PETER LA FRANCHI / CANBERRA
Within the Royal Australian Air Force, economic service life issues are a major focus, particularly for the General Dynamics F-111, Boeing F/A-18 Hornet and Lockheed AP-3C Orion.
RAAF F-111 maintenance and operating costs average A$1 billion ($644 million) a year, a major drain on limited Department of Defence funds.
F-111 fleet flying hours have been dropping in the past five years, and are nearing half the total targeted in 1997. In October 2002, the Australian DoD reported a shortfall of 1,041h against the targeted 3,600h after the fleet was grounded due to wing-cracking problems and an in-flight explosion in a fuel tank. The report also stated: "With the exception of the [BAE Systems] Hawk and the [Lockheed Martin] C-130J, all air force aircraft have exhibited ageing factors that have resulted in increased inspections, repair, and fatigue and corrosion management. Accordingly, support costs have risen and will continue to increase year by year."
RAAF planning calls for the F-111 to remain in service until at least 2015-20, while the F/A-18 will continue until 2012-15. Revised studies of the effective service life of both types were ordered by the government last year, and the findings are due late next year or early 2005. But if the current F-111 availability trend continues, the type will face a major crisis as early as 2005-7. This has led to speculation about an interim strike aircraft requirement - a move Canberra is seeking to downplay.
In Australia, the Defence Science and Technology Organisation (DSTO) plays a major role supporting the RAAF fleet, conducting structural integrity research and monitoring specific platform corrosion issues. The corrosion research programme is multi-faceted, combining data surveys, information exchange, re-evaluation of the RAAF's corrosion management programmes and a variety of applied and strategic level research.
In parallel, an active co-operation programme has been established with the USAF Research Laboratories. This includes joint fleet corrosion and structural integrity studies, and the devel- opment of an equivalent initial flaw model of exfoliation and pitting. The model is being applied to develop more effective aircraft durability and damage tolerance analysis models for use on various platforms.
In 1997, the DSTO issued a report that warned that although most advanced aircraft life management philosophies were based on fracture mechanics, "no analogous analytical framework exists for the management of corrosion defects from the viewpoint of aircraft structural integrity". But despite experimental and theoretical international research, it said no such analytical framework was likely to be achievable given "the dependence of the corrosion mechanism upon many factors, including the material, environment and loading; the dependence of the rate of corrosion upon many factors; and the environment an aircraft experiences depending on climatic factors and its operational role".
The DSTO concluded that the most effective approach would be multidisciplinary, and ideally would expand data-sharing and other co-operation to the OEMs and other air forces.
In the F-111's case, where Australia is the sole operator, the report argued that research was needed into assumptions on corrosion impact on airframes and identifying options for "allowing continued operation of aircraft with identified corrosion defects".
A 2000-1 DSTO study of F-111 structural materials assessed what data had emerged in the aircraft's life that could inform safety and economic decisions. The report highlighted the difference in the state of materials knowledge at the point of aircraft construction and the hurdles engineers and technicians were likely to face in maintaining a type for decades. The study found that a significant body of data on the two most common materials in the F-111 airframe - D6ac steel and 2024-T851 aluminium - existed before 1978 but had not been brought together in a coherent form, compared and analysed.
In some cases "data was so sparse that it was essentially limited to the same data sources" used by General Dynamics to perform initial durability and damage tolerance analysis. D6ac steel has proved to be susceptible to corrosion pitting, with the subsequent onset of fatigue cracking. There are similar problems with the 2024-T851 aluminium used for wing skins.
Source: Flight International
