Analyzing the trades between a single- and twin-aisle NMA

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By the Leeham News Team

Introduction

April 8, 2021, © Leeham News: Some people believe Boeing should launch a new single-aisle airplane about the size of the 757-200/300 to compete with the Airbus 321neo.

Others believe the new airplane should be a twin-aisle aircraft. A few, including LNA, believe the new airplane must be a three-member family and must be a twin-aisle.

The largest member of a single-aisle Boeing NMA would be longer than the Boeing 757-300. Photo: Delta Air Lines.

Whatever the new airplane is, the general specifications are aircraft up to 250 passengers in two classes and a range of up to 5,000nm.

There is also agreement the airplane must start across from the A321neo. Configurations vary widely, but 190-200 seats in two classes are common.

Boeing CEO David Calhoun said on an earnings call that the next new airplane will compete with the A321 and cover the Middle of the Market.

Summary
  • A single-aisle, 250-passenger airplane would be longer than the 757-300.
  • Technical build challenges, while not insurmountable, exist with long, thin airplane.
  • Gate and ramp space constraints accompany a long airplane.
  • Other trades exist for a twin-aisle design.

Discussion
Defining the size

Boeing’s product development historically jumps family members by about 15% in seating capacity.

Using this rule of thumb and two classes, a three-member family starting at 190 passengers, the step-up would produce 220 and 252 passenger siblings.

A single-aisle NMA-7 would be as long as the McDonnell Douglas DC-8-71/73. Photo: Wikimedia.

A 250-passenger, single-aisle airplane would be longer than the 757-300 (178 ft)—about the length of the Douglas DC-8-61/63 (187 ft).

But there are technical trades between a pencil-like single-aisle airplane and a short if wider, twin-aisle aircraft.

For Boeing, the 737 MAX family is sub-optimal. The MAX 8 is the optimized aircraft. The MAX 7 is a shrink. The MAX 9’s stretch presented take-off rotation constrictions. The MAX 10 exacerbates this restriction. The MAX 10’s range also is reduced from the MAX 9.

Boeing needs to bring two aircraft to market simultaneously. The 737 needs a Non-Derivative replacement (737R) and the NMA Lite (NMAL) needs to fill the void created by the retirement of the 757/767 fleets in the spaces where the 787 is too much airplane. Given its financial constraints from the prolonged MAX grounding and the COVID-19 pandemic, developing two new aircraft families simultaneously is probably out of the question.

With the pressing need to be competitive with the A321, Boeing needs to move first in this sector. With aging aircraft above the A321/MAX 10, a replacement aircraft also is needed for this sector. But the business case for a two-member NMA is not viable. Hence, the need for a competitor to the A321 and a three-member family.

The NMAL in focus

LNA previously described the two divergent paths NMAL designs could take. The first fork in the road is the single-aisle or twin-aisle decision because everything else rolls out of it.

Manufacturing the NMAL erects no roadblocks. Boeing has already done carbon wings and fuselages, FBW Control Systems, and Multiple Center Wing Boxes, all the cornerstones of building an up-to-date aircraft. If Boeing focuses on best practices and stays away from technology for technology’s sake, it should go together very quickly with minimal risk.

The single-aisle airplane

Structurally, the single-aisle airplane will weigh slightly more per passenger to build than the twin-aisle aircraft. This is a byproduct of the smaller fuselage circumference being less stiff compared to a larger circumference and the greater fuselage length causing higher keel beam structure loads to support the cantilever structure aft of the main landing gear trunnions.

Thicker fuselage skin panels will weigh more due to increased loads and the increased number of windows installed. The wing being longer will also weigh a bit more. The single-aisle airplane will have a longer fuselage. This could hamper takeoff rotation angles unless the gear is sufficiently tall. Taller gear comes with a weight penalty. It may also carry one more set of escape doors (and their weight) aft of the wing because excessive distances to an exit may come into play.

The fuselage will have more floor beams and intercostals to support them, but they will be shorter and possibly be of lighter gage since each beams loads will be lower. The tail surfaces will weigh less than a twin-aisle airplane because they will be smaller for any tail volume coefficient due to their greater moment.

Single-aisle advantages

On the plus side, the aircraft will be slightly narrower, reducing drag and allowing the wing to have a slightly higher aspect ratio for the same area than the twin-aisle airplane when gate span is limited. This provides a better fuel burn per seat/mile.

A big benefit to flyers will be longer overhead bins. On the minus side, in turbulence, the ride in the single-aisle airplane is quite likely to be worse than the twin-aisle airplane. The further away from the center of gravity (CG) you sit, the further you may move in space as the aircraft pivots around the CG. Not all turbulence rotates the aircraft that way, but when it does, it is really uncomfortable. Baggage loading/unloading will take a bit longer and have less weight distribution flexibility since the cargo bays are longer and narrower and the bulk cargo doors smaller. Installation of a future XLR fuel tank consumes more baggage area cutting belly cargo revenue more than the same fuel tank volume in a twin-aisle airplane.

