Bjorn’s Corner: Increasing an aircraft’s range, further considerations

 

By Bjorn Fehrm

By Bjorn Fehrm

23 October 2015, ©. Leeham Co: In last week’s Corner, we went through how Airbus can offer an Ultra Long Haul (ULH) aircraft to Singapore Airlines by increasing the Maximum Take-Off Weight of its A350-900, increasing the tankage and lower the payload. There are a couple of other considerations when extending the range of an aircraft that we did not touch upon. For completeness, we go through them here.

When increasing the allowed weights (really, masses) of a certified airliner, there are a few things that need to be re-evaluated and perhaps modified. First of all, the airframe needs to withstand the higher loads caused by the higher weights. Secondly, the aircraft’s field performance will be affected by higher weights. Required take-off field length must stay within usable limits, as must landing performance.

If the increase in flying weights are significant, it will also require a check on what happens to the aircraft’s flight profile when fully loaded. A heavier aircraft will cruise at lower flight levels and the One Engine Inoperative (OEI) service ceiling will diminish.

We now go through these additional areas and evaluate their impact on overall aircraft performance in general and on an A350-900ULR in particular.

Structural considerations

Last week we saw that by increasing the Max Take-Off Weight (MTOW) by 12 tonnes to 280 tonnes, decreasing the payload and increasing the max tankage, the A350-900 could be transformed from a Long Haul aircraft to an Ultra Long Haul (ULH) model, Figure 1.

A350-900 to -90ULR

Figure 1. Weight and fuel situation for Airbus A350-900 operating for Singapore Airlines. Source: Leeham Co.

We focused on how to reach the endurance necessary (up to 19 hours) to reach the US West Coast and New York area reliably from Singapore. We will now go through the other areas Airbus needed to look into before it could offer the variant to Singapore Airlines.

The most obvious area is the extra structural loads that will be imposed on the aircraft when weights increase. In the case of the A350-900ULR, there are a number of factors that minimize the structural consequences of making a ULR from the standard A350-900.

What has been increased is the aircraft’s fuel fraction weight. When the fuel increase is confined to the standard wing tanks (i.e. no cargo bay tanks are needed), the fuselage will not be affected. Figure 2 shows the fuel system for the A350-900.

A350-900 fuel tanks

Figure 2. Placement of fuel tanks in A350-900. Source: Airbus. Click to see better.

There are wing tanks and a center tank. Fuel increase in the wing tanks stresses the aircraft at a minimum, as the weight increases where the lift is generated. Therefore, additional bending moments in the wings are minimized. Fuel level increases in the center tank will cause higher bending loads on the wing. A gust will push up the wing halves and the center fuel weight will act like a weight that does not want to follow the movement.  There will be increased stresses in the wing-root area.

Weight increase in the fuselage is worse still as the increased load has to be transported from the fuselage area where the load originated to the wing halves where the increased lift needed to counterbalance the load is generated. For the A350-900ULR, Airbus increases the fuel in the wing (not clear where, but most likely in the center area, as this is the tank that an OEM fills up last for the reasons we just discussed).

At the same time, the payload, which is placed in the fuselage, is decreased, see Figure 1. This lowers the stresses in the wing-root area; one says that the wing-bending moment is lowered.

An OEM controls how much can be loaded in the fuselage with the Max Zero Fuel Weight (MZFW). It limits the maximum payload that can be taken onboard. It could well be that the A350-900ULR has a lower MZFW than a standard 268t version for the above reasons. It would not limit its usefulness, as the aircraft is specially configured to fly with a low payload.

We now assume that center fuel tank is filled to a higher level, which increases the wing bending moment. By lowering the MZFW, Airbus can regulate the max wing bending moment of the -900ULR to be at the same level as the standard version despite having a higher MTOW. This technique can be seen at work in the different weight variants that Airbus has available for its aircraft. There are higher MTOW variants that have lower MZFW and vice versa.

Other areas that get affected would be the landing gear and the aircraft’s brakes. Structurally a landing gear often has margin for a modest weight increase, 4.5% in this case. Increased brake load in case of a rejected take-off is more critical. We will now discuss this and the other aspects of take-off and landing performance when the aircraft has a higher take-off mass.

Take-Off and Landing performance

When an aircraft’s range performance is extended through weight increase, the take-off performance is the main area that is affected besides the structural considerations. Take-off performance limitations are all about what happens when one has a bad day at the office.

The A350-900 take-off performance diagram in Figure 3 tells us what MTOW we can have for different airport runway lengths and altitudes on an ISA +15°C day i.e. 30°C/86° Fahrenheit.

