Plain Aviation

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What is a trijet in airliner terms?

private boeing 727 trijet

A private Boeing 727 trijet. Photo by Andrew W. Sieber CC 2.0

As you can probably tell already, the term trijet refers to any commercial jetliner with three engines. Normally they either have all three engines mounted aft of the cabin and near the tail as seen on the Boeing 727, or two mounted on the wings and the third mounted at the tail as seen on the McDonnell Douglas MD-11. One common feature of all trijets is that their third engine is always going to be embedded inside the vertical stabilizer to ensure symmetrical thrust output. While this feature increases the difficulty of design, production, and maintenance, the cost-savings of operating trijets over four-engined airliners is definitely worth it.

Origins of trijets

Trijets were originally intended to be a cheaper alternative to four-engined jetliners in the 1960’s to 1980’s when restrictions were in place against twinjets due to concerns over reliability and redundancy. Trijets are much more cost-effective than their four-engined counterparts such as the Douglas DC-8 and more recently the Airbus A340 as their lack of a fourth engine decreases production and maintenance costs while saving fuel. These benefits were very attractive to airlines in a time of rapid growth in the industry and led to a “trijet craze” in the 80’s.

Design of the third engine

Just like boats moving in the water, aircraft need to have their engines thrust that is symmetrical on both sides in order to fly straight. Designs for twinjets and four-engined airliners are straightforward because they simply mount half of the engines on the left side and the other half on the right side to create symmetrical thrust. But with trijets, the only place that the third engine can be mounted is at the very center of the aircraft. As it would be impossible and extremely impractical to mount the engine anywhere on the fuselage, the only option left is to embed it into the vertical stabilizer. But with this design, engineers must also consider how to deal with the hot exhaust gas from the engine that could potentially cause structural damage. On the Lockheed Tristar and Boeing 727, engineers have designed an S-duct that runs through the vertical stabilizer and channels hot exhaust gasses from the engine to the very rear of the aircraft, where the APU exhaust would be.

lockheed tristar trijet

Rear-view of a Lockheed Tristar showing the S-duct of the third engine. Photo by Alan Wilson CC 2.0

On the Douglas DC-10 and McDonnell Douglas MD-11 however, third engine simply runs straight through the vertical stabilizer with its intake and exhaust at the front and rear of the stabilizer. This configuration is known as a “straight” layout, and while it may reduce design complexity and simplify maintenance, it is less aerodynamic than an S-duct.

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A KLM MD-11 with the distinctive “straight” layout. Photo by Maarten Visser CC 2.0

Decline of the trijet

The quickly dwindling numbers of trijets around the world can be attributed to the rise in popularity of twinjets. Trijets made their debut and peaked in popularity in an era when the FAA did not allow twinjets to fly on a route that would take it farther than 60 minutes worth of flying time away from any airport en-route. This regulation was known as the 60-minute rule, and essentially meant that twinjets were not permitted to fly long-haul routes over oceans or uninhabited terrain. The same restrictions did not apply to trijets, and therefore they were able to enjoy similar certifications compared to four-engined airliners while remaining more fuel efficient. But in the 1990’s, restrictions against twinjets laxed as technology improved. Twinjets were allowed to fly more remote and direct routes as long as they had the proper ETOPS certifications. As a result of this, the role and status of the trijet shifted from what was a cheaper alternative four-engined jet to a more expensive version of a twinjet; essentially these reduced restrictions stripped away the only real advantage that trijets had over twinjets. With the introduction of increasingly reliable and powerful engines such as the General Electric GE-90 and Rolls Royce Trent 1000, there is no longer a need for a third engine onboard traditional airliners that would only increase fuel burn and maintenance costs.

We can agree that trijets have an aesthetic appeal that cannot be found on any other type of airliner. While there are still several trijets in service with cargo airlines and minor carriers, their numbers are quickly declining. While I don’t think aircraft manufacturers are going to redesign an aircraft with three engines, I sure hope that I’m wrong.

 

 

2 Comments

  1. I think you have noted some very interesting points , thanks for the post.

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