Looking through my father’s old pictures of Heathrow Airport from the seventies and eighties, I can’t help but notice all the trijets. Aircraft like the 727, TriStar and DC-10 are seemingly everywhere, whilst twinjets an quadjets were a rarity.
Look at it today, however, it is the complete opposite; twinjets are the most common, quadjets, whilst still popular are nowhere near as common as before and trijets are seemingly nonexistent. So why did trijets fall so much out of favor?
Though the extension of ETOPS regulations is the most common reason why manufacturers stopped producing trijets, it was also a combination of comparable higher operating costs and a change in the way we fly that ultimately sealed the fate of trijets.
Most production of trijets ceased in the 1990s, with new twinjet aircraft such as the Boeing 777, 787 and Airbus A330 replacing them on most routes, whilst a small number of new A320 and 737 variants replaced them on some short-haul routes.
In this article, we will look at what a trijet is, what made them so popular on both sides of the Iron Curtain and the reasons why they are no longer produced with only one real exception.
What is a Trijet?
As the name implies, a trijet is any jet-powered aircraft which is propelled by three engines. Typical configuration sees an engine is mounted under each wing with the third mounted at the aft section of the fuselage.
Most commonly, the third engine (engine two) has an S-duct, where the air intake is above the fuselage and the exhaust is at the rear of the aircraft. Famous examples of this can be seen on the Tupolev Tu-154, Dassault Falcon 50 and Lockheed TriStar.
In other cases, such as on the McDonnell Douglas MD-11 and its predecessor the DC-10, engine two is built directly into the tailfin as a standalone engine like the two on the wings, rather than passing the air through the aircraft.
The origins of the trijet design can be traced to the trimotor design of the Golden Age of Flight during the Interwar Period.
During this time, manufacturers attempted to improve range and speed of their aircraft by adding an extra engine. After WWII, this design became irrelevant thanks to the advancement in piston and turboprop engine technology.
As time went on and jet engines became more common, aircraft manufacturers began toying with the idea of a three engine design similar to trimotors of decades past, just updated for the jet age.
It was this philosophy that led the Soviet Union’s Tupolev design bureau to design the Tu-73, the world’s first trijet design, in 1947.
Although this would not ever be put into production due to the Soviet military preferring the rival Ilyushin Il-28, it heavily influenced the later design of the similarly unsuccessful Martin XB-51 bomber of the late 1940s and 1950s.
This, combined with Tupolev’s choice to use British-made Rolls-Royce Nene turbojet engines had a profound affect upon British and American aircraft designers for years to come.
Once commercial jet engine technology had caught up and airlines beginning to demand short-to-medium haul jet airliners in the 1960s, aircraft manufacturers began to produce a slew of trijet airliners.
The first of these new batch of trijets was the Hawker Siddeley Trident in 1962, followed by the Boeing 727 in 1963. In turn, the development and subsequent initial success led to the development of the Yakovlev Yak-40 in the USSR.
Why Were They so Popular?
Early trijets like the Hawker Siddeley Trident and Boeing 727 found considerably initial success as short-to-medium haul airliners, especially with large national and/or international airlines.
The main reason for this success was their ability to fit between short-haul airliners like the 737 and later the A320, and larger quadjets like the 707, DC-8 and especially the 747.
This status gave them great flexibility for airlines in terms of how they were used. Some airlines used them on routes with slightly more demand than what the 737 could handle (as the original 737-100 had a lower capacity than the 727).
In other cases, the comparably longer range of trijets saw them employed on routes with a similar level of demand as a route served by a 737, but over a range the 737 physically could not fly.
Later trijets such as the DC-10 and TriStar were popular because they acted as the bridge between twinjets and quadjets.
Because twinjets couldn’t do long range transoceanic flights due to regulations (more on that in a minute) airlines were faced with a problem: Use inefficient quadjets on less demanded long haul flights and make a loss, or don’t serve them at all and risk losing passengers’ business forever.
Trijets, however, provided that convenient bridge for airlines; they could now operate these less demanded transoceanic flights and make a profit, all without the risk of losing passengers’ business for not serving that route.
Why Are Trijets no Longer in Production?
