"The old lady needs a hip replacement", a phrase uttered by Ted Petty, long time team leader of the volunteers that look after the Sultan of Oman's VC10 in The Brooklands Museum, during the team's annual Christmas Dinner that was held (a bit late perhaps) in January 2000. Unfortunately Ted never got to see the 'old lady' with her 'new hip', as he passed away on 1st February 2002. The problem did get fixed though, by a team of volunteers from DARA/St. Athan, the RAF maintenance base that carries out all sorts of major repairs on VC10s and many other types. This is the story of a VC10 with a 'damaged hip'.
Standard VC10 A4O-AB has been in the Brooklands Museum collection since August 1987. On the 6th of that month she made her final touchdown on the Brooklands runway that she once made her first flight from, was pushed back towards the end of the runway and parked there. Ever since that day she has been lovingly cared for, principally by a team of volunteers led by Ted Petty since the day she arrived at the museum. But unknown to everybody a small problem lurked in her that was meant to become a big problem eventually! That day came in September of 1999, thirteen years on from her final flight. Sometime during that month a large bang must have scared off many a bird that had found a quiet place to sit on the large white airliner, as the main hinge tube on the left main gear suddenly snapped, fracturing over three-quarters of its circumference. A what you say? Let me explain.
The main gear of the VC10 retracts inwards into the fuselage, and in order to do this the main gear incorporates a horizontal tube to which the main strut and support are attached. This horizontal tube is supported in bearings at both ends allowing it to rotate and thus to retract the gear.
Apart from enabling gear retraction, this tube needs to be pretty sturdy as all the forces on the aircraft's undercarriage are carried through this tube. This means everything from just standing still to a pretty firm landing of a heavy aircraft in crosswind conditions. To be able to do all this, the actual part is a piece of steel weighing over 350 lbs! Not something you would want dropped on your head, but also not something that should snap just like that. Because it did. Just look at the photos below.
The initial reaction of everyone who saw the problem after it had been spotted on 30th September 1999 was disbelief. How could such a massive piece of steel have snapped so suddenly? The second thought was: what next. Would the whole gear collapse? The aircraft still looked the same from the outside, you had to get close to the left main gear and look up into the gear bay to spot the problem. The left wing was probably a couple of inches closer to the ground than it should be, but not noticably so. To prevent further disaster a jack was quickly brought to the scene and erected beneath the left wing. This did not enable everyone to sleep soundly though, as the only jack available in the museum that would fit was still not strong enough to hold the entire weight of the aircraft should the gear really collapse! It did however take some of the load off the weakened gear and also it steadied the aircraft, as a stiff wind could still move the entire bulk on its oleos! The question whether this would protect the aircraft from further disaster was suitably answered around Christmas time 1999 when gale force winds swept over southern England and France. Much damage was caused, but A4O-AB was still standing strong!
After the initial shock had worn off, the questions started. Unfortunately, the answers were not always easy to provide, and a quick solution seemed far off. The hinge tube needed replacement, that was clear, but how to go about it? And who was to carry out this task? A quick glance at the maintenance manuals provided a detailed account of how to fit a replacement tube, but it was a daunting task as quite a few 'special tools' would be needed (which the museum didn't have), and it involved removing the entire undercarriage assembly, which also necessitated jacking the entire aircraft (jacks that, again, the museum didn't have)! Also the manpower needed for such a task had to be assembled. As I said before, the aircraft had been lovingly looked after since its arrival at the museum, but the team of volunteers that looks after it was an unlikely choice for this job with the youngest member close to pensionable age, and a team leader who had passed eighty years of age, even though they were probably most willing to help. The obvious team to carry out the job would of course be the RAF, but even though we all pay tax, expecting them to turn up to fix an airplane for us was a bit steep. Some things did get accomplished early on though. Mr. Hugh Tyrer did a preliminary examination of the failed hinge tube in October to discern the cause of the problem, around the same time the other hinge tube in the right main gear was NDT tested to see whether that one had a similar undetected problem. Not much later a replacement part was also found. A hinge tube that was written off for use in the RAF's flying aircraft, but which could still provide years of sterling service for A4O-AB. Also the search for a set of jacks was still on. It took quite a bit of time but although not completely the right type, a pair of Tristar jacks was obtained, one of which was eventually installed under the tail of the aircraft to further steady it. And so the situation stayed for a while. Negotiations with the RAF and possible other parties were underway, the aircraft remained in one piece, nothing to worry about surely?
