RAAF F-111 Pig - Maintenance
The ageing of the F-111 fleet in conjunction with a significant increase in planned retirement date has presented a new challenge to the RAAF, namely: maintaining the level of safety from catastrophic failure while still being able to economically operate the fleet. Such a goal requires an in-depth understanding of the structure, materials, and possible failure modes. For instance, structural optimisation aimed to increase inspection intervals for critical structure (which will reduce the maintenance burden) should only be practiced if time-dependant failure modes, such as corrosion, will not become the new life-limiting scenario for that location.
Safety by inspection was the key to protecting the F-111 fleet since the early 1970s. The ageing of the F-111 fleet in conjunction with a significant increase in planned retirement date presented a new challenge to the RAAF, namely: maintaining the level of safety from catastrophic failure while still being able to economically operate the fleet. Such a goal required an in-depth understanding of the structure, materials, and possible failure modes. For instance, structural optimisation aimed to increase inspection intervals for critical structure (which would reduce the maintenance burden) should only be practiced if time-dependant failure modes, such as corrosion, would not become the new life-limiting scenario for that location.
The F-111 was Australia's primary air strike weapon over the latter part of the 20th century and into the early part of the 21st century. The aircraft possessed a number of special and even unique capabilities. One of these was its long range capability, enabling the aircraft to operate without refuelling over very long distances. To accomplish this, the F-111 maximises the storage of fuel in a way not adopted with any other aircraft in the RAAF. It is in one sense a 'flying fuel tank' with armaments attached and a cockpit for the crew. Unlike many other aircraft, there is no fuel bladder in the F-111.
One consequence of the unique fuel storage system on the F-111 is a requirement for repair work in an environment not replicated on any other RAAF aircraft. Australia's 24 F-111 aircraft required significant fuel tank repair and maintenance work from the moment of delivery in 1973. The discovery of the deteriorating fuel tank sealant, coupled with the fact that the Australian aircraft had spent 5 years in storage, before delivery, meant that the RAAF was required to rectify major fuel leak issues on the aircraft from the outset. This repair work in cramped, hot and humid conditions, was carried out for some decades by F-111 fuel tank maintenance workers - primarily in the formal deseal/reseal programs but also through ad hoc maintenance. There were four formal deseal/reseal programs (conducted in 1977-1982, 1985-1993, 1991-1993 and 1996-1999). These programs used a range of different techniques and chemicals and involved personnel working full time on fuel tank maintenance with long periods inside the tanks.
In late 1999 the fuel tank repair section at RAAF Base Amberley became concerned at the health effects being experienced by members of the section, with some 400 personnel reporting a range of symptoms and illnesses. In 2009 a Parliamentary Inquiry examined the health and compensation issues surrounding F-111 deseal/reseal workers and their families, and other F-111 fuel tank maintenance workers, with particular reference to the unique fuselage repair work undertaken and the health risks involved. The Inquiry took evidence and histories from the workers themselves. It also reviewed scientific and health studies to date, and considered the Department of Defence and the Department of Veterans' Affairs services and support provided to affected workers - compensation payments; health care schemes; ex-gratia payments. The Inquiry report included a number of recommendations and in May 2010 Budget the Government announced the measures funded in the Budget in response to the Inquiry report.
The Sole Operator Program [SOP] was established in the late 1990s in response to the United States' 1996 decision to retire the USAF F-111 fleet. The RAAF, in partnership with the Defence Science and Technology Organisation (DSTO), and Australian industry ran the tripartite sole operator program between 1997 and 2008. The SOP was stood up the program to deal with the knowledge issues that were going to arise when required to operate the aircraft alone and to fill in the gaps that existed in taking the aircraft past where the USAF had been. The SOP was responsible for developing and maintaining the in-country capability needed for the continuing safe operation of the RAAF's F-111 fleet in light of rapidly diminishing US support. The USAF retirement of their F-111s also enabled the RAAF to acquire an expanded inventory of spares and to invest in a number of test programs to identify future maintenance issues.
Many of the fixed and removable panels on the RAAF F-111 aircraft are made up of bonded honeycomb sandwich panels. Experience with the RAAF fleet has shown that a serious problem exists with degradation and damage of these panels. A review of the literature was undertaken to gain an understanding of the extent of this problem. It was found that panels were subject to large areas of adhesive bond separation and corrosion damage. This damage was believed to be caused by the ingress of water in the panel through poor sealing at the edges or after repair of the panels. Moisture in the panel is also believed to cause adhesive degradation that may reduce the strength of the bonds in such panels.
An extensive Block Upgrade Program was completed in 2005. Over several years, 1960-1970 technology was replaced with state-of-the-art digital equipment that involved ongoing upgrades of F-111 aircraft with new systems and weapons. This significantly reduced aircraft downtimeas well as costs by combining systems upgrades with planned maintenance tasks.
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