Bjorn’s Corner: Weight or fuel limited, what is this all about?

By Bjorn Fehrm

By Bjorn Fehrm

24 July 2015, ©. Leeham Co: In recent articles around the Ultra Long Haul (ULH) needs of Singapore Airlines, there have been many references to aircraft being either fuel or weight limited. It is not so evident what this all means and what the practical consequences are of one or the other limitation.

Let’s go through what it all means with a practical example and show how it will affect the performance of the aircraft and what one can do about it.

As an example we will pick Boeing’s ULH 777-200LR. It is known as the Worldliner since it can connect almost any two cities in the world with its ultimate range of 9,300nm. In practical use, the Worldliner has often been configured for less range. In such configurations it runs out of fuel tank space before it reaches its Max Take-Off Weight. This is “fuel limited.” Here is how it works.

The best way to understand how it all works is to learn to read an aircraft’s Payload/Range chart.  Figure 1 shows the Payload/Range chart for the 777-200LR taken from the Airplane Characteristics for Airport Planning (ACAP) document that the OEMs issue.

777-200LR Paload Range chart annotated

Figure 1. 777-200LR’s Payload/Range chart. Source: Boeing. Click for better view.

It is a very good chart, as Boeing has historically included a lot of information in its charts that other aircraft manufacturers don’t. We will now show that this information is quite useful. The chart looks busy but is quite straight forward to understand.

We have the aircraft’s empty weight + payload on the Y axis and the range on the X axis. As we fill the aircraft with fuel we have useful range to fly. How much depends on how much fuel is filled in relation to empty weight + payload. The resulting range is shown by the sloped lines named “Brake Release Gross Weight” spreading over the diagram from left to right. The more the “Gross Weight”, or Take-Off Weight (TOW) as it is more commonly called, consists of payload the less range. Therefore the lines starts at high payload and little range as it only allows a little fuel and goes to longer range the more the TOW is a mix of less payload and more fuel.

On the lower left I have entered the Operational Empty Weight (OEW) of the 777-200LR as 155t. The ACAP lists this as 145t but Boeing shows it as a more realistic 155t for operational aircraft in other documents. Now follow the OEW + Payload axis upward and you reach the line from A to B which describes the Maximum Zero Fuel Weight (MZFW). This is the maximum payload that can be loaded on top of the empty weight for the aircraft.

MZFW is a structural limit and is 461,000lb/209,106kg for the 777-200LR. With an empty weight of 155t the 777-200LR can load 54 tonnes of payload and can keep this high payload for flights up to 7,500nm, point B. This is an extremely good value. Normally airliners can only load the maximum payload to around 5,000nm.

Weight limit line

At point B we are at the aircraft’s sloped Gross Weight line called Maximum Take-Off Weight (MTOW). Here we have to start off-loading payload to be able to fill the tanks more to fly further, otherwise we would pass the MTOW. The aircraft is at its weight limit.

If we can’t fill more fuel because we pass MTOW this means we have to follow the sloping MTOW line downwards from B to H in the diagram, we trade payload for fuel. We can read on the line that this trade at MTOW is at 766*1000lb or 347*1000kg, i.e. 347 tonnes or 766,000lb.

The angle of this line, in fact all the Gross Weight lines, describes how fuel efficient the aircraft is for such a mission. The steeper the line the more fuel the aircraft consumes. Here’s how one reads the fuel consumption value:

We have reached the aircraft’s Max Take-Off Weight. To fly further we need to offload payload and fill fuel in the tanks instead. To fly 1,000nm further from B we need to off-load payload all the way down to F. Checking how much that is, we can see it is 209t – 196t = 13t, i.e., we trade 13t payload for 13t fuel and gain 1000nm in range. Therefore the fuel consumption for 777-200LR for a 8500nm route is 13t per 1000nm.

Fuel limit line

The diagram also has a steeper line that starts at C. This is the line that describes how we trade payload for range when we can no longer fill the tanks further. Our fuel tanks are too small, despite having weight margin, we can’t fill more fuel to fly further; the aircraft’s tanks are full.

In such a case we have to off-load more payload so that the aircraft gets lighter and can therefore fly longer as the drag due to weight (induced drag) is reduced. The line from C to E is the aircraft’s fuel limit line when equipped with standard tanks. It shows how much payload we must sacrifice to fly further once at the fuel limit. At E we have traded all payload to get the aircraft ferried to 9,600nm.

If you read the text on the steep sloping lines it says “Fuel capacity” and the base aircraft can take 145,541kg fuel. The additional lines are for aircraft equipped with Additional Cargo bay Tanks, ACTs. We can read the max is 162,636kg with 3 ACTs.

Standard range

To understand the OEM’s standard range situation; standard range is given with nominal passenger capacity with their bags and no cargo. In the case of -200LR this is 301 passengers times 210lbs or 95.3kg equaling 28.7t or close to 30 tonnes. Add 30 tonnes to the aircraft’s OEW of 155 tonnes and we are at 185t at point I. Follow the red 185t line to the right until you are at fuel limit lines, point D, or the MTOW line, point H.

You can see that we are hitting fuel limit lines at D way before we have reached the aircraft’s MTOW of 347t at H. If we have no ACTs installed, the fuel is enough to reach 8,650nm. If we have the max allowed three ACTs, we reach the max range of 9,400nm at H. The extra 100nm compared to official figures is because each ACT weighs around 500kg so we should have gone up 1.5t to hit the 3*ACT fuel limit line at 9,300nm.

