Popular Trijet: Boeing-727.
US-based planemaking juggernaut Boeing catalyzed the jet age when it launched the four-engine 707 in the 1950s.
However, going forward, it also had to consider what aircraft it might produce for shorter flights to and from smaller airports. Airlines had different demands, with some wanting proven four-engine jets while others preferred the idea of twin-engine aircraft.
Boeing came up with an alternative solution that effectively met both of these demands halfway. The result was the three-engined Boeing-727 design.
To meet customer demand, Boeing needed to create an aircraft that could access smaller airports, as short runways were unsuitable for the larger Boeing-707. Prospective customers included American Airlines, Eastern Air Lines (which would be the launch customer in 1964), and United Airlines. American was looking for a twinjet, but Eastern demanded a plane with at least three engines capable of serving its international network.
Boeing eventually decided on a three-engined design, which was satisfactory for the three aforementioned airlines. It would be the only trijet Boeing ever produced.
This new plane also needed to descend quickly into airports while avoiding obstacles (like buildings) in the runway's vicinity. To achieve this, Boeing developed a large and sophisticated flap system, which provided extra lift at low speeds.
In another first, the 727 also featured a small gas-turbine engine auxiliary power unit (APU). This innovation eliminated the need for a ground power supply to start the engines and became a particular selling point for airlines operating in developing countries.
As Boeing began recognizing the aircraft's potential in the late 1950s, its rival, Douglas, was working on the rear-engined DC-9. Meanwhile, over in Europe, the British Aircraft Corporation was also designing the BAC 1-11 around this time. Hawker Siddeley developed the Trident - arguably the closest matched performer to the 727 (production of the Trident concluded in 1984).
Boeing knew it needed to sell at least 200 examples of the aircraft to make the 727 a success. However, by December 1960, it had received orders for just 40 examples from Eastern Air Lines and United Airlines. Even when the aircraft made its first test flight in 1963, orders were still well below the break-even point.
As such, Boeing sent the 727 on a worldwide sales tour in an attempt to drum up business, during which the aircraft visited 26 countries. By the time it returned home, the jet had clocked up an impressive 122310km. Ultimately, the gamble paid off and Boeing far surpassed its original target.
The 727 first flew commercially with Eastern in 1964. However, tragedy plagued the new aircraft the following year, with Boeing witnessing three of its 727-100 aircraft crash within three months. Altogether, these accidents resulted in 131 fatalities.
Investigators looking into the crashes discovered that some pilots did not fully understand the flap system. As such, they reportedly allowed the planes to descend at too great a speed. Some politicians called for the type's grounding.
However, Boeing and safety regulators were adamant that nothing was wrong with the plane. Even so, the authorities did declare that 727 pilots needed more training and that Boeing should modify the flight manual procedures regarding the final approach. Despite reassurances that it was safe to fly in, the aircraft fell out of favor among passengers for around six months.
Boeing's original production run was for 250 aircraft, but the larger 189-seat 727-200 proved vastly more popular than expected. In total, across all 727 varieties, Boeing built and delivered 1832 of the trijets, far surpassing its original target. After 22 years in production, FedEx received the final Boeing 727, a 727-200F, in 1984.
By 2003, most airlines had retired the loud, thirsty planes in favor of quieter, more fuel-efficient aircraft. 727's last commercial flight took place in January 2019.
Despite its accident-plagued early years, the 727 became one of Boeing's (and, indeed, the world's) best-selling airliner families. While the 727 filled a niche in its day and overwhelmingly exceeded Boeing's expectations, a trijet commercial aircraft simply wouldn't work in today's world.