CSI Metallurgy
So! There were chip detectors fitted and working on that day, 1st April 2009, in the gearboxes of the Super Puma G-REDL. How was it then that they had given virtually no warning that the main gearbox was a time bomb that was about to explode at any minute?
Mike M, in the business, told the Lad that a helicopter was mostly a flying collection of gearboxes. The passenger cabin is not much more than an appendage bolted to the engines and the mechanisms.
For most engineering, structural failures, the earliest symptom is a crack. It, mostly, begins to open at the surface of a component. It extends, speedily or slowly, into the interior of the piece, some pieces [chips] breaking away as it does so. Final failure occurs when the crack makes the component unable to carry out its function. has not got enough sound material in its cross-section that is not cracked to cohere under tensile stresses. There is another type of failure that is particularly relevant to a helicopter. Underneath a rapidly rotating ball or roller in a bearing track [race], pieces of the track can begin to crumble away. This does not often proceed to fracture of a race as it is due to compressive stresses. These tend to keep the part pushed together rather than as tensile stresses tend to pull the component apart
The AAIB report tells us, in summary.
One chip had been found on one of the detectors some days before. The mechanics consulted the manual as to what should be done. They thought that it was just a piece of scale. They spoke to the manufacturer’s engineers by phone and email. At each end of this communication channel there misunderstandings as to what had been found, what had been done, had not been done and what should be done. The upshot was, terribly, nothing further was done because no significance was attributed to the ‘piece of scale’ In fact it was bearing steel – of dreadful import.
In the middle of the last flight, one of the eight planet gears abruptly shattered into massive fragments.
One fragment was over-run by the next planet gear. If it had been the size of a matchstick, a piece of metal would, at the speed of this gearbox, have been crushed, passed through and flung out. There would have been some damage to the gear teeth but no more. This fragment however was of an awful, irresistible size. In the subsequent examination of the wreckage it was never found but everything else was. The size of the gap of its absence tells us in itself a grim story. It would have been a hellish boulder the size of a man’s fist: quite uncrushable. It burst through the strong metal shell of the gearbox outer case, splitting it like an over-ripe tomato with a shattering explosion.
The crew saw the emergency, on-screen warning of the catastrophic fall in oil pressure to almost zero. Two seconds later the craft ceased to respond to any flight controls. The grip of the gearbox on the main rotor was broken. The helicopter rolled and yawed and pitched as though it were a paper windmill in the hand of a child. Twenty seconds later the rotor lifted – free – away from the fuselage; it tilted and then hacked and severed the helicopter tail and its control rotor. The rotor then spiralled away into the air; freed from the burden of the fuselage. What was now left, the cabin and engines together, streamlined like an egg: plunged uncontrollably towards the sea far below, carrying the desolate passengers and crew.
As there had been only one chip found in the oil system, how did the failure start and progress without many more? Call in the metallurgists.
The metallurgists are the brothers in arms of the engineers. Tony T told The Lad that he had the best job in the world, as a failure metallurgist in an engineering organisation with an international reach. He travelled widely to study forensically a variety of failures. Luckily, none of his problems involved loss of life but even if they had, perhaps he would have helped with an understanding as a kind of memorial. If the study of metals appeals to you, go for metallurgy.
Metals are complex structures when you look at them with a microscope. Most people imagine, if asked, that even at that level metal is smooth like a bar of chocolate is smooth. Far from it. Solid metals are crystalline. In a crystal, the overwhelming numbers of atoms are lined up neatly in rows, sheets or repeating patterns. Tiny examples of crystals of this sort are sugar and salt. However a piece of metal is not one single crystal. It is almost always made up of millions of tiny crystals jammed together and each adhering to its neighbours.
Why is this? When the metal is molten, every atom is rushing madly about in all directions. As it cools, pairs of atoms begin to adhere to each other in millions of places, called nuclei. Then other atoms arrive and stick to the first two but only in a neat, repeating array. Imagine 4 footballs: three touching each other to form a triangular pattern and the fourth sitting on top and in the middle of the triangle below.. Such an arrangement could extend as far as you like if you have enough footballs. As the atomic pattern grows, forming a crystal, it continues until it hits another nearby. This is happening in all the nuclei but in a direction depending on the orientation of the first pair of atoms. Each crystal array is at a different angle to that of its neighbour. Thus as the metal cools, each of the millions of atoms in a single crystal is rushing to slip into its place in the pattern. Until, thud, – thud, – thud – and every crystal is jammed up against its neighbours and the whole mass jerks into a solid, rigid mass of thousands and millions of crystals.
In recent, two or three decades some special components such as aero-engine gas turbine blades have each been made of a single crystal. But this can be done only with enormous difficulty, technical cleverness and great expense that are not justified for any other components.
In addition to the conventional processes that are equivalent to standard police work or ‘boots on the ground’ Lots of cutting, polishing and etching with acid to show the structure under the microscope.
Then they zoomed in on the critical areas. The materials engineer has an army or techniques and instruments to study his materials??? Because of the critical importance of the investigation of this accident, she brought to bear some of the Special Forces of the army. There was conventional optical microscopy but also 3D surface optical mapping, 3D X-ray tomography, high resolution Field Emission Scanning Electron Microscopy, and (FESEM) Transmission Electron Microscopy, TEM.
The pictures below show the view, like a geologist overflying a mountain range, of the fracture surface that they had to study to tease out answers.
One of the significant, engineering features of the planet gears is that the bearing track or race and the gear teeth are all in one piece. As we have said before, most of the stresses caused by the bearing rollers are mainly compressive and tending to close any cracks; but here’s the rub, with the integral gear teeth much of the stress field produced is tensile and crack opening. The result with the two fields is that in this double function component, once a crack has crept beyond the race stresses, it enters the gear, tensile stress region opening and, perhaps accelerating. It can eventually pass through the whole gear pinion and break it into pieces.
The morphology [the shape] of the fatigue crack in the second stage planet gear, 13. suggested that it had initiated from a point at or close to the surface of a highly loaded section of the bearing outer race,
The following pictures show a 3D view of their conclusions.
Ultimately, this suggests to The Lad that the failure and any chips began, if not entirely internally, then at least, within the compressed area of the gear pinion material body. The result? Nothing – no crack – broke the surface to provide chips until the fracture was almost complete and only minutes away from catastrophe.
What can we do about this problem, this type of behaviour? Let’s look at this in the next post.
Engineers’ responsibility
Engineering is one of the three drivers in the advancement of the human race. This blog aims to give to career seekers and also to the general public a taste of how this might be so. They are not well served by the current media. It is an engineer posting: not a ‘scientist’. It describes real professional engineering as it is in the real world usually in the present and occasionally as it was in the recent past.
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