David Learmount/London
Airline-accident figures for the first six months of 1997 will encourage the commercial air-transport industry, because the trends now show a gradual flight-safety improvement in absolute terms.
The fear has been that public perception of airline safety will be damaged by the numbers of accidents if these rise as the size of the industry increases. So, if the 1997 figures so far were to represent the start of a consistent downward trend, reflecting a more rapidly reducing risk, the industry could congratulate itself on being on course towards a long-sought objective.
Compared with the same period in 1996, the number of fatalities has fallen steeply, from 609 to 231, providing the drop which has gently tipped the trend for January-June figures over the past decade (see charts). The number of fatal accidents, at 20, also shows a drop, compared with the 1996 figure of 25, but neither the fatalities nor accident figures represent a decade best. Both figures, however, are better than those of the ten-year average, which are 461 and 22, respectively, and the three-year moving-average trend has also visibly tilted downward.
Six months is too short a period to be taken as a serious indicator, however, and the airlines are not attempting to deceive themselves that a major battle has been won, let alone the war. The first half of any year has not been shown to be a reliable predictor for the final six months.
Record blemished
There has been only one serious major-airline fatal accident so far this year: the China Southern Boeing 737-300 crash, in which bad weather appears to have been the main factor. The report will eventually reveal which of the three types of bad-weather accident it was: the type where the pilot made a bad decision in continuing the approach; the type where the weather information to the pilots was poor or wrong; or where there was an unpredictable, momentary, out-of-limits condition. Whichever it was, the event unfortunately spoiled a three-year accident-free period during which the concerted efforts of the Chinese authorities and industry to break a long record of poor safety- achievement had begun to show results. With luck, and China's continuing efforts to keep industry growth in line with infrastructure development and training capacity, the China Southern accident may prove to be a "sudden and momentary" departure from a consistently improving performance.
The only other fatal accident involving a large turbofan-powered airliner involved only one death, but, like the China Southern accident, it was weather-related. Weather was clearly a prime factor in seven of the 20 fatal accidents, which makes it the most common single cause in the list of events. Technical causes come higher up the list than they usually do in a world of increasingly more-reliable aircraft, with 25% of the fatal accidents involving some form of technical malfunction, usually involving an engine or propeller.
The accident responsible for the largest single number of fatalities involved a 1967-built Antonov An-24, which, it has just been confirmed by the Russian authorities, did indeed suffer from structural failure because of corrosion. The resulting crash in southern Chechnya, Russia, caused the death of all 50 people on board. The cause had been suspected soon after the accident because of the sudden loss of contact with air-traffic control, and the fact that the event occurred in the cruise. The most telling clue had been that the tail section was found a considerable distance from the rest of the wreckage, and that part of the aircraft had needed extensive maintenance when it had returned from subcontract work in the Congo.
Fuel tank safety
Many in the USA and within the US National Transportation Safety Board (NTSB) are voicing their concern that the precise cause of the mid-air explosion on the Trans World Airlines Boeing 747-100 accident on 17 July, 1996, has not been determined, and that the US Federal Aviation Administration has appeared to do nothing about those facts which are known.
The NTSB has established that an explosion took place in the almost-empty centre-wing fuel tank, and says that it believes that this explosion was the cause of the aircraft's structural break-up. In December 1996, the NTSB produced a summary of accidents and incidents involving fuel-tank explosions in Boeing aircraft, and recommended measures which airlines could take in the short term to reduce the likelihood of such an event.
Within the past month, the NTSB has written to the FAA criticising it for failing to make any of these procedures compulsory, because some involve modifying operational procedures rather than a need for design change. These include, for example, ensuring that fuel loaded is cold, and limiting the period, especially on hot days, between passenger loading and aircraft take-off so that the air-conditioning unit under the centre-wing tank does not have time to heat the tank contents too much.
The FAA, however, has confirmed that it is not ready to make decisions on what action to take. Meanwhile, the deadline for industry comment on action on fuel-tank safety is 1 August, and the NTSB has just begun flying an instrumented 747-100 in a sponsored test programme to determine what conditions increase the risk of fuel-tank explosions. Tests carried out by Boeing within a few months of the event had established that, in the TWA aircraft, fuel vapour would have been heated to the lower end of the temperature band in which, given even a weak ignition source, ignition risk is high. The FAA's defence is that, since carrying fuel involves inherent risks, its policy for the time being is to ensure that there are no ignition sources.
Another "explosive" event involving 747s took place during the first six months of 1997, but it involved good news. Since the December 1988 destruction of a Pan American World Airways 747 over Lockerbie, Scotland, by a terrorist bomb in flight, the aerospace community has been researching ways of "hardening" aircraft against critical damage from such events. Tests in the intervening years have involved the use of energy-attenuating baggage containers, cargo-hold liners, and the feasibility of designing aircraft structures in such a way that airframe damage momentarily inflicted does not propagate, leading to structural break-up.
Evidence of success in developments sponsored by the FAA and the UK Civil Aviation Authority was provided by controlled explosions on a fully pressurised 747-100 hull at Bruntingthorpe aerodrome, UK, in May. Four simultaneous explosions took place, one unprotected (as at Lockerbie), the others with different levels of protection. The scientific verdict was that, almost certainly, the hardened container or the protective hold-lining, had they been used at Lockerbie, would have prevented the tragedy. The analysis of the tests is expected to be complete before the end of 1997.
After that, says the CAA, the findings will be examined for the feasibility of making some form of protection compulsory, and will be made available to industry for product development and the aircraft manufacturers for possible future airframe-structure design changes.
Controlled-flight-into-terrain (CFIT) accidents are virtually absent so far this year, which is unusual. The Tar Heel Aviation cargo-flight accident on 2 January may be shown to be in the CFIT category, but that is arguable. An historical perspective may eventually prove that this absence of CFIT events can be attributed to consistent programmes to raise airline CFIT awareness devised and executed since 1990 by the Flight Safety Foundation and the International Civil Aviation Organisation.
No "new" accidents
The types of accident so far this year do not appear to have shown any trends or to have provided the industry with any specific alerts, except for the usual admonition that there is no such thing as a new type of accident. In the success of the aircraft-hardening experiments, however, and the continuing debate as to how fuel tanks may be made safer, the evidence is there that the industry continues to work to make aircraft more likely to survive when things go wrong.
Source: Flight International
