Scrapping VC10s was never going to be a popular topic of discussion amongst enthusiasts, even though this practice also served to keep the type in the air. Let's have a look at the projects that were set up to learn more about the ageing process of an airliner.
What is the problem with ageing aircraft?
As aircraft age, the moving bits will show the effects of wear over time, but the structural bits wear out in their own way, showing the effect of having been subjected to pressurisation cycles and landings through small fatigue cracks throughout the structure. The de Havilland Comet crashes had shown the industry what this could do to an airliner, but the Aloha Airlines flight 243 accident really reinforced this message, showing what an ageing aircraft had to cope with over time. After that accident in 1988 the term ‘Widespread Fatigue Damage’ or WFD came into vogue to describe the effects of a plethora of small cracks that could gang up over time, causing significant damage even though the structure should still be safe to fly based on conventional analysis methods.
The VC10's structure was designed in the late 50s to 'fail safe' principles, meaning that in a lot of places there was enough metal to accommodate a certain amount of fatigue cracking. From a design point of view, every metal structure is expected to develop some fatigue cracks but you can either make sure that the structure is sound enough to stay intact even if some elements develop cracks, or you can make sure that the structure is limited in the 'life' it may accumulate by testing for this life and ensuring that no unsafe cracks develop during this life.
Looking at Romeo
Super VC10 G-ASGR, or ‘Romeo’, was the last of the BOAC Super VC10s to enter service in May 1969. It flew with the airline for twelve years until its last ferry flight to Abingdon on 27 May 1981. After more than ten years in storage, the airframe was dismantled in 1992 and moved to Filton to serve as a spares source. Parts of the airframe were used in 1994/95 to figure out if corrosion was present in areas that were normally inaccessible. Only a limited number of places was investigated and fortunately few significant faults were found, which served to support the conversion programme that was busy refurbishing five other ex-airline Super VC10s into K4 tankers.
The first large teardown project
The main goal of this project was to validate a new Structural Examination Programme (SEP) that was based on a new damage tolerance assessment. The original VC10 design was aimed at the civil airline market and the structure had been designed to accommodate several years of commercial airline use. In the 1990s, the structure had to be reassessed as the typical RAF use was decidedly different compared to an airline career. In the military, the VC10 was used for both transport and air-to-air refuelling, which meant more landings, different flight cycles with more time spent at medium and lower altitudes and for some aircraft, this came on top of the accummulated stresses from its airline career.
On the RAF VC10, some significant fatigue cracking that had not been anticipated based on the original civil qualification was discovered during regular inspections. This prompted the development of the new SEP, but had this been implemented as originally conceived it would have meant a significant increase in the time and manpower needed for these inspections. Any metal structure can accommodate a bit of cracking, as there will always be a safety factor built in, but the challenge for the designer is to find the point at which there is enough metal to cope with these initial cracks without endangering the aircraft before they can be detected and rectified. In the design stage, you can always add more metal to avoid cracking, but this would create an aircraft that is too heavy, and by leaving more metal out the cracks would either be too small to be detected before becoming a problem, or you would have to inspect the structure between each and every flight. There is a middle ground somewhere in between, but this will always be based on a projected use for the type and this had changed for the VC10.
Because of this the decision was taken to use a teardown project to get a good look at the structure of the VC10, so that the SEP could be adjusted to a level that would not increase the maintenance burden too much, while still ensuring continued safe operation of the type in military service. Through the use of a teardown programme, the normally hidden joints in the structure can be microscopically examined so that the level of damage present can be ascertained. This can then be used as a basis for assessing the other airframes and adjusting the intervals in the SEP to a practical and safe level.
Both QinetiQ and BAE Systems were involved in the programme, bundling their expertise and knowledge. Between them they targeted which parts of the available airframes would need to be examined and this lead to specific tasks that could be carried out by the team responsible for collecting the samples. One of these areas concerned the main wing joints where the wings were attached to the centre section (Rib 0), and where the outboard wing section attached to the inboard section (Rib 22). These joints had been designed to be ‘fail-safe’, meaning that they could accommodate the failure of one part of the joint without endangering the overall integrity, which had been accomplished by using a butt joint covered by doubler plates on both sides, connected with two staggered rows of bolts on each side. With such a joint the important bits of metal are hidden from sight behind the doubler plates, and therefore the state of the joint can only be completely assessed by taking it apart. This was done to the joints from three VC10 K2s and one VC10 K4 (ZD235) and over 1500 bolt holes were examined in detail. The good news was that there was no sign of widespread fatigue damage. There were some issues though, as the sealant between the joint faces and the associated protective treatments had broken down, causing moisture and fuel to seep into the joints. This could potentially lead to corrosion issues in the joints. Another point of interest was the fin to fuselage attachment, which was studied using the metal from XV103. There was evidence of corrosion around the bolts that attached the fin, but the teardown showed that the bolts could be replaced on an individual basis. By reaming the holes and fitting a slightly larger new bolt, the corrosion inside the bolt holes could be removed and this was a significantly easier procedure than removing the entire fin.
