an aircraft that could fly longer, cheaper, and could compete with quadjets, such as the Boeing 707, Boeing 747, or Douglas DC-8, and were thus exempt from restrictions. So, the only feasible way was to remove one extra engine. To compete in that space, McDonnell Douglas had little choice but to develop a widebody that was not bound by the same limitations by creating a trijet.

Equally important was engine reliability. In the 1960s, Pratt & Whitney and General Electric were pushing out new high-bypass turbofan engines, but failures were not uncommon. The risk of losing one engine on a two-engine airliner, while rare, was severe enough to justify restrictions. By contrast, with three engines, the DC-10 could afford to lose one and still complete its flight. This design gave airlines confidence when flying across oceans, deserts, or polar regions, the areas where diversion airports were scarce.

Finally, market competition was critical. The DC-10 was not intended to replace the Boeing 747, but to complement it by offering slightly smaller capacity for high-demand routes. To fulfill that role, it needed range and unrestricted routing flexibility, something a twinjet of the time could not provide.

Experts And Airlines On DC-10

Photo: Paul Spijkers | Wikimedia Commons

Industry observers, former airline staff, and aviation historians broadly agree that the DC-10 Twin was an idea ahead of its time. The proposed variant was seriously studied and even marketed to airlines like North Central, which later became part of Northwest Airlines. As described by Northwest Airlines History, concept art and promotional materials showed how McDonnell Douglas envisioned the two-engine DC-10 serving domestic US routes, competing with the Airbus A300 in Europe. It was a logical idea on paper, but one blocked by regulation.

North Central Airlines showed some interest in the DC-10 Twin, but as a smaller regional player, it lacked the resources to commit to what would essentially be a new aircraft type within its fleet. Other airlines were cautious, too. The economic turbulence of the 1970s, marked by oil shocks and recession, meant carriers avoided experimental models with uncertain regulatory status. Instead, most preferred to buy the established trijet variant that was already certified for global routes.

Enthusiasts on forums such as Quora and Reddit and aviation history sites often speculate that if ETOPS had been introduced a decade earlier, the DC-10 Twin might have found buyers and carved out a profitable niche. But timing is everything in aviation. McDonnell Douglas had already struggled to compete financially with Boeing, and the company lacked the resources to launch an all-new derivative without guaranteed demand. In hindsight, the DC-10 Twin is a fascinating “what if,” but not a viable path for the company at that moment.

DC-10 Vs Other Aircraft From The Same Era

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The Airbus A300, launched in 1972, just two years after the DC-10, provides a striking comparison. It was the world’s first twin-engine widebody, but it was designed primarily for short- to medium-haul European routes. Europe’s regulatory environment was slightly more relaxed than the FAA’s, allowing twins to operate further from diversion airports. Thus, Airbus carved a niche where the DC-10 Twin might have competed, such as high-density, medium-range flying that did not require transoceanic capability.

However, the A300 was limited in scope. It could not initially fly across the Atlantic or operate long-haul to Asia. Airlines saw it as a regional supplement rather than a global flagship. By contrast, the DC-10, with three engines, matched the Lockheed L-1011 TriStar and even challenged Boeing’s smaller 747 variants on specific intercontinental routes. In this sense, the trijet design gave McDonnell Douglas more commercial flexibility.

DC-10 vs Early Twin Widebody Design Comparison:

This table highlights the DC-10 Twin’s limitations compared to its contemporaries, and why the design never advanced. It also illustrated how the Boeing 767, which arrived a decade later, gained ETOPS certification for cross-Atlantic flights. Had the DC-10 arrived a bit later or had the engine design been more reliable, we probably could have seen the DC-10 Twin in action, but history doesn’t like “what ifs”.

When ETOPS arrived in the 1980s, the tables turned. Suddenly, twin-engine jets like the Boeing 767 and Airbus A310 could cross oceans, offering lower operating costs than trijets. This shift rendered the DC-10 Twin obsolete before it could even fly, as newer twinjets occupied the very market McDonnell Douglas had once hoped to fill. The eventual dominance of twinjets highlights just how quickly the regulatory and technological environment changed.

A Slow Decline Of Trijets

Photo: D. Coleman | Wikimedia Commons

While the trijet solved regulatory problems, it introduced new challenges. Mounting an engine in the tail created structural and maintenance complexities. Critics argued that positioning a massive turbofan above the fuselage placed additional stress on the airframe and increased risks in case of failure. Pilots, however, generally adapted well, and the design became iconic not only on the DC-10 but also on its successor, the McDonnell Douglas MD-11, as discussed by the users of the Infinite Flight community.

The DC-10’s safety record also became controversial. Catastrophic accidents such as Turkish Airlines Flight 981 in 1974 and American Airlines Flight 191 in 1979 highlighted design flaws unrelated to engine placement, but they cast a shadow over the aircraft’s reputation. In reality, once these design issues were corrected, the DC-10 went on to have a long and generally safe career. Still, the public perception of the DC-10 as “unsafe” was difficult to shake, and some critics unfairly blamed the trijet layout for its troubled early years.

Photo: © Raimond Spekking | CC BY-SA 4.0 | Wikimedia Commons

Had McDonnell Douglas launched the DC-10 Twin, it might have faced its own drawbacks. A two-engine widebody without ETOPS approval would have been restricted to short- and medium-haul flights, directly competing with the Airbus A300 and later Boeing 767. That market was already crowded and would have forced McDonnell Douglas into a more challenging competitive space. Ironically, by avoiding this move, the company prolonged the relevance of the DC-10 as a long-haul workhorse.

The Bottom Line

Photo: Tomas Del Coro | Wikimedia Commons

Ultimately, the DC-10’s three-engine design was a product of its time. McDonnell Douglas needed a widebody aircraft that could compete with Boeing and Lockheed while bypassing regulatory restrictions on twinjets. The trijet provided performance, compliance, and commercial flexibility in an era before ETOPS, ensuring the DC-10 was viable for global service.

At the same time, the unbuilt DC-10 Twin illustrates that McDonnell Douglas saw the potential for a twin-engine future, but the company was constrained by timing and circumstance. Once ETOPS became a reality, twins like the Boeing 767, 777, and Airbus A330 took over the long-haul market, relegating trijets to history and also ousting many quadjets. Yet in the early 1970s, a trijet was the only way forward in the long-haul market dominated by quadjets.

Today, long-range flights are dominated by ETOPS-rated twins like the Boeing 787 and Airbus A350, capable of flying more than six hours from a diversion airport. The DC-10, though now retired from passenger service, continues in limited cargo services and remains an important milestone in aviation history. Its trijet design reflected the best solution available in its time, bridging the gap between the four-engine giants of the 1960s and the twin-engine revolution that defines aviation today.