Monday, October 7, 2013

A network analysis of airplane disasters

I hate heights. This is a well-known syndrome (acrophobia), and so I am not alone. However, it does mean that I dislike being in airplanes, especially small ones. In turn, this means that I am interested in air disasters, because it gives me a very good reason to feel that I should dislike being in planes.

Airplane crashes are publicly documented in a way that car crashes, for example, are not. The latter are all too common, sadly, and so you will not find any lists online detailing them. You will, however, find plenty of information about airplane crashes, including a lot of details that you might be better off not knowing about. I will skip most of these morbid details, since this is a family blog, but in this post I will be looking at some of the actual data.

The only airline never to have been involved in a fatal accident in the jet age.

Airplane incidents

One internet site that you might like to peruse is the Aviation Safety Network. which has a database with details of all known aviation incidents worldwide. From 2 August 1919 to 1 October 2013, there were 16,844 recorded incidents, including 13,785 accidents, 1,045 hijackings, and 708 other criminal occurrences.

The information is mostly taken from the reports that arise from the official investigations (if there was one). If you read some of the descriptions, not only will you never fly again, you will never even set foot in another airborne conveyance, even while it is still on the ground. What this database does is itemize every single thing that could possibly go wrong with a plane, and what effect this has on the people in it.

There is a long-standing rumour that the most dangerous parts of a flight are take off and landing. However, the data make it clear that this is complete nonsense. Consider, for example, the circumstances of the 40 worst accidents in terms of number of fatalities per plane (excluding ground fatalities):
Taxi phase
Take off phase
Initial climb phase
En route phase
Approach phase
Landing phase
If all planes ever did was take off and then immediately land, the passengers and crew would all be much better off.

However, the worst double-accident did occur while one plane was taxiing and another was taking off. Both were Boeing 747s, and their collision killed 335 of the 396 people in the taxiing plane and all 248 people in the plane that was taking off. This was in the Canary Islands in 1977.

The worst accident involving a single plane occurred in Japan in 1985, when 520 of 524 people died. The 747 plane had previously suffered damage, which apparently was not repaired properly, and the plane therefore ruptured in mid-air. [Note: 747s on domestic routes in Japan are configured to carry close to the maximum number of passengers.] The next worst accident (all 346 people died) occurred when the luggage compartment of a DC-10 opened shortly after taking off in France in 1974.

And so the list goes on, usually involving the failure of some part of the aircraft systems. However, an all too common cause of fatal airplane accidents is what is euphemistically called Controlled Flight Into Terrain (CFIT), which means that the pilot was in control of an undamaged plane at the time of the crash. This problem has been partly addressed by the introduction of Traffic Collision Avoidance Systems (TCAS) and Minimum Safe Altitude Warning (MSAW) devices.

Actually, when you look at it, 35 of the top 40 accidents occurred from 1972 to 1999, inclusive. Only 5 of them occurred after that, and none of them after June 2009. So, air safety is officially considered to be improving continuously through time. Prior to 1970, when the Boeing 747 was introduced, accidents involved fewer people because there were far fewer passengers per plane. [The original 747 had 2.5 times the capacity of the previous Boeing 707.] So, only two accidents from the 1960s make it even into the top 100 list, and none of them were prior to 1962.

A network

The Aviation Safety Network site also provides summary lists concerning some of the accident situations, and it is this summary information that we are considering here. In particular, we are interested in the information regarding the nature of the flights. The data are the number of fatal hull-loss accidents (fuselage written off, damaged beyond repair) per year and the number of associated fatalities. The data cover the years 1942 to 2012, inclusive (71 years).

The flight categories include: Training flight (total of 136 accidents and 553 casualties over the 71 years), Ferry / positioning activity (137 accidents, 523 casualties), Cargo flight (712; 2,986), International scheduled passenger flight (367; 17,597), and Domestic scheduled passenger flight (1,299; 39,403). For the Training flights, Ferrying, and Cargo flights, this is an average of ~4 casualties per accident; for the Domestic passenger flights it is ~30 casualties per accident; and for the International passenger flights it is ~48 casualties per accident. [The summary data for both the International and Domestic Non-Scheduled Passenger flights are missing from the web site.]

I have analyzed the annual accident data using a phylogenetic network as a tool for exploratory data analysis. To create the network, I first calculated the similarity of the years using the Manhattan distance, and a Neighbor-net analysis was then used to display the between-year similarities as a phylogenetic network. So, years that are closely connected in the network are similar to each other based on the number and severity of the aircraft accidents, and those that are further apart are progressively more different from each other.

Basically, the accidents increase in number and severity from top to bottom in the network, with 1985 (see above) and 1972 being the worst years. The worst period was 1969-1980 (shown in purple), with one exception (1975). Note that 1977 was overall not a particularly bad year, in spite of the Canary Islands incident (see above).

Perhaps the most important message in the network is that the years 2000-2012 (in red) are generally clustered with the 1940s (green) and 1950s (blue). So, in spite of the massively greater volume of air traffic in this century, the number of fatal accidents is currently not much greater than it was before the advent of the jet age.

The years 2000, 2001 and 2009 are clustered away from the others (at the top left) because there were still a few bad accidents involving International flights even though the number of accidents involving Domestic flights was low. Indeed, 2000-2001 were the first years to return to the Domestic accident levels of 1942-1945 (which are clustered at the top right).

I presume that I should take great comfort from this overall trend. The reason for it is not hard to fathom, and indeed it is the purpose of the Aviation Safety Network. Every time there is a reported aviation incident it is investigated, and any lessons learned are disseminated. So, if the circumstances leading to an incident are avoidable, either by improving the technology or by changing the human operating procedures, then efforts are made by the authorities to implement those changes, so that the incidents will not be repeated. Safety must therefore increase, at least until radically different modes of transport are introduced. This is why the 1970s were so dangerous — the aviation authorities were suddenly confronted with the consequences of having jumbo jets in the skies.

This is the fundamental difference from car accidents, of course. On the ground, we insist on repeating the same types of incidents over and over again, with only improvements in technology to help us. The human operating procedures remain essentially the same, and fallable. I guess that is why car manufacturers have been developing Advanced Driver Assistance Systems (ADAS), as a step towards semi- or fully autonomous vehicles.

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