More on Cargolux: weights and fuel burn

Note: Jon Ostrower at Flight Global has this update.

The underlying cause for the Cargolux rejection (the airline’s word) of the delivery of the first two Boeing 747-8Fs is performance.

Performance is based, generally, on two things: weight of the airplane and specific fuel consumption (SFC). If the airplane is too heavy, it may not meet the payload and/or range guarantees. If SFC is below specifications, range/payload may be affected. If you combine the two issues, a larger problem exists.

It’s been well known for more than a year that the 747-8 was “heavy” and the GEnx engines burned too much fuel. Despite the two year delay, GE hasn’t developed a performance improvement package (PIP) for the 747-8 that will be ready before the end of 2013, according to sources familiar with the situation. GE’s priority has been the 787 program.

We asked Boeing about the weight and SFC issues. Here is its response:

Q — 748F is about 8pc overweight, more or less.
Boeing: It’s misleading to say we missed our specifications. After we set our original specification, we completely redesigned the wing, which is significantly heavier, but more than makes up for that in increased aerodynamic efficiency and lower fuel burn. Our flight loads survey testing presented us opportunities to further enhance the airplane’s performance through further weight savings as well as to improve our payload range capability by increasing our maximum taxi weight, maximum takeoff weight, maximum landing weight and maximum zero fuel weight. As with all our airplanes, Boeing will continue to seek further improvements in weight throughout the life of the program.

Additional information from us: We understand the 747-8F is 5,000-6,000 pounds heavier than the revised design.
Q — Airplane is 6 to 8 percent short on fuel burn specs.
Boeing: While the first airplanes we deliver will be somewhat short of initial specifications for fuel burn, your estimate is much too high. Even at entry into service, the 747-8 will be a great airplane for our customers, and give them unparalleled efficiency and low cost, including double-digit improvements in fuel burn and lowered emissions. And with the GE Performance Improvement Package in work for the GEnx-2B engine, coupled with a number of aerodynamic improvements we have identified for implementation, we are confident that the airplanes we are selling today will meet our customer specifications. As with all development programs, we continue to focus on performance improvement for future line numbers and retrofitable kits for all delivered aircraft to further improve the performance of each airplane.

Additional information: subsequent to our posing the original question to Boeing, we were informed the SFC shortfall is closer to about 2.5%, which actually tracks with information we received last year.
Q — Cargolux isn’t the only airline unhappy with the airplane.
Boeing: Cargolux has said that it has refused delivery because of unresolved contractual issues, not because it is disappointed in the airplane. The 747-8, whether Freighter or Intercontinental, is a great product, carrying more payload farther and with greatly reduced fuel burn over the very successful 747-400 that it replaces. Our customers realize the value this airplane will bring to their operations, and will be pleased to have the airplanes in service.
The first 747-8 Freighters to be delivered meet or exceed customer expectations for maximum payload capability, community noise, common pilot ratings, airport compatibility — including takeoff and landing-separation distances — and all low-speed takeoff and landing requirements. In addition, the 747-8 delivers greater that a double digit fuel burn improvement over the highly successful 747-400. The airplanes are better than promised in community noise reduction, achieving QC2 on departure and QC1 on approach — putting the 747-8 in one of the quietest noise categories for large airplanes.

Additional information: That being said, we reliably understand (as we reported in our previous post) that another customer is negotiating to bypass early, overweight freighters that require rework in favor of later production models that have in-line modifications.

38 Comments on “More on Cargolux: weights and fuel burn

  1. Scott, what does this mean?

    “We understand the 747-8F is 5,000-6,000 pounds heavier than the revised design.”

    What is the “747-8F” and what is the “revised design”?

    I assume you mean Cargolux’s aircraft is 5,000-6,000 pounds heavier than subsequent 747-8F’s, which have had design revisions incorporated. Am I interpreting this correctly?

