Bjorn’s Corner: New aircraft technologies. Part 20. Efficient flying

By Bjorn Fehrm

July 7, 2023, ©. Leeham News: We explore different technologies in the series that can make our next-generation airliners more efficient and, thus, less polluting.

We have discussed developments of engine and airframe technologies, such as Turbofans versus Open Rotors and different airframe configurations to minimize drag and, thus, energy consumption.

When utilizing these developments to increase efficiency we must fly the aircraft in a different way depending on the technology.

And how we fly the aircraft is not only influenced by the factors we have discussed. We must consider factors at the airplane level, at the airliner operational level, and finally, at the airline fleet level.

Figure 1. The NMA concepts included dual aisle airliners. Source: Leeham Co.

Operational efficiency means more than aircraft efficiency

We have discussed factors influencing airframe and engine efficiencies like optimal fuselage forms (single aisle vs. dual aisle, Figure 1), low drag airframe trends (lower parasitic and/or induced drag, Figures 2 and 3), and engine advances.

Figure 2. To minimize parasitic drag, the JetZero airliner uses a BWB configuration. Source: JetZero.

These are all aircraft technologies to lower the energy/fuel consumption of the airliner and, by it, emissions.

Figure 3. The NASA and Boeing Truss Braced Wing X-66A demonstrator aircraft shall fly in 2028. Source: Boeing.

But airplane operational cost minimization involves more than minimizing energy/fuel consumption. The optimal cruise speed for the airframe is not optimal speed for an aircraft with crew and passengers. Time-dependent cost factors now modify how the airliner shall be flown.

Seen at an even higher level, a flight profile that minimizes operational costs might not be the profile that optimizes the airline’s fleet utilization.

We will discuss all these different aspects and how the new technologies affect energy and cost optimizations at different levels.

We start by looking at what consequences different airframe/engine changes have for how the airliner shall fly. Example:

  • Why must a Truss Braced Wing fly higher than a normal airliner? What consequences does it have for its operation?
  • Why does an Open Rotor propulsion system benefit from a lower operating speed than a turbofan airliner, and what is the flight profile that optimizes an Open Rotor airliner’s efficiency?

Then we look at what this means on the next level of optimizations:

  • What factors of the airliner’s costs drive a higher cruise speed than the airframe’s long-range cruise speed?
  • Why would fleet management change these speeds further to optimize fleet efficiency?

We start by looking at airframe and engine-related factors next week.

14 Comments on “Bjorn’s Corner: New aircraft technologies. Part 20. Efficient flying

  1. To me, one significant factor in cost and fuel consumption for a given passenger trip is non-stop flights vs multi-flight trips.
    Multi-stage trips of necessity involve people traveling more miles in the air, which increases cost and fuel consumption.
    For example, a direct 10,000 km flight vs two 6,000 km flights.
    Even if the direct flight costs 20% more per available seat km, it would be worth it because of flying 10,000 km vs 12,000 km.

    • On the other hand, long direct flights necessitate carrying a larger load of fuel — which adds “dead weight” during the early portion of flight.
      It costs fuel to carry fuel.

      • Every T-O cycle cost as well, so it comes into effect when you want to exceed range at max T-O with a full pax and cargo load. Then you need to offload cargo of pax to go non-stop. To fill 1/2 tank and land halfway to fill up is a loss. The A380 was intended to fly LHR SYD non stop but the engines were not efficient enough. Such engines came later on the A350 and could maybe do it or you have to wait for the Ultrafan.

    • Your post is basing cost on “available seat km” but if the load factor on the mooted 10,000 km route is 60% and the load factor of the two 6,000 km routes is 85%, it would be cheaper (and greener) to operate the two 6,000 km routes.

    • When I worked for an airline ages ago, emergency plans were there that if fuel costs “exploded” for some reason, we would introduce tech stops. Higher payloads/ lower fuel consumption. In that situation utilization/ travel time / frequencies would have become less important anyway..

      Strategically located airlines like Air Astana and Icelandair, Play try to establish their business models around it.

    • “Even if the direct flight costs 20% more per available seat km, it would be worth it because of flying 10,000 km vs 12,000 km.”
      -Cost is always a strong indicator of fuel
      Burn and emissions. A carefully activity based costing is carried out so I’d remain skeptical.

  2. Mr. Wales ,
    Passenger-hours squandered in-transit is also a form of cost , and leads directly to lower passenger satisfaction .
    You are totally correct , and most pax are willing to vote ‘yes’ with their wallet .
    D. 😎

    • I don’t see how passenger hours lost in transit makes for any difference and certainly not a calculation an airline would consider.

      I figured out a long time ago trips of any length pretty much took up a day. Or like you can do from where I live and get up and take a flight out at 1 am or so. That in turn had its own cost as off schedule, muddled brain and messed up in general. I won’t take those flights anymore (some up front savings at the time, maybe not now).

      If I need to be someplace I just leave a day early and at times I would do two days. I got questioned about that but it worked and paid off.

      Pax satisfaction? Hmmm. I don’t see smiles on faces anymore. It used to e an adventure and even exciting, now, meah, whatever the trip end is for is the goal and just a means to an end. In our case its vastly better than 2500 miles driving to just reach Seattle.

      I don’t even vote with my wallet, its what works that fits the end goal needs and no worry about the cost (yes it has to be affordable). Last trip was I don’t care about $100, I want it to work to suit what I am after.

      My wife gets extra leg room she needs when she goes and while that cost added, its more than worth it.

      If we could not afford it, then we would not go.

      Keep in mind plane travel was and is a way of life for Alaskans. There simply is no alternative that makes sense with how far we are from anywhere else US.