Another item of concern was that several wing spar parts showed signs of damage in multiple locations. Small cracks were visible, using the appropriate techniques, extending from rivet and bolt holes. One important issue with WFD is that these cracks are liable to create a zipper effect, where they combine into one failure extending over several fastener holes. For this, the cracks should all run in the same direction towards each other, but in this case the cracks were seen to run parallel to each other, so that the ends would never meet up. Because of this, the VC10's wing structure was deemed to be safe from WFD. Also, this area was already seen as needing additional inspections and repairs/strengthening, so the results did not lead to any unexpected extra work.
The single K4 available at that point, ZD235, became an important tool in assessing all the repairs that had been carried out over the years in the Repair Assessment Programme (RAP). By taking apart the various bits of extra metal and strengthening, the condition of the underlying structure could be assessed and this was used to avoid having to take apart similar repairs on the flying K4s and other VC10s. It also played an important part in validating the new fixes that were developed as a result of the teardown inspections. All the new inspections and/or repairs were trialled on this airframe to make sure that they could be carried out and yield the desired result. A particularly challenging task was removing all the sealant that had built up inside the fuel tanks. A specialised company was contracted to do this, as it involved workers inside the cramped wing structure using high pressure water jets to strip the sealant from the joints and interfaces within the structure. Once this had been carried out, the underlying structure could be examined in detail without taking it apart. All the trial inspections were used to validate the new SEP.
The second phase of the teardown programme
From 2008 on, the second phase of the teardown project got started after ZD242 had been delivered to Boscombe Down. The first project had encountered some logistical challenges with the subject airframes being at St. Athan, the QinetiQ team 150 miles away, and the RAF Aircraft Recovery & Transportation flight having to fit the work into the rest of their schedule. Also, there were no hangars available so all the work had to be carried out outside. Because of this, the required samples were taken from the partially dismantled airframes and transported to storage, with an extensive trail of documentation to ensure traceability. By having ZD242 delivered to Boscombe Down, the aircraft, the personnel and the laboratory facilities would be closer together, which should lead to a more easily managed project.
While the first phase was, at least partially, focused on customising and streamlining the maintenance program for the remaining years of the VC10’s RAF career, the second phase continued on from the other aspects that were related to ageing aircraft. ZD242 was considered to be an excellent example of a high-time airframe, having served as an airliner for thirteen years, clocking up over 43000 flying hours, and accumulating an additional 7000 hours as a RAF tanker over the fourteen years of its military service. Initially, ZD242 was kept live, with regular engine runs carried out to keep the systems in good condition so that the system condition could be assessed as well. With ageing airframes the major concerns are structural, but the opportunity was taken to look at system deterioration as well.
Once this was done, the phase A teardown collected samples from the fuselage window apertures, the pressure floor over the main undercarriage bay, the wing-to-fuselage main frames, the rear-fuselage main frames, the rear passenger door aperture and the fin spars. This was still a limited teardown as the majority of the airframe stayed intact, but it did use intrusive tactics whereby the items of interest were cut out, enabling both the structural items and the system condition to be eveluated under laboratory conditions if necessary.
During the phase B teardown, the work was carried out in concert with GJD Services, the company responsible for scrapping the airframe, so that the airframe could be completely dismantled and the non-essential items scrapped as needed. The main area of focus during phase B was the center section torque box, which is the wing structure that continues through the fuselage to connect both wings together and form one long box-like structure from tip to tip. Also, the wing skins and wing spars were sampled in several locations, as the previous work on the K2s had shown problems in this area with multi-site damage.
One of the challenges for GJD Services was getting to the important centre section torque box, and also the keelbeam around and below it. The solution was to work from the top to the bottom, slowly cutting away the fuselage from the top down until the areas of interest were revealed. By removing the rear fuselage and later on the front fuselage, only the centre section remained. The good news was that there was no sign of any susceptibility to WFD, and there were no new surprises compared to the previous teardowns. Overall, the VISSAGE project, for VC10 Integrated Systems & Structures Ageing Exploration, had a successful outcome in that it confirmed the structural integrity of the VC10 design, even after more than 50,000 hours and 40 years of flying.