With the exception of certain members of the Dassault Falcon family of business jets, there are no three-engine airliners currently in production. These are the main reasons why they are no longer produced:
ETOPS Regulations
One of the reasons trijets first came about was due to the FAA’s “60-minute rule” which stated no twin-engine airliner could fly more than 60 minutes away from an airport in case of engine failure.
And this made sense for the time. Early jet engine technology was thought to be unreliable (as piston and turboprop engines had been) and regulators didn’t want to risk a plane full of passengers crashing into the water because one of its engines cut out.
Due to these rules, twin-engine aircraft were mostly relegated to cross-land routes, whilst trijets and quadjets were used on routes the FAA 60-minute rule prevented twinjets from flying on.
As jet engines proved to be more reliable than their piston and turboprop forbearers had been, the “60-minute rule” was extended to 90 minutes and eventually 180 minutes which it is today. Collectively, these regulations are known as ETOPS.
When ETOPS regulations were at 60 and even 90 minutes, it made it infeasible for airlines to use twinjets on less-demanded long-haul routes, hence the need for trijets.
But when ETOPS were raised to 180 minutes in 1985, airlines began to realize that a simple low-demand transatlantic or transpacific route – the kind of routes three-engine airliners often flew – were no longer needed.
For that reason, airlines stopped placing orders for trijets en masse and aircraft manufacturers followed suit by phasing them out of production.
Higher Costs
It should probably come as no surprise to learn that trijets aren’t particularly fuel efficient… at least when compared to their twinjet counterparts, almost exclusively a byproduct of their third engine.
Increasing the weight of the aircraft as a whole, trijets require more power to get off the ground and reach the same speeds a twinjet rival can, to gain this extra power, trijets are forced to consume more fuel.
Because of this, trijets are forced to carry much larger quantities of fuel, which also increases the weight of the aircraft, thus forcing it carry more fuel to carry the additional fuel and creating something of a negative multiplier effect.
Aside from the lower fuel efficiency, the position of the third engine above the fuselage made maintenance much more dangerous, not to mention expensive.
Whilst the engines on the wings could be easily maintained like on any other aircraft, mechanics were required to use specialist apparatus (namely scissor lifts) to reach the third engine.
Perhaps unsurprisingly, these lifts weren’t cheap and always carried the risk of a mechanic falling overboard whilst doing maintenance (even if a harness connected him to the lift) causing serious damage to the mechanic.
Using these lifts also required quite a bit of time on the ground, no matter how small the job was. As flying is the main way airliners earn money, every second trijets spent in the hangar being maintained was a second it wasn’t earning money.
All this meant that trijets have a much higher cost per mile than their twinjet rivals, translating to higher operating costs for airlines and thus, comparably lower profits for them.
Point-to-Point
When trijets were first invented, airlines operated on the “Point-to-Point” theory of air travel, whereby passengers would travel directly to their destination, rather than do multiple connecting flights.
Under this method, it gave airlines a huge number of long-haul routes that had enough demand to warrant a twinjet (though ETOPS regulations prevented them from being used on these routes) but not enough for a quadjet.
As such, the trijet was the easy go-between.
Although this theory has seen a resurgence in recent years thanks to the emergence of low-cost carriers, the point-to-point method was mostly abandoned after airline deregulation in the late 1970s in favor of the “Hub-and-Spoke” theory.
The direct opposite to the point-to-point theory, the hub-and-spoke theory sees passengers fly from their local airport (the “Spoke”) to a larger one (the “Hub” – airports like JFK, Heathrow, Charles de Gaulle etc.) before flying to their final destination.
With this, trijets became somewhat irrelevant.
The aircraft that provided transfers from the spokes to the hubs were usually regional aircraft, or even a short-haul airliner like the A320 or 737 if the route is particularly popular, whilst the hub to destination flights were done by quadjets, and later, the first long range twinjets.
Due to this model, most airlines saw no need for a fleet of three engine airliners as the routes they’d been bought for either no longer existed, or had too much demand compared to their capacity.
Similarly, low-cost carriers simply couldn’t afford to run them, meaning most found themselves send to aircraft graveyards or repurposed as cargo aircraft.
What do you think about the humble trijet? Should it make a resurgence? If so, how? Tell me in the comments!
Featured image courtesy of Dmitry Terekhov via Flickr.