      Sure some of us have driven the Alcan for various reasons (one resulted in a wreck for me and a really bad trip as well as scary low temps in one location of an estimated -70 Deg f). I guess I have been up and or down it 4-6 times.

      But barring some special circumstances its not viable and rent vehicle when you get to where you are going if you have to.

      And for the US there is not a whole lot West of the Mississippi so 1500 mile (ANC to SEA as well) are not uncommon and trans Con US is a benchmark for single aisle range (granted that is a fair amount higher now)

  3. Bjorn: Again thank you, that is the tech end stuff I am interested in.

    In the end moving pax and making money doing so is the driver for the Airlines but I am not interested in Mile Per Gallon or KPH for Pax cost other than the metric allows an understanding of what drives the tech end in best speed.

    Speeds have come down since my early days of tube engines, some may be economics but some may be designs of engines and wings that are driven by economics in the end. Highly interested in that part

  4. Thks for this very interesting subject. Cost index (CI) defined by each Airline ops dept take care of the balance between fuel cost and time related cost. With new airframe/engine technology it is clear both family of cost will evolve ( TBW=higher FL, open rotor=lower TAS. )
    What are factors which are difficult to integrate by Airbus/Boeing in our crowded sky ?
    At individual flight level , my main concern is the
    impact of the real time ATC constraint on the initial FMS profile. With the 4D computation performed separately on the ground and onboard and the introduction of new datalink protocole ( in Europe ADS-EDP) to coordinate above two computations I am confused about the optimum CI , may be a customised
    CI for each flight defined at take off through this coordination?
    At fleet level the impact of contractual clauses of leasing contract might be a concern to define the optimum CI on the middle term.

    • mikeul:

      Bjron of course will respond or not but I don’t see ahything in the crowded sky aspecdt that Airbus or Boeing can’t acomodate – at that point its a ground base traffic management and someone has to come up with a fix if it gets to the lock up point.

      I don’t see it as an aircraft tech issue, crowded sky would be a consequance of air travel and its ground based to deal with it even if some tech needs to go into the aircraft to make it work (aka best example would be a transponder, it gives (or gave) ground managment the tool to feed into the system that was needed)

      Text com has been another aspect that is far better than voice com. ADS another one (Alaska FAA was actually the start of that to deal with nav issues out in Western Alaska).

      I could see only artificial gravity being the only thing to allow a totally bogged down system (not happening of course) and then you have to come up with a solution or wind up with restrictions on traffic numbers, ie slots)

      Slower aircraft actually free up room but the only tech that accommodates slow is turbo prop or maybe open rotor. Helicopters sort of work as well in that regard (slow and can use all sorts of other routes) but reasonable is different than possible but not realistic.

  5. TransWorld : Thks for your answer which I share. To clarify my point:
    The idea in my post is to give some food for thought on Bjorn’s open-ended question: “The way we fly the aircraft efficiently is not only influenced by the factors we have discussed. We need to consider factors at the aircraft level, at the operational level of the airliner and, finally, at the airline fleet level.”
    In an increasingly congested airspace, with aircraft offering increasingly different performance levels, I’d like to focus on two ideas:

    -new technologies (TBW , Open rotor , UAV ,…) will effectively change the basic parameters (true air speed, flight level , climb and descent gradient…), which will be taken into account by the airline in defining the Cost Index of the FMS, which is in parallel increasingly 4D connected .
    – At the operational level of the flight number, the new connectivity-enhanced protocols enabling more efficient management by ATC (ETA, fuel consumption, etc.) will lead to a real-time computation of the CI , moving away from the aircraft manufacturer’s model, and why not an updated CI just before leaving the gate? which would be a significant change for the pilot and the airline’s OCC.

    To simplify the current non accurate ATC a/c performance model are gradually replaced by “better” on board FMS data but at the gate ATC will provide the final CI taken care of all known ATC and airport constraints .
    Exchange between above three parts ( ATC , crew, OCC ) is complex.

    On long haul flights updated wind forecast provided to FMS through datalink are already a major improvement for efficiency.

    At the upper level of a sub-fleet, a variable CI has an impact on the assignment of each aircraft to the airline flight schedule and subsequently maintenance schedule, and we could also imagine the possible impact of leasing contract clauses.
    I concede above is strongly influenced by my european background . North america airspace and ATC might be different.

  6. The railroads were told they had to order new tier 4 locomotives. The problem is the Tier 3 locomotives are cheaper to operate. So a bunch of Tier 3 locomotives are getting rebuilt instead of Tier 4 bought.

    These concepts are neat and nerdy, but if it costs too much, or not reliable, do we really believe the airlines will buy it?

  7. This is what I think
    An Airline operations efficiency depends on many factors:

    GDP of the country of origen, and destination
    Country’s population
    High speed train availability
    Airport infrastructures
    Highways infrastructure
    Country’s weather patterns
    Size of the country
    Economy of the country
    Type of short, medium, and long-haul flights, that this Airline operates
    Type of passengers
    Average number of business class passengers, of the Airline
    Does the Airline carry cargo?
    What type of cargo?
    Socio-economic relations of the country of origen, and its destinations
    Airline’s agreements with other airlines
    Does the Airline operate in a main international hub?
    Does the Airline offers an excellent on-board service?
    Is the Airline focused in safety?
    Is it a legacy carrier, with a lot of experience?
    How far are the Airline’s main destinations?
    How many passengers does this Airline carry every year?
    Is this Airline backed by a solid economy?
    Is this Airline interested in taking good care of the passengers?
    Is the customer’s experience important for this Airline?

    And so on…

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