Can we really compare the efficiency of different airliners, relying on raw numbers? Do numbers tell the whole story or are there more key factors to consider?

We often list percentage improvements in efficiency, when we compare different aircraft. This requires a bit of explaining. Basically, this isn’t like fuel consumption in a car. For that, we measure either miles to the gallon (mpg) or litres of fuel per 100 kilometres. But making such a comparison between aircraft wouldn’t tell us much. This is because we need to factor in the number of people in this aircraft.

So when we compare the efficiency of different planes, we use ‘passenger miles’ per gallon (or litre) of fuel. So when we say that the 737 MAX is 14-15% more efficient than the previous generation, we mean that it will use 14-15% less fuel, to carry the same number of people, to the same distance. But having ‘passenger miles’ means that we can make similar comparisons for planes carrying more or fewer people.

The above becomes important, for example when we want to compare an Airbus A220 with an A320 or 737. But where it gets really important, is when we look at longer flights. For example, transatlantic flights, or flights from Hawaii to the mainland US… with single-aisle aircraft. For a quick look at what it means, we will examine two carriers: Southwest and Hawaiian Airlines.

Southwest: Do they Compare Efficiency or Utility?

As we know, Southwest is a single-type operator. They have a Boeing 737 fleet, comprising of 737-700 and 737-800 models. They also have MAX-8 aircraft, and will re-start using them soon. And one of the destinations they are keen to go back to, is Hawaii. So, to decide which of the two generations to use, they will compare what their efficiency means to their range.

Southwest’s 737NGs can fly from California to Hawaii. But against strong seasonal prevailing winds, they need to carry extra fuel. This is not only for the extra flight time, but also for safety, in over-water operations. And that means restricting the aircraft’s load, i.e. their passengers. So in this case we compare not just efficiency, but utility. Simply-put, there are some routes that Southwest’s NG can’t do, with a given number of passengers. But the 15% more efficient MAX, can.

So efficiency of the 737 NG and MAX don’t really compare in such routes. The difference isn’t 14-15%. When we factor in the number of empty seats in the NG, the gains in the new jet will be much greater. However, during the pandemic this matters less. Andrew Watterson, the airline’s Chief Commercial Officer, explained:

“…the NGs have payload restrictions going westbound in the high wind time of the year, which is actually right now. We’ve not got the demand to fill the aircraft right now, so we’re not really having to restrict the payloads, but normally we would have to restrict the payloads this time of the year with the NG. The MAX with the greater range will be able to go full year-round to all the airfields they service, so that will also be an increment to our business case in Hawaii.”

Hawaiian Airlines Compares The Efficiency Of Very Different Jets

Still in the same part of the world, but Hawaiian Airlines is quite a different operation these days. The airline used to operate exclusively between the Hawaiian Islands. This has stopped for some years now. The airline flies not only to California, but much further into the US mainland, and plenty of places beyond.

This means that Hawaiian has quite a disparate fleet these days. So, they now have some Airbus A321neo aircraft. And their efficiency and long legs have interesting implications, when the airline compares them with the rest of their fleet. That’s because Hawaiian’s A321neos are its first single-aisle Airbuses. Its Boeing 717-200s don’t see use beyond the islands.

So Hawaiian has to compare the efficiency of its A321neos with its bigger jets. These would be their Airbus A330-200s. The airline had orders for the ‘neo’ version of this jet, called the A330-800. However, they eventually canceled them, in favor of Boeing 787-9s. But these won’t arrive for another 18 months.

Obviously, the A321neos can’t do many of the routes the airline is going for these days. Hawaiian fly around the whole Pacific and beyond. Nor would the A321s have enough seat capacity for all of these routes. But they can do many of them. And for those destinations, the older A330s don’t compare at all well, in terms of efficiency. Worse, the low demand exacerbates this effect further.

The Little Matter Of Cargo

So Hawaiian are happy with the way the efficiency of their A321s compares with their other fleet – big and small. Hawaiian’s CEO Peter Ingram said:

“For a while we’ve actually flown it on some interisland routes, very short haul, and we’re using it for some of those flights; while, at the same time, we can put it into a market on the mainland US that might have been an A330 market in prior times, but doesn’t have the volume of demand to support that right now.

“And, the cost per cycle of an A321 is considerably below that of a widebody aircraft. If you don’t have the cargo revenue to offset that and you can’t fill all those seats, then absolutely we’d like to go ahead and use the 321 as much as we can.”

And this brings us to the question of cargo. As we have seen in the pandemic, some airlines were able to limit their losses, by carrying hold cargo. Obviously the Hawaiian islands can use many supplies from the mainland. So to compare the efficiency of an A321neo vs an A330, the airline has to factor in the extra cargo revenue of the latter. However, the A321 can carry a few Load Devices (cargo boxes) in its hold, unlike, say, the 737.

Overall, newer aircraft give airlines new options. They need to consider multiple factors, in order to compare both the efficiency and the utility of their fleets. The numbers don’t tell the whole story. But sometimes efficiency gains allow smaller jets to perform roles of bigger ones. And when this happens, overall efficiency in the fleet leaps forward. Hopefully, these are improvements that airlines will be able to put to good use, even beyond the pandemic.