      • The wing is a new design from the outset, just the planform remained from the 744.
        Half of the weight growth was due to the stretch, which was not on the original cooking recipee, the other half is weight growth due the ‘revised design’ of the wing which is a misnomer for the repercussions of flawed assumptions regarding the wing loads. The ‘revision’ of the wing structure then spread into the fuselage, which also oboleted the 744 production tooling that was planned to be re-used for the 747-8 and spoiled the floating transition to the -8 in final assembly. But Boeing knows how to utrn a turd into a cake. They now advertise the 747-8F as “70% new by airframe weight”

  2. Also, it should be noted the consequence of an overweight aircraft will not manifest itself in meaningfully higher fuel burn; the noticeable impact is on payload/range. On an aircraft the size of the 747-8F, you can add thousands of lbs to OEW and the change in fuel burn will be barely measurable. Assuming the overweight 747-8F has not increased MTOW since the OEW increase was realized, every added pound of OEW is one less pound of payload or fuel you can get onboard when operating at max structural loads. Mission capability will be the real consequence of any extra fat on Cargolux’s 747-8F, not fuel burn/economics.

    • Your explanation helped me to understand the Boeing reply to the first question:

      “We completely redesigned the wing, which is significantly heavier, but more than makes up for that in increased aerodynamic efficiency and lower furl burn.”

      Initially I had some difficulty to reconcile “significantly heavier” with “lower fuel burn”. But it’s true that the bigger the aircraft is the less impact the overweight problem will proportionally have on fuel burn. And the increased aerodynamic efficiency of the wing appears to compensate for that with reduced drag, which would bring a corresponding reduction in fuel burn.

      • compare to the A380 : a rather heavy plane for its payload
        that has nonetheless very good specs due to utilising that
        weight in the right places ( large, efficient wing ).
        My understanding up to now was the basic wing structure
        on the 748 had not been changed ( but span extended,
        reprofiled and the movable surfaces majorly redesigned ).

  3. “.. we are confident that the airplanes we are selling today
    will meet our customer specifications. ”

    Is my interpretation correct that these improvements are for planes
    not sold yet ( i.e. unavailable for the sold and in production frames ) ?

    What is the weight change from original projected spec
    to redesigned wing projected spec ?

  4. CM :Also, it should be noted the consequence of an overweight aircraft will not manifest itself in meaningfully higher fuel burn;

    This is incorrect. When flying at MZFW, every added kg of MEW substitutes your payload, hence you lose revenue.

    When flying below MZFW, MEW increase adds to your fuel burn. At 6,000lbs overweight, you burn approximately 455t more fuel and pay roughly $385,000 in extra fuel costs per annum per aircraft (assumes 4,500 flight hours, avg fuel price 250USc/USg).

    Not a small amount!

  5. Funny, GE is on quote in Aviation Week that the GEnx-2B has a shortfall in SFC of 2.7% alone. If you add the effect of the empty weight growth of 8-15t (depends on whether you count the stretch or not), 6% isn’t that unrealistic to assume.

  6. Not wishing to be unkind, but I found the answers by Boeing to the questions asked to be “glib”

  7. When answers to questions fail to actually answer anything, I tend to loose confidence in what they say.
    Why not put an engineer up and have him provide actual figures. I know they can get bogged down in endless numbers, but they at least provide some actual basis. All this management communication training dribble is not helping anyone.

    • I am sure Boeing has an engineer talking to the CV engineers. Putting a Boeing engineer before the unwashed masses of the public will only confuse them with all those numbers.

      The bottom line seems to be the B-747-8F is not 8% over weight, it is about 6,000 lbs over weight, which is closer to about 1.5% to 3%. The SFC was not missed by 6%-8%, it is 2.5% -2.7% total, which includes the 6,000 lbs over weight airframe.

      These types of numbers are very typical for any new airplane at EIS.

      • 1) The 2,7% figure is engine SFC. Airframe weight effects come on top.
        2) The 5,000-6000lbs figure is what future (‘revised design’) 747-8s are going to be lighter than early 747-8s. That shouldn’t distract from the fact that early 747-8s are significantly above spec.

  8. Always wiling to take the risk of doing an off-point post, one thing that interests me is that CV’s agreement to accept the first two planes now (and apparently wait for future improvement) was certainly in contract form; ie the parties amended their contract to allow delivery and acceptance now, before U-Turn Al showed up at the board meeting. If I am correct, then U-Turn’s position is that he is telling B to change Qatar’s contract for calculation of late delivery damages to the formula CV got which he seems to like bettter. He is trying to extort more from B by making threats that are among the worst possible for B: Further delaying or cancelling delivery of planes from B’s two most deeply troubled programs. He has no legal right to do this, but knows B will never enforce their contract rights in court because they are, I think, truly desperate to settle with U-Turn ASAP, not litigate with him for years. B is not without advantages, including U-Turn’s need for the planes. It would be great to be a fly on the wall during those negociations. So much for the rule of law.

