Why The Aviation Industry Abandoned Tri-Jet Commercial Planes

From the 1970s through the 1990s, tri-jets were a common sight in our skies. Aircraft with three jet engines were capable of transoceanic flights and more fuel-efficient than four-engine aircraft like the Boeing 747, with its oddball hump over the cockpit. Smaller tri-jets like the Boeing 727 were known for their exceptional short-field capabilities. 

You don't see many tri-jets anymore, though. Time has marched on, and classics like the 727, McDonnell Douglas DC-10 (take a look inside the factory that built those 1960s airliners), and Lockheed L-1011 are outdated and inefficient by modern standards. Even the MD-11, one of the most modern tri-jets produced, has been relegated to cargo duty after its final passenger flight in 2014. 

Twin-engine jets have taken over the skies. Two engines use less fuel than three, and modern turbofan engines are much more powerful and efficient than the turbojets of old. Regulations have changed to allow twin-engine airliners to fly farther and farther from diversion airports, so that now they're allowed to fly across oceans — sealing the fate of three- and four-engine aircraft for long-haul flights.

It's not often that you get better fuel efficiency and more power at the same time, but that's exactly what the turbofan does. Its core is still a turbojet engine, but a turbofan adds a large fan to the front to propel some air into the turbine while another stream of air bypasses it. The bigger the fan, the more air bypasses the jet, producing much more thrust than the turbojet alone without using much more fuel. This is an oversimplified explanation, but you get the idea.

High-bypass turbofans have become quite popular in modern airliners. They can produce a great deal of power when needed during takeoffs and go-arounds. Turbofans also burn less fuel when cruising because the fans contribute so much thrust, making them far more efficient than older turbojets. About 23% of an airline's operating expenses pay for fuel, so aircraft running only two high-bypass turbofan engines instead of three or four turbojets could save a lot of money. The only problem was flight restrictions for twin-engine airliners.

Engines weren't always as reliable as they are today. In the early days of commercial aviation, it was fairly common for an aircraft with four piston engines to arrive with only three of them still working. Even into the jet age, twin-engine aircraft were required to remain within a 60-minute flight to the nearest airport to make an emergency landing if an engine failed. This regulation — called ETOPS, or Extended-range Twin-engine Operational Performance Standards – made it impossible for commercial twin-engine aircraft to fly across oceans, relegating them to short or medium-haul routes.

Since tri-jets have three engines, they were exempt from this 60-minute requirement. This led to the development of widebody intercontinental tri-jets like the DC-10 and L-1011. Three engines were less expensive to run than four, so this was a chance for McDonnell Douglas and Lockheed to gain ground on Boeing with its less efficient four-engine designs, and tri-jets thrived for a while.

However, by the 1980s, turbofan engines had proven to be extremely reliable, and ETOPS rules were changed to allow commercial twin-engine aircraft to fly first 120 minutes, then 180 minutes away from possible diversion airports, and then even further, making transoceanic flights possible. Tri-jets fell out of favor, except for the private planes of the ultra-rich, for the same reason they became popular in the first place: fewer engines use less fuel and are less expensive to run. The rest is history.