The Twin-Aisle Airplane.

Structurally, the twin-aisle airplane can be built lighter than a single-aisle airplane of equal capacity. This happens for many reasons.

A three-member, twin-aisle NMA family has advantages over a three-member, single-aisle NMA family. Photo: Leeham News.

The shape of the fuselage itself allows for the use of thinner keel structures as the skins shaped in a larger circumference are much stiffer than the smaller diameter single-aisle airplane. There are fewer floor beams in the fuselage, but they are a bit longer and may have more gage-related weight.

A big weight savings will be the reduced number of windows, as well as the potential to use one fewer pair of escape doors and the structure needed to support them. The larger fuselage diameter will be shorter for the same capacity. This means that its takeoff rotation clearance angles are much better and may allow the use of shorter/lighter landing gear struts than the single-aisle airplane. This could really help high hot performance of the airplane vs the single-aisle.

The tail surfaces on the airplane will weigh more because they need to be larger for the same tail volume coefficient due to the reduced moment. On the plus side, the larger fuselage diameter means that lower lobe bulk cargo doors are larger, allowing quick access to baggage and freight. The cargo loading would also be less sensitive to balance issues as the cargo would be loaded closer to the CG than a single-aisle airplane. This also provides a superior place for the inevitable LR fuel auxiliary tank to be installed without sacrificing as much in the way of belly cargo revenue.

Inside, the larger cabin diameter makes window seats more comfortable as the fuselage curvature doesn’t intrude into the seat space as it does on smaller diameter fuselages. Baggage stow bins can be deeper and installed down the cabin center and cabin sides as opposed to only against the cabin wall on a single-aisle airplane. Two aisles allow passengers and crew far better access to lavs and galleys, improving the trip experience. The wide fuselage allows the installation of premium lie-flat seating with more placement flexibility than a single-aisle airplane.

Profitability

We need to look at how the new airplane can capture more revenue for the operator. The paradigm shift here is that reducing costs, while important, is a dead end. You cannot save your way into profitability. The best path to profitability is to generate more revenue. The airplane must be designed to be a superior revenue generator. If we look at the total picture, the twin-aisle airplane has so much room to do so. You can even fly it harder than a single-aisle airplane because it can fly more legs per week.

In past articles, we have seen the impact the airport and the gate places on the airplane design. Both the 767 and the 757 are FAA Design Group IV aircraft. This means that internationally, they fit in an ICAO Code D Gate with a wingspan between 36 and 52 meters and a main landing gear track width between 9 and 14 meters. In US domestic service, they use a Gate Type B or C. These gates both accept aircraft with wingspans between 118 Ft and 171 ft, with the Type B Gate being limited to a fuselage length of 160 Ft.

These gate limitations have significant impacts that affect the single-aisle airplane in more adverse ways than a twin-aisle airplane. Airport congestion on the ramp hampers longer airplanes more than shorter airplanes. When you push back off the gate, regulations say that the maximum nose wheel steering used shall not exceed 55 degrees. There are wingtip clearance requirements, minimum nose to building distances, and incredible variability airport-to- airport gate configurations. Congestion is everywhere and ground controllers often find aircraft movements blocked. Shorter aircraft will always be less impacted by the knock-on effects of this.

However, there’s also the issue of slot availability at most airports outside the US. Virtually every major airport in Europe, Asia, and Australasia is slot-constrained at peak hours–if not most hours. This means a carrier can’t shorten its scheduled turn-times unless it can obtain better-timed slots, which is often difficult outside its hubs (where it controls a critical mass of slots). Ramp control and ATC might let you depart early on occasion — but if you can’t schedule a quicker turn, you can’t monetize it.

Generating Additional Revenue

Enplaning and deplaning the airplane is a key revenue-generating opportunity. Time is money. The faster you get off the gate, the faster you fly and make money. A single-aisle airplane is constrained by the cabin door. Its door size is almost irrelevant when you have every passenger going down the same aisle path. Door size matters a lot on a twin-aisle airplane and reduces onload and offload times dramatically when sized to allow both aisles to load/unload simultaneously. The twin-aisle airplane needs a big door, a crucial first step in the rapid turnaround process that is not available to a single-aisle airplane. A big door twin-aisle airplane gives you faster passenger exchange, faster baggage handling because the bulk cargo access is better, faster loading of food and drinks, and faster cleaning because the workers have two aisles to work from. All of this means that you can get more trips per year out of the airplane. Those trips fly more people and belly cargo. Revenue increases happen that are not available to a single-aisle airplane.