A350-900 take-off performance ISA +15

Figure 3. A350-900 take-off runway length requirements at different aircraft weights and airport altitudes on an ISA +15°C day. Source: Airbus.

I have marked the -900ULR MTOW with a red line at 280t take-off weight. It is easy to imagine how the curves extend from the given max certified weight of 275t to 280t. We can see that we stay inside the usual 10-12kft runways at airports which have an altitude up to 2,000ft. Singapore’s Changi airport has 13kft runways and are at sea level, as are LAX (11kft and 12kft runways) and Newark (10kft and 11kft runways). So runway performance should be ok at 280t for the A350-900ULR for these airports (and many others).

It is important to point out that the lengths in Figure 3 are not the normal take-off distances the aircraft has; they are shorter. The figure shows the worst possible runway length the aircraft will need when an engine goes bang just before or after the V1 speed (the speed of continuing the take-off, see Robert Bucholz comment). In case of the engine failing before V1 one stays down and applies full brakes. This is where the maximum braking capacity comes in and this might have to be tweaked in the case of the A350-900ULR. If one loses an engine after V1, the take-off continues on one engine. The runway length is then the distance needed to an obstacle which is max. 50ft high in the direction of the runway.

Contrary to take-off the Max Landing Weight (MLW) of the aircraft can still stay the same for a variant that has other weights increased. Even if MLW was increased for reasons of landing with more of fuel on-board, landing performance is seldom critical for a modern airliner.

Flight level and service ceiling

An airliner which has a very high wingloading (700-800 kg/m2) will have to start its cruise on very low Flight Levels (FL), sometimes below flight level 300. Optimum flight levels for an aircraft’s efficiency is around FL370-410. In the case of the A350-900, the max wingloading goes from 605 kg/m2 to 632 kg/m2. There will therefore be no problems with the A350-900URL’s capability to reach optimum flight levels.

A final consideration would be the max service ceiling that the aircraft would have with one engine inoperative (OEI). I don’t have the OEI ceiling for A350-900 but these are normally FL 130-200. Once again, the low wingloading increase of the A350-900ULR and a small decrease in max continous powerloading (max continous OEI engine thrust compared to MTOW) of 0.121 (-900) reducing to 0.116 (-900ULR) should guarantee a high enough OEI service ceiling for most routes.

24 Comments on “Bjorn’s Corner: Increasing an aircraft’s range, further considerations

  1. Very informative.

    Structural capabilities for loading more fuel in the center wingbox should top the offload from a reduced payload.
    Payload is a linear load along the fuselage introducing a
    moment arm at the wingbox in addition to transferring increased weight into the wing.

  2. Great write-up as always! I learn so much from this site!

    One question I have is, will this aircraft be able to carry higher (±250 passengers, baggage plus some cargo) on ULH routes or will it only be able to fly these routes with the reduced seat counts that SQ is planning? In other words, how practical is this aircraft outside of low density, high premium configurations?

    • Airbus has said you can return this aircraft to a standard A350-900 specification. This means the fuel computers get loaded with new software to cut off the inflow of fuel to the center tank earlier when tanking the aircraft and you get the normal weight spec of the aircraft i.e. 268t MTOW and 192t MZFW and so on.

      Principally you could get the values in-between for how much you could load i.e. a scaling of fuel vs. MZFW and MTOW but it is a design and certification effort that has to have a return of investment.

      We don’t have the actual MZFW of the -900ULR, it could very well be that a more normal cabin config would be practical. The ULH range would be cut when you load more payload. The sum of OEW+Payload+Fuel will have to be 280t for the -900ULR variant. With a cabin with 250-300 seats, I don’t think the OEW increases as you get a more normal amount of heavy premium seats but the payload will eat into the fuel for the mission. The trade is; increase payload with 5.8t and you loose 490nm range.

      • Still curious if Boeing could do the same thing with a 787-9, take the reduced pax load and get the same or better range at a lower fuel cost as you would have more fuel efficient engines getting less overall load into the air.

        • Hi Transworld,

          Boeing could do an ULR aircraft out of the 787-9 but there are few things that make it a bit harder than for an A350-900. The -900 is the base model designed to be stretched to -1000, it therefore has margins on wingloading (605kg/m2), wing tank volume (can increase 15% without needing cargo bay tanks) and spanloading (weight per effective m span, 4075kg/m). For 787-9, which is the stretch of the 787-8, we have 700kg/m2, need for cargo bay tanks and 4270kg/m effective spanloading. Furthermore the A350 engines are 2% more efficient than the GEnx/Trent 1000 engines (they are one generation younger). On the other hand the A350 is heavier.

          in summary it is possible but require more job from Boeing to reach the same result, not because the 787 is a less good aircraft but that the -9 is a stretch and therefore has less margins.