Summary

We now know that the oft quoted 9,300nm of the 777-200LR is a max value and will only be achievable if the aircraft has installed three extra cargo bay tanks to move the fuel limit lines out. The standard 8,650nm represents flying times of 18 hours and there are very few -200LRs used on longer routes. Operators therefore seldom install ACTs and if they do they typically install one.

We now know that those that talk about the 9,300nm range of the Wordliner probably don’t understand that is normally fuel limited to shorter range (although 8,650nm can by no means be called short). After 18 hours in an aircraft, one is pretty happy to step off and stretch one’s legs.

An airliner is either weight limited or fuel limited at its max range. Of the two, the fuel limited aircraft can add ACTs to fly further. Once at MTOW, we need another aircraft to fly further with our given payload.

9 Comments on “Bjorn’s Corner: Weight or fuel limited, what is this all about?

  1. Very interesting post. My former job was flight dispatcher Since 1970 up to 1987, during my active occupation I was lucky to start manual flight plan calculations so this graphics would really useful I worked on B707, DC-8, B747, Dc-10 and all others types of aircraft. By the end of my career my job was easier by using computerized flight plan and took very short time. During all year of 1989 I finally was assigned to design automatic ATC flight plan. For that I have to update all around the world FIR according to AIP of each country
    Thank you again for post

  2. Thanks for the article. The payload-range diagram is a useful tool to analyze a number of things. The main error it has is the underestimated OEW, as Björn pointed out.
    The slope between A and H does give an indicator about efficiency. In a mission as described above (TOW: 351t; LW: ~195t) the aircraft actually burns between 20.8t/1000nm and 11.3t/1000nm (if no ATC-restriction in altitude exists) .
    The 13t/1000nm constitute an average. The average is not easily found by seeking the mean value of the both given values. The B777-200LR/300ER is highly restricted in initial cruise altitude when flying full weight. The initial fuel burn is hence very high. That is the reason why the B777X gets a new wing.

  3. Just an addition:
    The difference between 20.8 and 11.3, 9.5t/1000nm, is the fuel required to haul the trip fuel. That starts at 9.5t/1000nm, and ends up at 0 (when all trip fuel is burned).
    When landing in between and refueling, the half the trip fuel due to payload and OEW needs to be carried on each section. The average flying weight is reduced.
    If a 9000nm trip is flown as two 4500nm sections with intermediate landing, the fuel consumption for the entire trip per seat is roughly cut by 15-25%, depending on aircraft model.
    That is why the ME3 are so well situated (cutting many global connections roughly in half), and why ULR will always remain a niche market.

    • Singapore Air has to compete with ME3, partly because they will eat its lunch and for Singapore’s role as a global business hub. They want to be the one with a dozen direct flights to US cities rather than Emirates. This is the only way to do it by getting a sizeable fleet of ULR.

  4. Dear Bjorn

    Thank you for an easy to read and understandable explanation of reading a range payload diagram. This is the best one I’ve found.

    One point I don’t get in your example of having to swap 13 t payload for 13 t fuel to get another 1000 nm range. My interpretation says this is valid for an aircraft with 1 ACT, not a standard capacity aircraft. For the standard capacity aircraft, it becomes fuel limited at C and one must follow line C E to the intersection of the 8500 nm range line. Reading across this will give payload plus OEW of 189 t, a payload reduction of 20 t. Looking another way, for a standard capacity aircraft everything to the right of line C E is not valid and must be hidden.

    Please point me in the right direction.

    • Hi Andrew,

      you have got it! Of course a standard aircraft flies along the points A-B-C-E, this is the standard aircraft diagram. Add one ACT and your diagram moves out one line to A-B-E-G and so on.

      What I wrote about is that I wanted to have the angle on the MTOW line which is the same as the fuel consumption per nm. Therefore you can take arbitrarily short intervals fuel/range or long ones to measure that. 8,500nm which is the nominal range is not involved in that. You just want the angle = fuel traded for range. I thought 1000nm was convenient and could read 13t for that range increase, you could also say 6.5t per 500nm or 1.3t per 100nm, it is all the same angle and fuel consumption which is 0.013t per nm.

  5. It gets very interesting when fuel consumption is reduced for very long ranges with better engines as the payload fraction increases. Since these long range aircraft maintenance costs are cycle driven, engines; landing gears; nacelle are the big hitters and you get more benefit from less cycling on wheels and brakes, fuselage pressure cycling and some other systems that idle along at criusing altitude. Hence there is a tradeoff between flying non stop heavy fuel loaded v.s. with one extra cycle with less fuel and more payload. I suspect here is a 2 engine aircraft break point in fuel efficiency and maintennce cost when flying long range non-stop is more profitable. Björns model might show it. Besides you gain more flighthrs per week per aircraft compared flying a 2 stop route.

  6. In 2005 a 777-200LR with the ‘standard’ three aux fuel tanks and 35 people on board flew 11,664nm. Apparently there were no further modifications to the aircraft. This flight was not possible according to your chart – can you please explain why this is? I would be surprised if the extra distance was solely attributable to favourable jet stream winds? Thanks

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