08 December 2010
What does “busiest” mean?
Michael Jennings

In response to my recent post on Japanese airports, in which I mentioned that Haneda airport had had four new runways built since 1988, Patrick left the following comment

Huh?  And there must have already been at least one to start off with bringing us up to a minimum of five.

As it happens, Haneda has closed runways as well as opened new ones, and so Haneda at present has four runways.

But Heathrow, as I understand it a vast, if not the vastest airport in the world, has only two.

How come?

I will get back to the runways in a bit, but the belief that Heathrow is the busiest airport in the world is often heard in the UK. At least it was - I don’t think you hear it quite as often as you once did. As it happens, this belief is false, and it has been false for the 25 years of so that I have been paying attention to this kind of thing. The belief seems to have its origin in a somewhat disingenuous phrasing that was often heard in the aviation and engineering press a decade or two back, specifically “Heathrow is the busiest international airport in the world”. Note the word “international” in this. What was meant was that more passengers passed through Heathrow when flying on international routes than passed through any other airport flying on international routes. The reason for this is very simple. Britain is a small island with one giant city on it, and almost all of the places one might want to fly to from that giant city are in other countries. Domestic services make up a significant percentage of services from every other major airport in the world, but from Heathrow the percentage is miniscule. (In addition to this, for another reason that I will get to in a bit, a significant percentage of those domestic routes that do fly from London tend to be from airports other than Heathrow). Therefore, although more international passengers used (and still use) Heathrow than any other airport, there have always been other airports that handled more passengers in total.

However, many people have missed this distinction over the years, and have just heard it as “Heathrow is the busiest airport in the world”. (After all, aren’t most airports international airports? The phrasing of the stated statistic has often been deliberately misleading). Many people in the general media have made the same mistake, thus further spreading the misunderstanding.

As it happens, though, Heathrow is a massive airport. The most common legitimate statistic by which airports are compared is “Total passenger movements per year”. Every passenger who departs, arrives at, or passes through (transits) the airport countes as one passenger movement. By this measure, Heathrow has been number three in the world for most of the last twenty years. The only airports that have exceeded its traffic have been in the US: most notably the main airports of Atlanta (which has five runways) and Chicago (seven). In 2009, Heathrow actually moved up to number two in the world by this measure. Statistics for 2010 suggest that it will move down to number four, behind Atlanta, Chicago, and Beijing Capital, the first Asian airport to exceed Heathrow’s traffic. although Tokyo Haneda has at times been in the top five in the world.

However, although this is a useful statistic for determining how much terminal capacity is needed, this is still not the right statistic for determining how much runway capacity is needed. There are two reasons for this. Firstly, not all aircraft are the same size. An airport that hosts flights that use smaller aircraft is going to have more takeoffs and landings, and is going to need more runways. Long haul flights tend to use larger aircraft than short haul flights, and for various reasons Heathrow hosts almost all long haul flights out of London, whereas short haul flights are shared around London’s airports. (This has recently become even more pronounced with recent decisions of the EU to deregulate landing rights at Heathrow for foreign airlines, which led to most long haul traffic that had previously used Gatwick to switch to Heathrow). Plus there is a vicious circle. If runway capacity is at a premium but terminal capacity is not, and a lot of passengers want to use an airport, then airlines will use larger aircraft rather than putting on more flights. Since the opening of Terminal Five, Heathrow has been in this position. Heathrow has fewer runways than other airports. Rather than having fewer passengers use the airport, Heathrow simply hosts larger aircraft.

Secondly, there is an issue in the nature of the statistic: a passenger movemet is a passenger who starts a journey at the airport, a passenger who ends a journey at the airport, and a journey who changes planes at an airport. This last passenger is only counted once, even though he has both landed and taken off again at the same airport. Therefore, an airport that hosts lots of transfer passengers is going to have more take-offs and landings than one that is the start or the end of most journeys. As it happens, the airlines with the most dramatic hub and spoke systems in the world are the large US carriers, and their busiest airports are hubs in the middle of the US where vast numbers of passengers change planes. Due to a lack of political and airline consolidation and the preponderance of discount airlines, Europeans are much more likely to fly point to point. As it happens, the largest hub of the largest airline in the world (Delta) is Atlanta. The second and third largest airlines in the world (American and United) have hubs in Chicago. Atlanta and Chicago are still the two busiest airports by this statistic. Heathrow, on the other hand, comes in at number 12. The top six airports in the world by this measure and eight of the top nine are in the United States. With the exception of Los Angeles (which is a hub for traffic from the East Coast changing for aircraft to Asia - no such hub is necessary on the East Coast as modern aircraft can fly directly from anywhere in the US to anywhere in Europe) all of these airports are inland hubs where many passengers change from one flight to another.

By this measure, Heathrow is not the busiest airport in Europe. Paris Charles de Gaul has this distinction. This airport has four runways.

Before I bring this to a close, one more factor has to be mentioned. Not all runways point in the same direction. Some airports have runways that actually cross one another. It is not always possible to use all runways at the same time, or all runways at full capacity at the same time. An arrangement of parallel runways allows the highest flow of traffic, but this can make capacity dependent on weather conditions. If wind is blowing hard in the wrong direction, particularly perpendicular to the runways, this can reduce the number of flights possible, or perhaps even close runways completely. In places where weather is extreme and comes from unpredictable directions, having multiple runways pointing in different directions can make operations more robust to changes in weather conditions. In places where weather is less extreme and more predictable, this is not necessary. Heathrow has only two runways, but mild and predictable weather. Not all two runway airports are equal. Not all four runway airports are equal. All four runways at Charles de Gaul point in the same direction, which is a lot of capacity. Those four at Haneda consist of two pairs of parallel runways at an angle of 60 degrees to each other, meaning that total capacity is probably a little lower but robustness to changes in weather conditions is probably better. Proposals for another runway at Heathrow (theoretically cancelled by the coalition government, but they will no doubt be back at some point) are for another parallel runway, which is quite a lot of extra capacity. 

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