dan asked in Cars & TransportationAircraft · 4 weeks ago

Why do we no longer use tri-engine aircraft?

I like the aircraft of the 1930s and 1940s and the tri-engines look interesting. I am just wondering why they stopped making theme? The only thing I have heard is that they had bad glide if one engine went out. Is that true or is there more to it?

12 Answers

  • 3 weeks ago

    If you're talking about the aircraft that used to have two engines on the wings and one on the nose, it's because engines got more powerful and reliable, and removing the engine from the nose improved the pilot's forward vision.

  • 3 weeks ago

    Both the DC10 and the 727 were tri engine. One of the issues decades ago is to fly Trans Atlantic or Trans Pacific a passenger plane had to have more than two engines. An early way around it was just to add a third engine in the middle. However that created problems. Assume all three engines rotated in a clockwise fashion when viewed from the cabin, you would get more thrust from the left engine between it and the cabin more thrust from the center engine between the engine and the third engine and the right engine would give more thrust from the engine to the outside of the wing. In other words, the right wing would bet better thrust in a normal situation. That required you to either run the left engine harder OR to keep using right rudder to keep the plane straight. If the left engine went out then you had a LOT of thrust on the right wing and none on the left. That would tend to move the plane to the left and often would cause it to flip over to the left. Three engine prop planes just were accidents waiting to happen so they went with four engines. That carried over to the jet age and again, we had four engines. However prop thrust was not a problem and Both Boeing and Lockheed went to three engines. They could still fly across the ocean because they had more than two engines and it made it less expensive to make the plane. Eventually jet engines got so reliable the standard was changed to two engines and now almost all planes coming out for passenger service have just two engines. The exception to new planes is the A380 which is so big it needs four engines.

  • 3 weeks ago

    There are many possible answers to this question. Firstly there’s the Etops rule as already explained in another answer, then there’s fuel efficiency as a 3 engine aircraft obviously burns more fuel than a 2 engine one, and also engines have become more reliable, reducing the need for them. Lastly, there were a few incidents when the L1011 tristar and the DC10 were in service where the engine in the tail had a failure and had an explosion. Due to the location of the engine, the shrapnel from the explosion was able to cut the hydraulic cords in the tail causing massive control problems which caused a crash where people actually died. Hope that answers your question! Iolo

  • 4 weeks ago

    Probably the biggest factor in why tri-jets were designed, built, and highly popular was regulations from the FAA and other global regulators.

    In 1936 the FAA implemented a rule that aircraft must plan their routes so that they were never more than 100 miles from an airport where they could potentially make an emergency landing if an engine failed. Originally it did not matter how many engines you had or what type. The piston powered aircraft if that time period could fly about 100 miles per hour with one failed engine, so being 100 miles from an airport meant being within 60 minutes.

    Jet engines were introduced in the early 1950's which meant aircraft could fly faster. So the 100 mile radius from emergency airports was outdated. so in 1954 the FAA updated the rule so that jet aircraft could fly 60 minutes from the nearest diversion airport. So however far they could fly in 60 minutes with one failed engine was how far they could be from an airport.

    Initially this modified "60 minute rule" applied to all jet aircraft. But by the early 1960's jet engines had proven their reliability to be far superior to the old piston engines. The idea of a "tri-jet" was already proven with the 3 jet Boeing 727 successfully and reliably flying shorter domestic routes.

    So in 1964 the FAA lifted the 60-minute restriction for jet aircraft with more than 2 engines. This opened the door for the wide body tri-jet aircraft. With the rule change manufacturers quickly created the wide body tri-jets. The overall operating cost of a tri-jet was far less than a 4 engine aircraft, and twin jets couldn't legally fly over oceans, so the Tri-jets quickly gained a significant presence in the wide-body fleet.

    By the mid 70's twin jets were powerful enough to fly cross-ocean routes, but regulations still would not allow it. In 1980 Boeing approached the FAA seeking an exception to allow the twin engine 767 to fly longer routes farther from airports than the required 60 minute radius. But FAA director J. Lynn Helms response was "It'll be a cold day in hell before I let twins fly long haul, over-water routes."