Yet, the revenue benefit of faster turn times only works if you’re a tightly scheduled airline on long-haul, which most network carriers aren’t–and aren’t likely to be in the future. If the network is hub-centric, the first goal is to minimize connection times by bringing all your arrivals in a tight “bank” or “wave,” and departing in a similar fashion 45-90 minutes later. Perfecting this hub timing naturally means sitting airplanes at non-hubs for longer than the minimum, even if you can physically deplane and board passengers in less time. And faster hub turns aren’t advantageous if they cut off inbound connections arriving later in the bank/wave.

Advantage Twin-aisle.

LNA believes that the additional revenue-generating opportunities provided by the twin-aisle NMAL far outweigh the difference in cruise flight seat/mile costs. The combination of fast turnarounds providing the time for more legs to be flown each week and increased bulk cargo space providing an added revenue source is a potent weapon for shrewd operators to employ moving forward.

 

8 Comments on “Analyzing the trades between a single- and twin-aisle NMA

  1. Thank you for your excellent article. It explains the trade offs between single aisle and twin aisle very clearly.
    With respect to Boeing’s design philosophy, I hope it utilizes the latest in cockpit design. In the March 22 issue of
    AW&ST, Jens Flottau and Guy Norris wrote that Boeing would likely keep conventional cockpit controls for design simplicity and low cost. It would behoove Boeing to look at the latest Grumman cockpits with active side sticks. Why constrain a newly hired cockpit design team with conventional controls? I believe the last AirBus aircraft to use them were the A300 and A310 series.

  2. How competitive would the 190-seat twin-aisle variant be compared to the a321neo?

    • Any new design has to achieve a clear economic benefit over the present benchmark, the A321neo. We can expect a fuel consumption improvement per seat of around 20%.

      • Bjorn, is 15%-20% achievable considering the twin aisle model LNA generated?
        Previously there are speculation that despite the clean sheet design and lightweight composite materials benefit, the Boeing’s twin-aisle NMA would at best be on par with the A321neo fuel consumption per seat IF no new engine technology is available. In other words whatever the % of improvement the RR Ultrafan or next CFM engine brings to the table that will be the only gap differentiating the NMA and the A321XLR. Do you agree? If not why?
        If so could airbus easily re-engine the A321XLR in the later half of the decade to close the fuel consumption gap against boeing’s clean sheet NMA? Or would the higher price tag for boeing limit its sales and popularity?

    • The comparrisons between the single and twin aisle aircraft should also include its revenue creation potential as opposed focusing only on cost. Airlines operate on margin, not cost, and the ability of the twin aisle aircraft to create more revenue from multiple revenue streams is powerful….. The possibilities that premium lay flat seating bring to the twin aisle airplane is just the tip of the iceberg……

  3. Re: ” Perfecting this hub timing naturally means sitting airplanes at non-hubs for longer than the minimum, even if you can physically deplane and board passengers in less time. ”

    Over several years of flying on Delta once a month or more pre-pandemic, on a wide variety of main-line aircraft, including A319’s, A320’s, A321’s, 717’s, 737’s, 757’s, 767’s, MD-88’s and MD-90’s, the “boarding starts” time printed on my boarding passes for domestic mainline flights was always 40 minutes before departure time. Actual boarding took less time and often started later for smaller aircraft. The longer single aisle types (757’s, A321’s) and twin aisle 767’s came closest to using up all the allotted 40 minute boarding time, with 261 seat domestic 767-300’s (now retired) boarding about as fast as 190 to 200 seat A321’s and 757-200’s. Only regional jets had “boarding start” times of less than 40 minutes before departure time (25 to 30 minutes if memory serves me correctly).

    I would guess that Delta would reduce its system wide mainline “boarding starts” times from 40 minutes to 30 minutes if it thought it could, and definitely doesn’t want to increase it above 40 minutes. Perhaps the way Delta thinks about boarding times of future aircraft purchases is that any aircraft that it buys for use in its domestic network, other than a regional jet, needs to be able to consistently and reliably board in 40 minutes, with a few minutes of slack to accommodate days when airplanes, employees, or other things, are running a few minutes behind schedule.

    At spokes served by regional jets or smaller mainline aircraft, I usually saw the aircraft pull in, unload, and then immediately reload for a return flight to a hub, except for the last flight of the day, which would sometimes park at the gate overnight for an an early morning return flight to the hub. It was at hubs where I more typically saw aircraft parked at the gate for a longer time than it would have taken them to load and unload, while they were waiting for passengers to walk across the airport form arriving flights, with an allowance for some of the flights being a few minutes late.

    • It only takes one slow de-planing and boarding aircraft to slow down an entire hub ramp, or stated differently, a hub ramp can be no faster than the slowest de-planing and boarding aircraft if passengers are not to be left behind.

  4. A twin aisle with 2-2-2 seating at 175′ length solves the problem of having a bigger freight lobe and a deeper fuselage for bending, but less drag than a 2-3-2. If 160′ length is a limiting dimension, a 2-2-2 at that length would be optimal with good circulation for 215 mixed class to 240 single class configurations.

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