In the 1970s, airlines wanted to operate long-haul flights over the ocean, but twin-engine aircraft were largely not permitted to do so.

The regulations at the time required that for flights far from alternate airports, the aircraft must have at least three engines. It was believed that if one engine failed, the remaining two could safely complete the flight.

This led to the development of aircraft like Boeing 727, McDonnell Douglas DC-10, McDonnell Douglas MD-11, and the famous Lockheed L-1011 TriStar. The third engine was placed in the tail section of the fuselage — at the base of the vertical stabilizer.

Over time, however, jet engines became so reliable that the regulations changed.

Thus, the ETOPS standard was introduced. It allowed twin-engine aircraft to perform long flights over the ocean, flying hundreds of minutes away from alternate airports.

The two modern engines provided the same thrust as three engines did previously, while consuming less fuel and being significantly cheaper to maintain.

Three-engine aircraft did not become any worse — twin-engine ones simply became better.

The last three-engine passenger airliner was produced in 2000. Since then, new passenger aircraft have only been manufactured with two or four engines.

Have you ever flown on a three-engine aircraft? Share in the comments.