    But Helms retired in 1984 and in 1985 the FAA approved the new ETOPS 120 rule, which allowed qualifying twin jets to fly as far as 120 minutes (based on their single engine speed) from the nearest airport. that radius eventually grew to 180 minutes and now aircraft can be certified to even higher ratings if the manufacturers demonstrate capability and reliability.

    The new ETOPS 120+ ratings in the mid 1980's allowed twin jets such as the Airbus A300 and Boeing 767 to fly routes that previously required a trijet. And just like 3 engines are cheaper than 4, the industry quickly adjusted their fleet plans since 2 engines are cheaper than 3. And thus, the end of the tri-jet era.

    By now, the ETOPS regulations basically allow twin jets to fly any route in the world as long as they have the range, and with large twinjets such as the 777 and A350 now capable of flying (almost) halfway around the globe, the 4-engine era is coming to an end also, with the A380 already planned for shutdown, and the 747 hanging by a thread with only a few orders for freighter or specialty models (airforce one, etc) trickling in. In fact Boeing hasn't sold a 747 passenger aircraft in several years, and will probably never sell another one.

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  • 4 weeks ago

    The fewer engines you hang off an aircraft, the more economical it becomes both in terms of capital outlay for the engine and on going maintenance.

    With modern engines being more powerful and reliable, two engines can now be used where previously three were required for power and redundancy.

  • Anonymous
    4 weeks ago

    We Did, up to 1985.The 737.

    No , all aircraft must be designed to safely fly on one engine out, whichever one. And, a specific glide--lose only so many feet in elevation/mile.

  • 4 weeks ago

    The early center engine design was not preferred because of the oil spray on the windscreen and the noxious fumes in the cockpit and passenger cabin. So when the early passenger planes had two or four more powerful engines on the wings, that was much better design for safety and comfort.

  • 4 weeks ago

    The main problem with a tri-engine configuration is that you need two different nacelle installations, and that drives the cost up. Piston engines used in the 1930's do not require a clear path behind them, unlike jet engines, which gave designers perhaps a bit more freedom.

    When the 747 was designed in the late 1960's, it made use of 4 of the then largest engines available. To make something smaller would either force it to be half the size with two of the same engines as the 747, or to compromise with a 3 engine configuration. That is why there were 2 three engine wide body designed to bridge the gap between the large twins (A300, Boeing 767) and the 4 engine -- namely the DC-10 and the Lockheed 1011.

    Since then, engines got a lot bigger still, to the point that there can be twin-engine jetliners as large as early 747 variants. And the limited success of the A380 seem to imply that the real market for jetliner essentially tops at 400 seats or so, a size that A350 and Boeing 777 occupy with their twin engine configuration.

    Moreover, demonstrated safety and certification of reliability allow twin engine aircraft to fly ETOPS routes, getting far enough from alternate emergency airport to permit point to point route across oceans, something that was not allowed 40 years ago, and made 3 or 4 engines jetliner a requirement.

    The "bad glide with an engine out" thing is not true.

  • 4 weeks ago

    It's down to progress.  Improvements in design and technology mean that modern engines are much more powerful, reliable and economical than used to be the case, so the third engine is no longer required.  If the designers of the Trimotor or Ju52, for instance, had had more modern engine designs available they could have designed them for only two.

  • JetDoc
    Lv 7
    4 weeks ago

    Tri-motor airplanes were made that way because the aircraft engines of the day were fairly low powered, and the aircraft needed three engines to perform properly. As engines and aircraft design technology improved, there was less need for the third engine.

    Three engine jets like the L1011 and the MD-11 were built because Government regulations said the airlines couldn't use two engine aircraft to fly across the oceans. Once that rule was changed, the three-engine airplanes quickly were retired.

    • Periferalist
      Lv 7
      3 weeks agoReport

      And the change to allow two-engine airliners also had to do with the fact that eventually, jet engine technology made it safe to fly airliners on two engines instead of three or four.

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