      • Thanks so much for elaborating a bit more. That is what I love most about this site – the interactive nature of it all!

        So for EK, for example, to fly the 16.5hr route to LAX on most days, they could possibly load another 11.6t more in this aircraft? This is assuming that 490nm could be flown in an hour?

        Am I understanding this correctly?

        • Yes you are right, I checked it in the model, with a still air distance of 7750nm (7250 + wind) you could have 30t payload with a SQ config.

          • Thanks Bjorn! The A359 is shaping up to be a real beast of an aircraft!

  3. Great piece of work! Difficult things explained so clearly and logically! Thanks!

  4. One minor point. You stated that the TOFL charts are calculated when an engine fails just before or just after lift off. They actually show the balanced field length (ie stop or go distance) for an engine failure at V1.

    For heavy weight takeoffs, V1 will be significantly lower than Vloff.

    Otherwise, a good description of the design considerations for airplane range increases.

    • Thanks Bob,

      great to have a real pro like you chip in here, you have established this data for several of our airliners out there.

  5. Thanks as always Bjorn.

    I am wondering why tanking fuel in the center tank should have any implication for bending moment at MZFW at all. Wouldn’t that fuel be gone at MZFW, with “unusable fuel” stored in the wing tanks?

    Random suggestion for another “fundamentals” post: the effect of cruise altitude on flight efficiency, and the reasons for a given plane’s optimal FL.

    • Hi Eric,

      think in the following sequence: 1. Morning. Aircraft empty. 2. First mission. Load of passenger and cargo to close to or at MZFW. 3. Fuel is added including to center tank, it is a long mission. 4. Take-Off and initial cruise. Aircraft hit gust shortly after take-off, there is still a lot of fuel in the center tank.

      OEMs must always dimension for worst possible scenario.

      Thanks for the fundamentals tip, it will come.

  6. Great, I look forward to the fundamentals section.

    Re the bending moment shortly after takeoff – I was assuming that the fuel in the wing tanks would provide enough bending relief to avoid reaching the critical MZFW load. I was assuming that MZFW defined the maximal bending moment at ultimate load, but I suppose that assumption isn’t valid.

    • The OEMs always have bending relief from any fuel in the wingtanks in their calculations, they analyze hundreds of cases. They have to stay below deforming stress levels at limit load which is the defining case. They then have to check that the aircraft does not break up at ultimate load gusts or hard landings (1.5*limit load).

  7. Bjorn,

    I’ve really enjoyed reading this article and your posts.

    Thanks for the great work!

  8. All three 787 have same wing and same max fuel capability (except -10 by 2018 because 789 MTOW-253 metric tons; later, he could fill the tank for about 263 t).
    Like 787-8 since 4 years at 228 t MTOW, more later (about 10 tons) to fill the tank if necessary, to go farther than 787-9 8300 NM with 280 pax.

    Such heavy 787-10 will be about 810 kg/m², like today 777…

  9. Example for reduced initial cruise altitude: the B777-300ER at maximum take-off weight has an initial cruise altitude below FL300. That aircraft is a good example of “stretch to the limits”, also in terms of wing loading.

    • Yes, but more powerfull bi WB fly higher vs quad soon in cruise.
      Here is “splash down” of 340-500/600 that fly no farther (a little less), than 777-200LR/777-300ER , because heavier and more fuel need.

      Boeing choice work to go far if you have best engines and aerodynamic.

  10. I wonder what re-engining the 280T A350 late next decade will do to the range…

  11. There have been several A350-900 versions considered over the last few years. The A350-900R that was to be based on the -1000, the A350-900F the future freighter variant similar to the A350-900R and the A350-900 Regional that was to be launched by SQ as well.

    This A350-900ULR now launched by SQ seems relatively new variant. Meanwhile the talk about the A350-1100 won’t go away. Now even Randy Tinseth is discussing it 🙂


    Boeing marketing vice-president Mike Tinseth expect Airbus to add a further, large derivative to its A350 widebody family.

    “Based on the A350-1000, it would be hard to believe they wouldn´t do something” he said. Tinseth labeled the in-development A350-1000 a “disaster”, arguing it has “compromised” engines and wings.

    This, he said, is partly down to the decision to increase thrust through changes in the Trent XWB engine core without enlargement of the powerplant´s nacelle when Airbus redesigned the variant in 2011.

    “I don´t think they can live long with being outsold … in that segment”, said Tinseth. But he believes that a A350-1100 would not pose a threat, because such a move would be “something we contemplated early in the 777X development process”.

    http://bloga350.blogspot.nl/2015/10/boeing-also-expects-a350-1100.html

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