Two titanium alloy wing attachment bolts from a commercial airline failed during the course of a routine service operation.
And there was a series of aircraft bolts fitted on a landing gear assembly that failed within a few months of each other.
This was a trigger to look closer.
Especially when these bolts are made of Titanium…and Titanium parts are expensive for a very good reason.
‘A picture is worth a 1,000 words’
As with every failure investigation an analysis of the failed parts is the starting point. This includes the use of an optical microscope, which is quite revealing in most cases.
Employing a more extended array of tools during a failure investigation may seem like a good strategy to tackle complex problems.
But then you must think…
Questions can lead to answer, with common ones being:
Is it fatigue?
What about excessive static load during service?
Any material quality issues or flaws?
The Human Factor
Coming up with the right questions is difficult though and it actually requires very well developed engineering judgment. This does not involve simply assembling all the data but most importantly connecting the dots between the failure investigation findings and probable causes.
History can be helpful in that regard…
“A number of aircraft have been operated over a period of three years of service following a major maintenance and system upgrade visit at an MRO facility.”
“Three out of these aircraft are now experiencing a fracture bolt fitted on the landing gear assembly.”
Looking At Things From A Different Perspective
When you bring this all back to the microscopes with more clues in mind, things will make more sense to the engineer.
The fracture surface does shows of a ductile fracture mechanism.
What remains is a confirmation with a tool that can go ‘deeper’, a Scanning Electron Microscope (SEM) illuminating the dimpled surface, characteristic of ductile fracture.
In the absence of other findings, such as cracks or defects, creep is something one needs to consider, bearing in mind that there are “three failed bolts at pretty much the same time, some years after a major maintenance”.
A Shifting Mindset
There is a common misconception among engineers that creep goes hand-in-hand with high temperature; definitely higher than the usual operating temperatures that an aircraft experiences during normal service.
For this reason, creep would be excluded straightaway from the list of probable causes or not even included in the list. Most engineers would (and do) think that way, which in turn would cause unnecessary further examination and a longer bill for the aircraft operator or maintenance organization.
What about over-torque?
Overstress can deteriorate the strength of any material but in most cases the failure comes rather quickly…not after three years. However, over-torque, at the time of a bolt installation, to a certain extent and over a prolonged time can actually cause creep, even at room temperature.
Practical failure analysis reports and research not only suggest, but have actually confirmed that this can happen for Titanium bolts.
The information is out there and engineers should find the time to dig for answers.
After all no problem is brand new!
Good engineering practice can actually save money and time.
A warranty claim to the maintenance organization can have very good chances when data and well-rounded justification is presented.
Titanium bolts are expensive, especially when the same issue has affected a large portion of your fleet and it’s likely to affect the rest of your fleet …if the same organization and maintainers followed the same (over-) torquing process.
Looking closer and through a smarter lens will save you money, time and will definitely contribute to safer aircraft operations in the long run.
Have you had similar problems with titanium bolts? Tell us your story in the comments below.
Some useful further reading on titanium bolts failure case studies / research papers:
- Stevenson, M.E., McDougall, J.L., Cline, K.G., 2003. Metallurgical failure analysis of titanium wing attachment bolts, Practical Failure Analysis, 3(4), 75-80.
- Jha, A., Sign, S.K., Kiranmayee, M.S., Sreekumar, K., Sinha, P.P., 2010. Failure analysis of titanium alloy (Ti6Al4V) fastener used in aerospace application, Engineering Failure Analysis, 17, 1457-1465.
- Croccolo, D., Massimiliano D.A., Vincenzi, N., 2012. Influence of tightening procedures and lubrication conditions on titanium screw joints for lightweight applications, Tribology International, 55, 68-76.