07 August 2015, ©. Leeham Co: Now that we have explained the range consequences of weight and fuel limited airplane operations, we might as well explain the last important part of the range of an airliner: Why the practical range is always shorter than what the OEM says.
When an airliner OEM gives the design or brochure maximum range of an aircraft, they do that with an aircraft in a “show-room” configuration and which is loaded with a filled cabin only; no cargo is included in the calculation. Further, in the cargo area, there is only bulk-loaded passenger bags. Container loading of the bags would have cost tare weight for the containers used and weight is to be avoided when stipulating the maximum design range.
In practice, we would have to consider tare weight for bags containers and possible cargo when discussing what practical range an airline can plan for a certain aircraft model. But this is far from the whole story. Here is what has to be considered in addition.
The OEMs calculate the design range with a full cabin and no extra cargo or bags containers. When they do the fuel calculation to get the maximum range, they have to increase the fuel they will have to pump in the aircraft for the mission to cater for minimum reserves. The reserves are there to cater for uncertainties in the mission’s completion, especially for the weather’s variability along the route but also route changes due to air traffic problems.
There are several types of fuel reserves, some stipulated by the regulators as bare minimum, others which are part of the airline’s company rules and procedures. The bare minimum, as stipulated by regulators, are the ones included in the OEMs range calculations and these are divided in en-route reserves and the reserves for manoeuvring when at destination or flying to an alternate landing place if the weather was too bad at the destination.
The en-route contingency reserve is to cater for “unexpected” factors, such as stronger headwinds, ATC or weather reroute etc and shall be the higher of:
As 2 makes for little fuel it is 1 that is valid for all but a few cases (the higher reseve alternative). To the contingency fuel, the aircraft’s captain shall add fuel for 30 minutes circling when arriving at destination airport and fuel to fly to an alternate airport should weather not permit that landing can be done at the destination.
The most usual OEM alternate fuel planning distance is 200nm for brochure data but regional aircraft OEMs often use 100nm alternate distance.
Airlines planning range
To understand how much of a nominal design range an airline’s route planning department can plan for on their new long range routes, let’s assume they have a number of Airbus A330-200s of the 235t version for long range missions which had a design range as given by Airbus of 6,500nm. We have learned that A330-200 has a range of over 7,000nm but that is for the 242t version that is not even in production yet. Airbus has just delivered the first 242t A330-300.
Airlines use the fleet they bought and got delivered years ago and for our case this was the rather recent 235t version of A330-200 with 6,500nm nominal range. For this acquisition, the airline specified a cabin which was two tonnes heavier than the nominal one and its practical crew and catering for the long range routes would be another one tonne heavier than what Airbus included in the brochure mission.
This shaves half an hour of endurance from the aircraft because the typical hourly fuel consumption is six tonnes of fuel. Half an hour means a loss of 235nm as the typical cruise speed is 470kts at FL 350 and higher. So the aircraft has a nominal range for the airline of 6,265nm.
Another factor which cuts maximum range for a certain passenger number is the assumed weight for Pax + Bags. Airbus and Boeing both used to assume 95kg/210lb for long range flights. This is a bit low and Boeing just changed to 102kg/225lb which is a more typical value an airline would use (it depends a bit on the region, an Asian operator might stay with 210lb). Should our airline use 225lb as planning standard we lose another 200nm from nominal range.
OEMs design range missions are flown on routes with no wind. In practice there is always wind and the route planning department has to plan for the part of the mission with the worst prevailing wind conditions. Let’s say this is a new Trans-Pacific route and that the worst planning wind going west would be 50kts. This takes our cruise speed from 470kts to 420kts, a decline in our covered distance per hour of flight of 11%. This means our 6,050nm range has now shrunken to 5,400nm if we use 225lb pax+bags weight.
We learned that in a real planning case the alternate fuel has to be for the actual alternate that has a Terminal Area weather Forecast that says that landing shall be possible at time of reaching the destination plus the time it takes to get to the alternate. The winds we talk about are for the winter period and this is also when we have the worst weather at our destination. Let’s say we have to plan with at least 300nm alternate distance, shaving another 150nm of our planning range. We are now at 5,250nm.
In practice airlines have to add additional reserves onto the minimum reserves that the regulators stipulate and the reserves they need for real weather. There are several reasons for that, the first being the state of the aircraft used for the flight.
The design range for an aircraft is calculated for the average produced individual. An airline cannot plan like that. It has to plan for the worst performing individual aircraft in its fleet for the mission type, otherwise it has to reserve particular individuals for certain missions and this is not practical.
When the flight crew arrives in the morning, they can get any of the company’s A330-200, they do not ask for “individual 17,” as this is the one which has especially good fuel consumption. Aircraft come out of the factory with a spread in their fuel consumption. The airline has to accept all individuals that are within the OEM’s contractually stipulated margins. Let’s say these margins for airframe and engine can be a total of +/- 1.5%. This means the airline has to plan the route structures with 5% plus 1.5% contingency fuel to be on the safe side.
Things would now be fine if there would be only factory-new airframe and engines in our world. But reality is that the planned new route could be flown with any of the airline’s A330-200, also the one which is just one flight from overhaul of both airframe and engines. Airframes can deteriorate between overhauls with up to 2% and engines with up to 6%.
Airlines keep track on the fuel consumption of each individual and the wear and tear condition of the individual can be catered for when planning the fuel before the actual mission. But the route planning department has to plan for flying the route with the worst individual of the fleet.
To the reserves we have described an airline can add additional reserves for special reasons. An example would be that an aircraft type is new to the airline and its pilots and the airline therefore decide to keep higher margins until they have learned how to fly the aircraft to get to its nominal fuel consumption.
In actual practical, airline route planning, there are many factors that contribute to the fact that if an OEM says that an A330-200 has a design range of 6,500nm, the practical range that an airline can plan with could be as low as 5,000nm or even lower. That is a decline in brochure range of 23%. The figure of 20% range planning margin to OEM’s figures is plausible, as we can see.
It is quite instructive to do this whole chain of practical considerations when discussing new airliners’ range and what routes they could be used for. Taking 20% off the brochure figures can be a good rule of thumb, it is not exaggerating.