    • “Somebody” is trying to get a better balance between money paid and the bought airplanes.
      (Independent?) from this QR is trying to get acceptable compensation for their under spec 748s and 787s.
      Assume that CV has good legal standing for a cancelation.

      My guess is AlBaker is trying to leverage some synergy in a “class action” type suit against
      Boeing.
      As I said before: I am surprised airlines are so meek towards Boeing about these projects.

  9. Concerning the added fuel burn: assume a lift-over-drag of 20 and an SFC of .6, then every additional lb will cause 0.03lbs additional fuel burn per hour of operation.
    Extending this to 4500 hours a year and 6000lbs additional weight, the added fuel burn will be in the vicinity of 370t additional fuel.
    If you have a 2.5% SFC-increase, the fuel needed simply increases by 2.5% (first approximation), which would roughly be 1200t additional fuel per year.
    1600t additional fuel total. Roughly.

    We see that the additional weight is less significant than the loss in SFC.

    The increased wing weight is indeed well invested when it helps the aerodynamics. The original B747 wing was quite light, but also not well optimized given 21st century fuel prices.

    • If I understand your figures this would mean that in this case any reduction in fuel burn due to improved aerodynamic efficiency of the wing would be offset approximately 30% by the corresponding increased weight.

    • The 6,000 lbs are not additional weight, they are the (planned) weight improvement between early and future airplanes. The spec(!) weight grew by more than 7,000 kg after(!) the fuselage stretch, and the early airplanes are probably 2,000 kg over that.
      So an early customer who may have bought the aircraft before even the fuselage stretch has seen quite a performance shift – from spec creep and ‘fat’ on top. In all fairness, the stretch added capabilities, so it is not so simple to identify ‘good fat’ from ‘bad fat’.

  10. Uwe :
    compare to the A380 : a rather heavy plane for its payload
    that has nonetheless very good specs due to utilising that
    weight in the right places ( large, efficient wing ).
    My understanding up to now was the basic wing structure
    on the 748 had not been changed ( but span extended,
    reprofiled and the movable surfaces majorly redesigned ).

    According to EK’s Clark in an interview a few years ago(maybe 2) , the A380 hit its “targets” because the SFC was better than expected.

    • That’s correct, the A380 had hit its contractual performance guarantees at EIS. Clark also said at the time that the early A380s were 6T overweight, however that was offset by better aerodynamics (which also allowed higher Mc) and better SFC.

    • The major structural members of the wing (ribs, spars, beams) are still in the same places in planview, but the parts are completely new designs, as the wing profile has been changed entirely and profile thickness increased. Twist/loft also changed. Despite being completely new, a fact that Boeing doesn’t deny, it was still certified under an ATC, without full-scale static load or fatigue testing.
      With the increased thickness, the same planform could have been built much lighter, but the potential weight saving was traded in for an increased MTOW (i.e. payload/range capability)

      • @KDX125: You seem very knowledgeable. Clearly, the t/c changed a lot on the same planform and of course, all parts had to be drawn from scratch. What I don’t understand is that they stuck to the same design principles… i.e.: built-up spars ad ribs (instead of integrally machined parts like in “modern” a/c). They even carried over the wing tip extension from the -400… why that? They could have built an entirely “new” wing. That might have saved them also from other problems like the flutter issue. Was it all to convince the FAA to do another “Grandfathering”? The 37NG got new wings and is 80% different from the Classic and they got it certified as a derivative also.

  11. What’s 2.5% of the fuel bill for a Cargolux 748 for one year, if Boeing had to pay the difference?

  12. How much can the fuel burn be attributed separately to the engine or the airframe? Do they measure the SFC of the engine on a test stand?

    • So since Boeing has flown the plane around for a year and a half, they know how much the airplane is missing its aerodynamic performance in terms total thrust used over a given range for a given payload.

  13. Boeing guarantees weight and performance to each customer but the numbers are very closely held between Boeing and the customer. It is none of the public’s business. Never mind what’s in the press; it’s what’s in the contract that counts.

    Them what knows ain’t saying and them what says don’t know..

  14. Whilst Al-Baker, may appear to have thrown his toys out of his pram, the industry understanding is that he’s crying foul with some justification. Few new build aircraft projects can claim immunity from weight creep these days, but the double whammy here is fact a triple one, weight, design & performance.

