Introduction to Intermediate Stop Operations (ISO)

The world of commercial aviation is full of trends. One could say that the entire industry is driven by these trends. The first two decades of the 21st century were driven by trends that included the development of more point to point routes, which were made possible by the innovations from Boeing’s 787 Dreamliner and more recently, the Airbus A350. This was coupled with the decrease in popularity of the hub-and-spoke model, whose routes were operated by larger, more expensive aircraft to operate, such as the Airbus A380 and Boeing 747.

China Airlines A350. Photo by Duy Khang Tran | AeroNewsX.

Recently, we have also seen the trend of airlines moving away entirely from three and four engined aircraft to twin-engined aircraft. The past decade marked the beginning of the end for the Airbus A380’s production, hundreds of Boeing 747 retirements, as well the retirements of many other now inefficient aircraft such as MD-11s and DC-10s, all in favor of newer, more efficient twin-engine planes. This is thanks to improvements in turbine engine reliability and safety, improving ETOPS (Extended Over Water Operations) ratings, and therefore allowing for increased direct routes to more destinations around the world.

Looking ahead to this decade, we can already see emerging trends for airlines, with the focus being on striking a balance between environmental sustainability and positive economics. For instance, airlines may utilize carbon off-setting mechanisms, achieve net neutral carbon growth such as EasyJet and JetBlue, use more sustainable plant-based jet biofuels, or introduce plastic-free flights on a trial basis such as Etihad.

Etihad's Boeing 787 'Greenliner'. Photo by Devin Ruhotina | AeroNewsX.

There is also a trend that is not very well known due to a host of logistical and potentially diplomatic hazards for airlines: The introduction of Intermediate Stop Operations, or ISO for short. Although scheduled flights over 4,000nm (7500km) account for only around 3% of total flights globally, they account for 25% of airlines' total fuel expenses. One potential solution to not only cut down airlines' fuel expenses but also carbon emissions would be to impose one or two intermediate stops along the way depending on the route length to minimize the transport losses of the flight.

That however begs the question, what are transport losses? To put the issue into perspective, let’s say an American Airlines Boeing 777-300ER is flying from Dallas-Fort Worth International Airport (DFW) to Hong Kong International Airport (HKG). It’s American’s longest route and covers almost 8000 miles (13,000km). The aircraft has to carry enough fuel to be able to make it all the way to Hong Kong, nonstop, without refueling. The plane is therefore near its maximum takeoff weight and for a good part of the first half of the flight, the engines have to cruise at a higher thrust setting, thus using more fuel (in terms of tons per hour) than if the plane was lighter.

Transport loss is essentially the fuel penalty for carrying additional fuel. Like how all jets work, they start out burning more fuel than usual at cruise altitude, but as that fuel gets used up, the plane becomes lighter, and the engines therefore no longer have to work as hard to keep the same pitch and performance as before.

In our example, if American integrated a fuel stop in Anchorage, Alaska, and Tokyo, Japan, the 777 would only have to take off with enough fuel to reach Anchorage before refueling, and then on to Tokyo, and finally Hong Kong.

Although promising, ISO could present a potential logistical nightmare for airline dispatchers and flight crews, and drive up maintenance and airport fees. However, with the proper preflight planning and careful analysis of potential intermediate stops over airports based on landing fees, fuel prices, weather, and other factors, the positives of this practice could have the potential to significantly outweigh the negatives associated with increased maintenance on aircraft due to an increased rate of takeoffs and landings, as well as other logistical items such as landing fees.

This practice is already being adopted in the cargo industry, mainly due to increased aircraft payloads and reduced weight limitations for carrying more fuel. Cargo aircraft and airlines are able to utilize intermediate stops along their routes that take them all the way from Hong Kong, Japan, China, Taiwan, all the way to places in America and Europe such as Atlanta, Miami, Luxembourg, Frankfurt, and many other places due to the mostly inanimate nature of cargo.

Atlas Air Boeing 747-8F. Photo by Matt Lino | AeroNewsX.

One would also have to take into account the passenger experience and satisfaction. The introduction of intermediate stops may be stressful and even more fatigue-inducing for many, even at the benefit of having reduced ticket prices due to the increased operational profit margins experienced by airlines.

Should this trend be introduced into the commercial air transport market, it would open up the aviation industry to more efficient and environmentally conscious airline operations and increased profits thanks to minimized transport losses.

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