Commercial aircraft today can cover some seriously impressive distances without the need to stop to refuel. As airlines have looked to expand their long-range operations, aircraft manufacturers have developed ER (‘Extended Range’) variants of their planes to increasingly facilitate their customers’ needs. But what exactly goes into developing an ER variant?
A popular development with Boeing
Since the 1980s, American manufacturer Boeing, in particular, has become known for developing ER versions of certain variants of its airliner families. The first family that it introduced ER versions to was the 767, its medium to long-range widebody twinjet airliner. The original model, the 767-200, entered service in 1982 with US legacy carrier United Airlines. However, the -200ER followed with Israeli flag carrier El Al just two years later, in 1984.
As Boeing developed stretched versions of the 767, it also produced corresponding ER sub-variants. In the case of the -300, it was, once again, only two years between the original (Japan Airlines, 1986) and ER (American Airlines, 1988) versions being launched. The longest model of the 767 was the -400ER, which Continental Airlines launched in 2000. In this instance, there was no standard -400, and the -400ER, therefore, became the only version of this variant.
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Other Boeing models also received the ER treatment, even if only on a small scale. The iconic 747-400, for example, also saw a very limited -400ER sub-variant. Boeing produced just six of these for Australian flag carrier Qantas in the early 2000s. The extended range provided the airline with a safety net on its lengthy transpacific routes. Qantas retired its first 747-400ER last February, one of the first of a series of high-profile 747 retirements throughout 2020.
Meanwhile, the Boeing 777-200 had both ER and LR (‘Long Range’) variants developed from the standard model. The -200ER entered service with British Airways in 1997, three years after the original -200 first took to the skies with United in 1994. Meanwhile, the stretched 777-300 had its first delivery to Air France in 2004. This has since become the best-selling 777 variant.
Not just a widebody phenomenon
Boeing’s ER models are not solely confined to widebody designs. Indeed, the largest and newest model of its 737NG (‘Next Generation’) family is none other than the 737-900ER. Boeing developed this variant to give airlines a narrowbody aircraft with a similar range and capacity to the now-discontinued 757. It also developed and launched the 737-700ER in the mid-2000s. This aircraft has similar specifications to the Airbus A319LR.
One can also find ER aircraft in certain regional jet families. One example of this is Brazilian manufacturer Embraer’s ERJ series. On a basic level, jets from this family are split into ERJ135, 140, and 145 models. However, all of these also have both ER and LR variants. This gives operators an edge when operating longer regional routes with lower demand levels.
But, now that we have seen the sorts of aircraft that can have an ER version, what exactly is it that manufacturers do to a plane to certify it as ‘extended range?’
Additional fuel tanks
Perhaps the most obvious way of increasing an aircraft’s range is to correspondingly increase its fuel capacity. This is generally achieved by installing additional fuel tanks, and it can have a significant impact on an aircraft’s overall range.
Using the example of the 767-200ER, Boeing’s first extended range model, we can see that, rather than adding auxiliary fuel tanks, the American manufacturer instead utilized the space it already had in the existing design. Specifically, this entailed using the center tank’s dry dock as extra space for carrying fuel. The result was an increased range of 12,200 km (6,590 NM). This represented a 5,000 km (2,700 NM) increase over the standard model.
Meanwhile, the Boeing 747-400ER can partly attribute its extended range to the presence of an additional 12,300-liter fuel tank in the forward cargo hold. Boeing did give customers the option of a second additional tank, but Qantas was the only customer for the -400ER, and chose one.
The result was an 800km (430 NM) increase in the -400ER’s range. While this is not as significant an increase as the 767-200ER, the aircraft could also carry nearly seven tonnes of extra cargo compared to the standard 747-400. This brings us nicely onto a crucial aspect of developing extra range models. With the burden of the additional fuel to consider, how do manufacturers increase an aircraft’s maximum takeoff weight?
Achieving a higher MTOW
There are many ways in which an aircraft manufacturer can increase its MTOW to safely carry the weight of additional fuel and tanks. Staying with Qantas’s 747-400ER aircraft, we can see that Boeing strengthened areas such as its fuselage and wings. It also reinforced the landing gear, as well as fitting it with larger tires. The result was an increased maximum takeoff weight of nearly 413 tonnes, compared to around 397 tonnes on the standard -400.
The Boeing 737-700ER also features structural modifications compared to the standard model. Specifically, it is fitted with the same wings and landing gear as the larger 737-800. These alterations allow it to safely bear the weight of the additional fuel.
Other factors in extending the range
Meanwhile, the 777-300ER features structural modifications in these areas as well, but also benefits from raked and extended wingtips to increase its efficiency. Therefore, aerodynamic factors also play a significant role in extending an airliner’s range.
Finally, an aircraft’s range can also be increased with a lower-density seating configuration. To briefly touch upon a different long-range variant, Planespotters.net reports that Singapore Airlines operates seven examples of the Airbus A350-900ULR (‘Ultra-Long Range’). According to SeatGuru, this features a premium-heavy configuration, seating just 161 passengers, freeing up weight for additional fuel. Consequently, the airline was able to operate the world’s longest scheduled commercial flight, directly connecting Singapore and Newark in just under 18 hours.