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.
how much would deliberate in-flight re-planning of the reserves extend the range again? And what would be a practical example of reduced range due to revenue cargo be?
Airbus use the JAR 3% reserve policy for their nominal range information, if I use 5% instead the A330-200 we have as example lose 150nm. For each tonne of cargo you lose around 100nm in range.
That is a nice, although understandably Euro-centric, explanation of the range requirements. The US rules are somewhat different with 45 minute domestic reserves and for international flight 30 minutes at 1500 feet, 10% en-route reserves reduced by re-dispatch, and ETOPS if required.
Speaking of performance, will the new Boeing data affect guarantees for orders already placed? Also, does that new data change the requirements or opportunities for the Middle-of-Market aircraft?
Interesting subject, well presented! Thanks.
thanks for the comments. Yes the US rules are different but the one I have described is the ones used by the OEMs for their data for markets in general. Every airline operate according to the rules of their local air worthiness authority, there are therefore many variants of the reserves rules. I tried to stay middle of the road not to make it to complicated, don’t think it was to Europe centric. Is there any other countries that have 10% contingency rules?, have not heard of it.
The changed Boeing data does not affect the aircrafts underlying performance, just under what rules they are presented. Therefore no effect on guarantees nor for the MOM work, it is just a more realistic presentation of their aircraft. All work with airlines has these more realistic rules or even harder as base.
Bjorn: well done again, some things I never thought about for dealing with real world range.
Ahh well, a Cessna 150 or 172 complexity range wise is not even a single aisle let alone a 777!
Apparently its the same unrealistic ‘nominal range’ for executive jets. In practice buyers go for heavier seats with more features and their tableware and other catering supplies are usually heavier.
Ive noticed in some discussions about particular airports and the runway lengths, often the paved runway is thought of as being the length available, when in every case the usable defined runway is quite a bit shorter. This is especially relevant in London City Airport
And fitting in with Bjorns series, this from P&W
No free lunch and be interesting to see how that engine and the original ones are good maint wise.
Finally someone speaks the truth. After all these years you are the first person to properly explain the truth about the numbers that are used in advertising. Being in sales myself I try to explain to anyone that will listen that you have to take anything and everything said in advertising with a little bit of skepticism. Reading any form of advertising is like listening to a drunk fisherman talking about the one monster fish he caught, a wee bit of exaggeration with a lot of drama.
Are the numbers Airbus has in its website comparable to Boeing’s new figures?
No, while Airbus is probably close on the cabin side they still use 95kg/210lb pax+bags and 3% JAR enroute reserves, Boeing is at 102kg/225lb pax+bags and 5% enroute reserves ie more conservative after the introduction of Standard rules.
The OEMs may argue that the ranges given are always unrealistic.
Added to the range reductions described in Bjorn’s article, there are further reductions if real-life flight profiles (restricted altitudes, non-optimal routing). Finally, especially twin-jet aircraft are prone to TOW-restrictions due to maximum field length.
I know that for campaigning the performance department do very accurate calculations for individual airlines. I therefore think that the advertised numbers do not really matter to real customers. If you are willing and able to shop an A330, you are probably not limited to comparing brochures. Hence, i am always puzzled why A&B care so much about what Average Joe (= the majority of us) thinks about their product.
Would it be possible for you to create a table of this more accurate comparison between A&B airplanes?
As said in the article, the Boeing Standard rules are only described in rough way to us, so we refrain from detailed comparisons until there is more details available.
I always wondered about the A340-500. It seems SQ was doing daily operations, with headwinds and reserves, out of the advertised range.