Quad Question: Why do airlines still fly four-engine jets when two will do?

While there have been a number of four-engine passenger jets over the years, there are – essentially – three major models today: Airbus’ A340 and A380 and the Boeing 747.

For a number of years, four-engine jets served the world’s most veritable long-haul routes. And while three-engine jets such as the Lockheed L-1011 and McDonnell Douglas’ DC-10 and MD-11 did serve some long-haul routes, quad-jets were generally preferred over tri-jets.

How Things Change

For a number of years, you’d be hard-pressed to find a jet with two engines operating intercontinental routes. Yet with the evolution of technology, the reliability of the engines that power twin-jets improved consistently. As a result, in 1985, ETOPS regulations were passed by the FAA, thus giving twins – which generally burn less fuel – the ability to operate on routes that previously required a tri or quad.

Given the reliability and power of these engines, and the massive savings in fuel costs, it would appear that the airlines have been vindicated. For example, the General Electric GE90 which powers Boeing 777s generates 115,000 lbs. of thrust – almost double what a GE CF-6 powering a 747 puts out. Indeed, the fact that a 777-300ER – whose maximum takeoff weight (MTOW) is approximately 700,000 lbs. – can still operate with an engine out speaks volumes to just how powerful the GE90 and other new engines are.

The gravitation towards the twin has been evident. For example, Delta and United are both set to retire their few remaining 747s before the end of the year, while both carriers possess fleets that are comprised of a number of twins which operate long-haul sectors. And given that no American carrier operates the A340 or A380, the retirement of those 747s is likely to be the end of the passenger quad-jet in the United States.

A Different Picture

In America, the reality is that quad-jets are almost obsolete. In Europe and Asia, however, there are still a significant number of quad-jets.

Between slower-than-anticipated orders and a number of other factors, it’s no secret that the A380 hasn’t been the success that Airbus would have hoped it to be. Despite this, a number of carriers are finding the planes useful. In Europe, British Airways, Air France, and Lufthansa all operate the A380. In Asia, there are nine carriers with the aircraft – Emirates alone has 93 within its fleet. Even though American carriers have found the A380 not to be a worthy investment, the same can’t be said for the rest of the world.

Though not as new as the A380, both the A340 and 747 are still used by carriers throughout Europe and Asia. In fact, British Airways remains the world’s largest operator of the 747, with 36 of the type still in service as of this writing. The carrier refurbished of a number of 747s in 2015, giving them new seats and upgraded in-flight entertainment systems and suggesting that those planes will remain active for some years to come.

What’s the Appeal?

Given that quads, which generally burn more fuel than twins, are still alive and well throughout the rest of the world, it begs the question: what is the appeal of a gas-guzzler over a more efficient aircraft?

There are a number of answers, but the one that I agree with the most is this: by using quads, airports operating close to maximum slot capacity can move more passengers using less slots.

For airports where slots are at a premium, airlines see the ability to operate one A380 per day rather than two 777s as a boon. After all, that slot is freed up to potentially be used by another aircraft, which could potentially allow a new destination to be served.

Though a 747 can’t fit as many people as an A380, they still boast superior capacity to two-engine models. This perhaps explains why BA – whose main base is at slot-constricted London Heathrow – still makes significant use of its 747s when other carriers have chosen to retire theirs. And though America has three of the world’s 10 busiest airports, as well as the busiest in Atlanta, I have to suspect that there are more slot-constricted airports in Europe and Asia than there are in America.

Ultimately, future developments like the 777-8 and 777-9 are anticipated to be able to seat significantly more people than the 777-300ER. Even so, we haven’t yet gotten to the point where we can fit, for example, 615 people – which Emirates has on its densely-configured A380s – on a twin. Moreover, airport congestion – and thus the need to alleviate it by using larger planes – is very much present. Until twins can seat a virtually identical number of passengers as their quad-jet counterparts, it’s likely that we’ll continue to see the latter in the sky for years to come.