    Despite our best intent we fail to see this project raising smiles on Boeing shareholders faces after the bulk of major carriers voted with their feet & sadly walked away.

    • Not enough info as to what is meant by ‘customer flight’. If this is a customer acceptance flight then this is part of the normal delivery process, along with inspections, technical acceptance, transfer of title etc. If indeed the normal delivery process is taking place, it would indicate to me that a financial settlement is being thrashed out and Boeing will in the end deliver the a/c.

  15. JPH :
    This is incorrect. When flying at MZFW, every added kg of MEW substitutes your payload, hence you lose revenue.

    Yes, cargo operations directly equate payload with revenue. That’s what I was trying to communicate when I said…

    CM :Mission capability will be the real consequence of any extra fat on Cargolux’s 747-8F

    As for the fuel burn consequence of an added 6,000 lbs OEW, your number is way off from my estimate. Using your mission rules and comparing two aircraft operating at equal payload and TOWs of 969,000 lbs and 975,000 lbs (the latter having a 6,000 lb OEW penalty), I come up with a fuel burn penalty of less than 1% for the fat bird – your estimate looks to be closer to 3%.

    Regardless of the amount, an SFC miss is less consequential to the operator because financial remedies are easier than trying to attach a value to mission shortfalls – especially if the mission shortfall means the operator cannot operate in a particular segment of the market. The OEM can write a check every month to the operator for the amount of fuel burned beyond the guaranteed performance. Proving the same kind of damages for business which cannot be realized due to a payload or range shortage is nearly impossible.

  16. Some general remarks:
    Weight is usually overrated, even by knowledgeable people. It is crucial when the added weight – say 2000lbs – reduce payload or range to a point where the aircraft cannot fly a key mission. Apart from that, each unit of weight increases fuel consumption by 3% times that unit per hour (as rule of thumb).

    Example: B787 is – let’s say – 5000lbs overweight, flying a 4500nm mission (at 450kts TAS), will require 5000lbs * 3.0% * 4500nm/450kts = 1500lbs (per flight). That’s something, but the B787 will burn around 120000lbs of fuel on that trip in total. That is about 1% additional trip fuel.
    Fun fact: the more efficient engine and aerodynamics are, the less painful is overweight. The B707-120 had to pay 5-6% of hourly trip fuel for each additional unit of weight.

    SFC is sometimes underestimated. A X% SFC increase directly goes into the fuel bill. A 2.0% increased SFC will increase the trip fuel by roughly 2%***. For our example, the 120000lbs will become 122400lbs, or 2400lbs more.

    ***: SFC is no physical unit, it is simply fuel flow divided by thrust (which are both physical units). The SFC is neither constant over altitude, or speed, or actual thrust.
    This graph is quite OK:
    http://upload.wikimedia.org/wikibooks/en/thumb/5/5a/Tfan-thrust-map-kk-20050824.png/640px-Tfan-thrust-map-kk-20050824.png

    • Thanks for the graph. It looks like the engine is less efficient on take-off and cruising at slower speeds.
      Since the thrust required at 12km is half that at 3km, does it burn half the fuel? It looks like cruising at 15km would cut the required thrust in half again.

      • Cruising higher is better, while 15km would probably stall most turbofans. The aerodynamics are also crucial: if the engine is more efficient, but the induced drag is higher due to lower air density, you don’t gain anything.

        Usually aircraft cruise best close to their maximum cruise altitude.

  17. @CRORPower Not sure about built-up vs integral ribs, that’s simply a cost/weight trade. One design driver for the -8 wing was the desire to keep the -400 fuselage largely unchanged in order to re-use the tooling. That meant the wing center-box, the part buried within the fuselage, had to be kept largely ‘as-is’ and observed as an existing interface to the new wing. Someone got carried away with ‘lean’ ideas like ‘re-use’ and ‘commonality’, but certification grandfathering was a driver as well.
    The wing tip extension are new, raked-style. The -400 has low-aspect ratio winglets.

  18. Schorsch :
    Some general remarks:
    Weight is usually overrated, even by knowledgeable people.

    Weight is usually underestimated by techie folk: It is not only the additional fuel burn that hurts. Every pound of empty weight growth means a pound less payload. Payload means revenue. Have a look at the MEW growth vs MTOW growth since launch and you see how much revenue potential the 747-8F has lost.

  19. Pingback: Initial 747-8 woes will not affect aircraft’s business case | Aspire Aviation

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