Everyone knows that the airlines are struggling financially. So does this affect your safety as an airline passenger? The answer is no, but it may affect your traveling convenience. My colleague at The Ohio State University and former head of materials at GE Aircraft Engines, Jim Williams, offers some elaboration on this point.

The price of jet fuel has more than doubled in the past two years and competition is restricting ticket prices. This leaves limited resources for new equipment. Are the aircraft unsafe? Categorically, the answer is ‘no’. Are the aircraft less reliable? The answer is probably ‘yes’.

As your car ages, it becomes less reliable, but not significantly less safe. Sure, failure on a busy highway can create some risk, but this is the exception, not the rule. For aircraft, the partnership between the US Federal Aviation Administration (FAA), equipment manufacturers, and airlines has effectively eliminated failure of aircraft (or their engines) during flight. However, aircraft and engines do develop ‘problems’ that render them temporarily unserviceable. Then, the aircraft is removed from service without endangering passengers, and some delay occurs while it is replaced. This may affect up to 30 flights over the next 36 hours, involving schedule changes and compensating adjustments.

While airlines are working to sustain the reliability of their aging fleet, regulatory pressures are mounting regarding engine emissions and, especially, noise. Also, higher fuel prices encourage acquisition of newer, more efficient equipment with cleaner, quieter engines. In this Catch-22 situation, both aircraft and engines have hard life limits established during FAA certification that are solely intended to prevent catastrophic failure. However, these limits can be superseded by the changing noise and emission regulations, making an aircraft prematurely obsolete for environmental compliance. Nevertheless, older, noisier, less fuel-efficient aircraft are still very serviceable and capable of generating revenue. So, they become ‘bargains’ for airlines where noise restrictions lag those in the USA, Europe, and parts of Asia. Hence, without comparable noise and emission standards, many Boeing 727s, 737-200s, and Douglas DC-9s will continue to operate on intracontinental routes in Africa and South America safely and

usefully for many years. Furthermore, many airlines are either state owned or subsidized, making rapid changes in regulatory constraints even less likely. Even hard life limits can be extended significantly by recertification following extensive disassembly, inspection, and replacement of cracked or corroded parts. The US Navy, and especially the Air Force, have instigated an effective program for life extension engineering, which merges empirical knowledge derived from fleet inspections with better stress analysis capability and the availability of improved materials. As an example, the B-52 entered service in the late 1950s with an expected life span of 20 years. Yet, 50 years later, it still provides a long-range strike capability.

In many contexts, information obtained by metallurgists during disassembly of an aircraft with 50 000 operating hours can reveal more about the operating environment than any stress analyses. For example, quantitative fractography, coupled with material property data, can provide an accurate understanding of the local operating service stresses in a component. Detailed examination of rotors from engines retired after their hard life limit (15-25 000 flights) is equally informative. Often, these parts show no cracks or other anomalies, suggesting that the hard life limits were too conservative. This costs operators a lot of money, which shows up in increased ticket prices.

A large US-funded study called ‘prognosis’ (which means, literally, ‘estimation of remaining useful life’) is underway to improve the precision with which lifespans of aircraft and engine components are calculated. The outcome could save hundreds of millions of dollars over the next decade. But this requires much more sophisticated modeling of damage incurred during service. The emergence of vastly improved computational tools and experimental methods promises to enable this. Transfer to commercial fleets will enhance confidence that unexpected failures will not occur. Later, longer life limits will provide direct societal benefit. The challenge to the airline is to improve the reliability of older equipment and enhance operating efficiency, while minimizing customer inconvenience. Here, safety is fundamentally not an issue, and those attempting to create concern among the traveling public are either misguided or disingenuous.

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DOI: 10.1016/S1369-7021(05)71105-6