The standard A350-900 is a long-range aircraft. Equipped with a three-class cabin with 325 seats Airbus claims a range of 8,100nm. For the A350-900URL version, the claim is up to 9,700nm by changes to the fuel system, adding 24,000l of fuel (from 141,000l to 165,000l).
How can one increase the range with 1,600nm by just increasing the amount of fuel? One can’t. This is a serious case of Apples and Oranges. The first case is equipped with a cabin holding 325 passengers and their bags. For the A350-900URL we talk about a cabin holding less than 200 passengers.
To understand what the real changes are and how they can be used, we have run the standard A350-900 as delivered to Singapore Airlines (Singapore, SQ) through our performance model. Then we ran the A350-900URL Singapore will get after the summer through the same model. Out came the performance charts in Figure 1.
The chart shows how long Singapore can fly with their A350-900s dependent on how much payload they have in the aircraft. The curves in the chart show what happen when we load the aircraft with more or fewer passengers (or a mix of passengers and cargo). The load of passengers is on the vertical axis.
If we assume each passenger weigh 100kg (which is the IATA recommendation) the standard Airbus passenger load weighs 32.5t (one line up from 30,000kg). One then follows this weight horizontally until one crosses a range curve, for the standard A350-900 at 7,700nm. This is the range of the aircraft with this payload.
At 325 passengers. we are at a weight situation for the aircraft where we can decrease the number of seats filled and gain range. At 250 passengers we can fly 8,500nm. Below 250 passengers our standard A350-900 can’t fill any more fuel. The fuel metering system says the tanks are full and we have to offload passengers at a higher rate so we can fly with a lighter aircraft and therefore gain range.
The A350-900 range curve is for the standard 280t aircraft as it has been delivered to Singapore since February 2016. The A350-900URL curve is for the aircraft with aerodynamic improvements we described here and with the extra 24,000l allowed in the tanks from a changed fuel metering system. The reduced drag from the improved winglets and the aerodynamic cleanup is giving the slopes a shallower angle (we get more range for the same payload).
The major gain from the increased fuel capacity kicks in at lower payloads. We discuss this in more detail below.
The charts are done with an assumed empty weight of the Premium heavy SQ A350-900 of 142.5t. One could argue the ULR variant should have a lower empty weight as it will hold below 200 seats. If we use the rumored seating of 162 seats for the SQ A350-900URL, divided between 68 Business and 94 Premium economy seats, we land on a cabin about 2t heavier than the standard SQ A350-900 cabin (which has 253 seats divided between 42 Business, 24 Premium economy and 187 Economy seats).
We will use an equal empty weight of 142.5t to simplify matters. Any difference will be small and will not change the discussion.
The chart is made with typical airline standards for fuel reserves. This means 5% enroute fuel margin for unforeseen winds or route changes, a circling of 30 minutes at the destination and the possibility to go to an alternate airport 200nm miles away, should the destination airport get unforeseen bad weather.
We now use the chart type to plot the effects of different SQ seating standards in Figure 2.
For the standard SQ A350-900, we assume it’s loaded at the typical load factor of 80% for its 253 seat cabin. This means we fly with 202 passengers with their bags, giving a payload of 20.2t. The 20.2t line crosses the standard A350-900’s payload-range curve at 9,200nm. The extra range compared to Airbus standard case is because we fly with 123 fewer passengers.
For the ULR version, we assume an 80% filled cabin of 162 seats, which equates to 130 passengers and 13t. We, therefore, plot a 13t line which crosses the URL curve at 9,800nm.
If we finally plot the Airbus claimed 325 passengers with bags in the diagram we have the payload line at 32.5t. This crosses the payload-range curve for the standard A350-900 at 7,750nm. This is not surprising, the enroute reserve used by Airbus is 3% and passengers+bags are counted as weighing 95kg.
The different lines in Figure 2 show why Airbus has increased the fuel capacity for the -900URL. At normal passenger loads the difference between the curves are small (it would have been nil if we had assumed an improved standard -900 as delivered after the summer).
Once we trade passengers for more fuel below the 250 line, we see the gains of still having room in the tanks to fill more fuel instead of just off-loading passengers to get more range. The ULR version of the A350-900 makes sense for extreme routes where you transport less than 200 passengers. For a normal passenger capacity of more than 250 passengers, the URL version gives no benefit (assuming both variants have the same improvement standard).