Tuesday, 11 December 2012

Fear of Flying and Safety of Gruyere Cheese

Tomorrow I’ll fly to San Antonio, Texas. How big a risk will that be?

Flying is the safest means of transport on a mile per mile basis. My chance of dying is a reassuringly low one in 8 million, perhaps even less than that. Travelling by Underground and by bus to the airport will be very safe, travelling by car to the hotel less so, yet still tolerable.

Why then do I feel any alarm about flying, joining the third of travellers who admit to some concerns? Several reasons have been advanced: the relative rarity of flying as an experience such that fears of the unfamiliar do not habituate, the lack of visible support, and claustrophobic confinement in a fuselage. However, much of the anxiety is engendered by turbulence, which triggers a startle response. It raises the prospect of falling out of the sky, while the same sort of disturbance in a car journey is of no emotional consequence.

At this point commentators often branch into an examination of human irrationality. Fear of flying makes people drive cars, which are more dangerous. On the contrary, I would argue that statistics have to be understood better before coming to a judgement.

Minute by minute, I will be at the same level of risk I submit myself to when driving. Driving is low risk, but tolerable, though with some scary moments. I will be in control, or so I imagine, and I imagine myself to be a safe driver. Sadly, most drivers over-estimate their own skills, and tend to discount being driven into by drivers of even lower skills. Nonetheless, many car crashes are survivable, and one may hope to have light injuries, and no more.

Flying is low risk, tolerable, but with some very scary moments. The accident profile tends to be all or nothing. Sometimes an entire plane is lost with everyone on board. That happens infrequently, but it is widely reported when it happens, and thus is easily called to mind. As a final insult to my ego, I will be provided with a seat, but not with my own set of controls. I cannot influence the outcome, so I am helpless. Apart from worrying about the rivets in the wings, I will also be worrying about pilot error.

Pilots are rigorously selected, highly trained, and regularly monitored. Despite that, they make mistakes. Many of those errors they confess anonymously on specialist websites, for the education of other pilots. This is a good system, which ought to be offered to politicians.

Sometime the error is so severe that the pilots die, and their mistakes have to be determined from the famous Black Box, which is in fact a bright orange ball or cylinder full of recording equipment.

In Human Error (1990) and The Human Contribution (2008) James Reason has looked at the psychological foundations of errors. He argued that we defend against accidents by creating defensive walls of Gruyere cheese. We accept that there are holes in the cheese, but believe that once we get to 2 or 3 defensive layers very few fatal errors should get through.

But consider the loss of Air France 447 which went down with all 228 souls on a flight from Rio to Paris in June 2009.  When the black box was finally recovered (using Bayesian statistics on flight paths, local conditions and past search data) the story could be put together. Far out over the Atlantic the plane flew into the normal equatorial thunderstorms. At this point the most senior pilot decided to take his rest break. This may have been very French, but it is slightly puzzling. The two less experienced but still very well trained pilots were left to fly the plane. The electric discharge known as St Elmo’s fire caused a blue glow in the cockpit, scaring one of the pilots. Unknown to them the air speed indicators then iced up, giving the autopilot such conflicting readings that it disconnected, passing control over to the pilots. All that was required at that stage was for the pilots to fly the plane till the weather calmed down. What in fact happened is that the junior pilot pulled back the nose (made the plane point upwards) and increased engine speed. This put the plane into a stall. In order to fly through the air a plane must have a well-judged angle of attack so as to create the required lift. Pointed too far downwards it dives, too far upwards it stalls. The stall warning alarms went off. They sent for the senior pilot.

At this point we ought to recall the snappy definition of intelligence: what you need when you don’t know what to do. The pilots were intelligent, but they were faced with a bewildering IQ problem with a severe time limit. The plane was falling out of the sky. The stall was so severe that the alarm stopped sounding because the inputs were so extreme as to seem invalid. That meant that later when they tried to level the plane by putting the nose down, which would have saved them from stalling, they moved from a Very Severe Stall (in which the alarm was switched off) to Severe Stall (which made the alarm switch on again). Paradoxically, when they tried to correct their angle of attack the warning system appeared to tell them off.  If they had persisted through the alarm zone the alarm would have finally turned off when they were back to level flight.

The senior pilot arrived back and tried to make sense of what was happening. They run through all the remaining indicators, and the juniors didn’t have time to explain the whole sequence of events in detail. Another piece of the jigsaw that has to be explained in this highly simplified account is that their plane had a side stick which works as a ratchet, not a control yoke joystick. As a consequence, the senior pilot could not see at a glance that his junior had put the nose up so far that they were in a major stall. Baffled, they looked at the instruments and did not realise their absurd angle of attack until the junior pilot said he had been pulling back on the side-stick all the time. They realised they were going to crash shortly before the impact, when the record ends.

In summary, this showed the limits of human intelligence, the capacity to rapidly understand an unusual, fast changing situation and make sense of it quickly. Experience is of little value unless it is active experience. Flying on autopilot is mostly an experience of boredom, interspersed by routine system management chores. The fearful, almost panic stricken reaction of the pilot pulling back on the joystick generated a fatal stall. The use of a side-stick which does not show its relative position, rather than a traditional and familiar joystick where you can see at a glance what the position is, may have been a contributing factor. However, the precipitating factor was a lack of reliable airspeed indicators, and those have been replaced with better instruments. Another little hole in the Gruyere cheese has been blocked up.

So, if you have read this far the story will be easily accessible to you, and you will realise why, frequency statistics notwithstanding, it is understandable that passengers should fear a machine whose mechanics they do not understand, supported by aerodynamic lifting forces they cannot see, guided by invisible pilots whose announcements are inaudible, and who are the best of humans, but humans nonetheless.


  1. I'm a corporate jet pilot. Pilots read about the mistakes others make and they like to say, "How stupid, I would never do that." But the truth is anybody can make a mistake. First world airlines are fairly safe mainly in my opinion because pilots and mechanics have unions and can withstand management pressure to do unsafe things.

  2. Thanks for your comment. Anybody can make a mistake, and all of us do. What counts is whether there are defenses in depth, and that people are well enough trained to use them. Safe flying!

  3. I didn't know that wings still have rivets. You live and learn.

  4. There are 6 million parts in the 747. Three million parts are fasteners, and about half of those are rivets.

  5. This comment is perhaps somewhat belated, but I could not let this pass. There are precious few holes in Gruyère cheese. Perhaps you were thinking of Emmentaler?