Well, not completely true actually. Because a degree of uncertainty remained over the stability of the gear the aircraft could not be opened up for visitors, which in the end is the purpose of it being in a museum. And for the same reason restoration work on the aircraft was also out of the question. While this does not seem like a big problem for an aircraft that's in one piece and looks to be in proper shape (apart from the jacks underneath then) it still may have caused some extra worries. The aircraft had been at Brooklands for 11 years before some large scale corrosion removal and protection was started on the outside surfaces in 1998. This was continued in 1999 with the result that the fuselage and wings were completely clean of any major corrosion issues. Although this was a major step forward it did not mean that we could sit back and relax. The engine nacelles and several other areas of the aircraft still needed major attention, and although some of these areas were not critical at that point, leaving them unattended would get them to that point eventually. Also although the wings looked clean, over time some small corrosion spots turned up again that had lain dormant during the previous years. Basically as long as an airplane is kept outside corrosion protection and removal is an ongoing battle and while the undercarriage was not repaired the corrosion had a free playing field.
To get an idea of the problems the repair crew would face, here are two images taken from the Maintenance manual for the VC10. The first shows that to get to the hinge tube, the main gear should be removed. In order to do this, hoisting gear is to be installed from the hinge tube. As the tube in this case cannot be used in such a way, another solution is needed. In the second image the 'official' way of removing the tube is seen, but with this particular tube being cracked it will not be possible to lower it in one piece, and as it weighs over 350 lbs taking it down by hand is not the perfect solution.
It took some time before definite plans could be made for the repair to the aircraft. Several companies were contacted to find the tools and expertise needed. In November 2001 the VC10 Maintenance Facility at St. Athan was contacted to see whether they could offer advice and/or help with the repair. After an initial discussion at St. Athan a party of volunteers was recruited to assist the museum with the repair. This team visited the museum in March 2002 to see the problem with their own eyes.
The magnitude of the task was immediately evident. The repair could be undertaken but a large amount of equipment had to be moved to Brooklands and the repair would have to be carried out outdoors. Also this kind of repair means the complete removal of the undercarriage assembly which is hard enough in a controlled environment but can be even trickier when you have to compete with the English weather as well. Still, everybody was confident that this could be done. A procedure was planned and replacement parts were obtained from a VC10 that was scheduled to be withdrawn from service, with help from the VC10 Integrated Project Team at RAF Wyton.
With some help from the Aircraft Recovery and Transportation Flight (also known as 'Crash and Smash') the equipment needed, amongst which the three jacks needed to get the aircraft's weight of its wheels, was moved to the Museum. And so on 14 June the volunteers set off for the long drive to Weybridge and the Brooklands Museum. The first afternoon was used to prepare the aircraft for jacking which would be done the next morning. On Saturday the weather was pleasant and the hard work could begin. With procedures being devised as the team worked, the job moved steadily on through the day and long into the evening to complete most of the repair. Another early start on Sunday, which was Father's day as well, meant that the aircraft was back on its wheels by Sunday afternoon with the new hinge tube in place. After all the ground equipment had been removed the fully repaired aircraft could officially be handed over to Julian Temple, the Curator of Aviation at the Brooklands Museum.
And so after almost 33 months of insecurity over the aircraft's future A4O-AB
was once again standing strong. Now the full potential of the aircraft as a
museum exhibit could once again be used, giving it a new lease of life.
What really happened?
The hinge tube on A4O-AB's left main gear was manufactured from high tensile stainless steel, being built up from three forgings which were then joined by argon arc welding as shown in the photo below. The tube installed in A4O-AB turned out to be a Super VC10 part, which means that it was replaced sometime during its life. Markings on the fractured hinge tube show that it was manufactured in September 1969, so obviously it cannot have been installed in A4O-AB when it made its first flight as G-ASIX on 16 October 1964.
