This is because commercial-aviation accidents are “very rare events”, according to Airbus. They are so rare, nowadays, that trying to compare one year’s aviation safety performance to that of another creates statistics which “are governed by ‘the law of small numbers’” and “most of the time … prove to be rather counter-intuitive”.
Airbus notes that “variations [in fatality and hull-loss rates] from one year to the next can be huge”, arguing that “the definition of safety is more subtle than a count of real accidents over a year”.
ICAO officially defines aviation safety as “the state in which the possibility of harm to persons or of property damage is reduced to, and maintained at or below, an acceptable level through a continuing process of hazard identification and safety risk management”. In other words, states Airbus, the official definition of air safety refers “to likelihood more than to real events”.
However, looking at accident data and causation over “a meaningful number of flights, reasonably at least a million flights per year” is useful, because it creates a large-enough statistical sample that useful inferences may be drawn from the data.
Because accident rates can vary widely from year to year – for instance, there were four accidents involving Western-built jets in 2004 but nine in 2005 – Airbus uses a 10-year moving average to describe the accident rate for any given year within that decade.
The company uses a 20-year timeframe in analysing the safety performance of each succeeding generation of commercial jets. Manufacturers are now producing the fourth generation, fly-by-wire aircraft with flight envelope protection. Airbus employs the 20-year window because “today’s operational conditions bear little resemblance to those at the beginning of the jet age”.
By aggregating data in these ways, Airbus says multi-year trends become evident. These trends “are less sensitive to yearly random variations” and so “they contribute to providing insights on the evolution of … air transport system safety”.
Parsed in these ways, the data show there continues “a steady decrease over time” in the accident rate; and “a virtually stable absolute number of accidents despite a massive increase in exposure” – i.e., the number of flights performed annually grows substantially from each year to the next.
Meanwhile, there have been “significant changes in both the number and the nature of aircraft”: aircraft are becoming technologically more sophisticated. At the same time the governance of airlines and aviation regulatory authorities continues to improve qualitatively.
“Advances in technology bring a decrease in accident rates,” Airbus finds: each succeeding generation of aircraft is safer than its predecessors. There is a large and growing improvement in safety from the second generation to the third, as remaining second-generation aircraft reach ages where they need frequent maintenance activity – which isn’t always performed.
On the basis of a 10-year moving-average, the latest generation of jets has half the accident rate of the previous generation.
Over the past 20 years, the rate of fatal accidents per million flights has fallen by a factor of five and the rate of hull-loss accidents has declined by a factor of three. Within those rates, “Nearly 90 per cent of all accidents happened during the descent/approach/landing or take-off/climb phases,” reports Airbus.
Loss of control in-flight (LOCI), controlled flight into terrain (CFIT) and runway excursions were responsible for more than 60 per cent of all fatal accidents over the past 20 years. Runway excursions and aircraft system/component failure or malfunction were responsible for some 44 per cent of hull losses.
But while CFIT accidents have fallen by a factor of seven over the past 20 years as new avionics have given pilots better situational awareness, and the rate of LOCI accidents has halved thanks to the advent of flight envelope protection, the rate of runway excursions has declined little. “The effect of recent technological breakthrough is not measurable … yet,” concludes Airbus.