Mr. Tyrer examined the tube in its installed position in early October 1999 and concluded initially that the crack was the result of a stress corrosion mechanism. Because of the repeated loading and unloading of the hinge tube during subsequent operations of the aircraft the tube is bent fractionally every time. Over a long period this can produce small cracks, invisible to the human eye at first, which slowly grow as the airplane continues to fly. These cracks must have been in place when the airplane arrived at the museum, after which the cracks continued to slowly grow under the load of the aircraft and after 12 years they got to a point where the beam couldn't take no more and snapped.
After the hinge tube had been replaced on 15th June 2002, the broken tube was collected by Mr. Tyrer for further metallurgical examination. By now it was in two pieces as the tube had completely fractured during the removal operation. During the subsequent examination several facts came to light.
At this point it is perhaps a good idea to say something about metal fatigue, as it is a tricky subject at best I'll try to keep things simple.
Metal fatigue: The occurance whereby a material fails after repeated applications of a stress that in itself should not be able to damage the material.
The problem first turned up on railways, axles failed before they were supposed to and engineers strove to find out what caused this. It is easy to imagine an axle being loaded by the railway carriage it is carrying, and this axle simultaneously turning as the train moves. Because of the load on the axle, it is slightly bent, but as the axle turns the side which is bent continually changes. The effect of this is that one specific region on the outside of the axle is continually loaded and unloaded and so on, and this variation in stress brings about metal fatigue. Initially nothing will show, but after a number of repetitions cracks form in the axle and if this is not spotted it will ultimately fail. A simple demonstration of the same principle can be gotten from a simple drinks can. If you push the sides flat it becomes possible to bend the can, if you go on bending the can from one side to the other long enough, the can will usually break in two.
To illustrate when metal fatigue will affect a material, a 'Wöhler curve' is used. This shows the number of repetitions on a logarithmic scale on the horizontal axis and the applied stress on the vertical axis. The curve itself then specifies the point at which the material will fail.
These curves are used when selecting a material for a specific purpose, and for many components the result is that a specific 'life' is set for that component, of a number of repetitions or operations after which the component needs inspection or replacement. Unfortunately several issues change the information in such a curve, corrosion being one. In the curve below the effect of corrosion on the curve is shown for both a low and a high repetition rate.
Other things that can affect a material's susceptibility to fatigue are imperfections and heat treatment. If we go back now to our hinge tube, it turned out that unfortunately it had both. The region where the cracking started was almost exactly where two individual parts had been welded together to create the complete tube. Also the surface of the tube seemed smooth enough just by looking at it, but when magnified it showed where machining had left very small circumferential ridges. The combination of all this, plus the corrosion and the small movements of the aircraft from the wind, people walking on and in it and undoubtedly some of the movements during the latter part of its flying career caused the minute cracks to appear, and over time they found each other and progressed to ever larger cracks.
The photo below shows the extent of the crack before it snapped completely. As this crack had been in place for some time the surface of it turned reddish from iron oxide, commonly known as rust.
In Mr. Tyrer's report he concludes that the fracture was the result of a long term stress fracture, which was caused by a long term stress corrossion mechanism. He then adds that no evidence has been found of substandard material or workmanship for either initiation or development of the cracking. As the bending stresses on the hinge tube since the aircraft has returned to Brooklands have been considerably lower than when the aircraft was in service it is reasonable to suggest that the tube could have been cracked before the aircraft arrived at Brooklands in 1987.
So what really happened? We will never really know. At some point during its career the tube began to crack, but it kept its secret until that day in September 1999. Thanks to the valiant efforts of the engineering team from St. Athan the aircraft has gotten a new lease on life, as surely with the tube as it was its career as a museum exhibit would have had to come to a drastic end at some point. Now all that remains from the incident is a notice on the left main gear reminding visitors of the dire straits the aircraft was once in.
This story could not be told without the help of The Brooklands Museum, Dave Cobbold and Jane Bisset at DARA/St. Athan and Hugh Tyrer of HT Consultants. Thanks!