During the course of a routine maintenance procedure, a commercial airline experienced the failure of two bolts made of titanium alloy that were used to attach wings.
In addition, there was a string of aircraft bolts that were used in the assembly of the landing gear that all broke within a period of a few months of one another.
This served as a motivating factor to look more closely.
Especially considering the fact that these nuts and bolts are made of titanium. . . and there is a very sound justification for the high cost of titanium components.
Investigation of the Failed Attempt
An examination of the components that were damaged in the failure is always the first step in any investigation into the cause of the failure. This includes the utilization of an optical microscope, which, in the majority of instances, is very illuminating.
During a failure investigation, utilizing a more extensive array of tools may appear to be a good strategy for addressing complex issues; however, this is not always the case.
But then, you have to consider. . .
But what about the clock? What about the price? When it comes to conducting a successful investigation, the tools are only as effective (or ineffective) as the engineer who is carrying out the task.
The following are some examples of questions that frequently result in answers:
Is it fatigue?
What about an excessive static load while the service is being performed?
Any problems or defects with the material's quality?
The Influence of People
Having the ability to formulate the appropriate inquiries is a challenging task, one that in fact calls for highly developed engineering judgment. This does not simply involve collecting all of the data; rather, the most important part is connecting the dots between the findings of the failure investigation and the possible causes.
In this regard, knowledge of history can be helpful. . .
After undergoing significant maintenance and having their systems upgraded at an MRO facility, a number of aircraft have been put back into service and have been in use for the past three years.
It has been determined that three of these aircraft are currently suffering from a fractured bolt that is fitted on the landing gear assembly.
Taking a Step Back and Looking at Things from a Different Angle
When you take everything back to the microscopes with additional information in mind, the engineer will have a better understanding of the situation.
There are indications on the fracture surface that point to a ductile fracture mechanism.
The only thing left to do is confirm the findings using a device that can go "deeper," such as a scanning electron microscope (SEM), which will shine light on the dimpled surface that is indicative of ductile fracture.
In the absence of other findings, such as cracks or defects, one must take into consideration the possibility of creep. This is especially important when taking into account the fact that there are "three failed bolts at pretty much the same time, some years after a major maintenance. "
A Change in Mental Attitude
There is a widespread misunderstanding among engineers that creep and high temperatures go hand in hand. This temperature is unquestionably higher than the typical operating temperatures that an aircraft encounters while it is being used for its intended purpose.
Because of this, creep would be removed immediately from the list of probable causes, or it wouldn't even make the cut for inclusion on the list in the first place. The majority of engineers would think that way, and in fact, they do think that way; as a result, the aircraft operator or maintenance organization would rack up an unnecessary amount of additional expense paying for additional examination.
What about using too much torque?
The strength of any material can be reduced by excessive stress, but in the vast majority of instances, the failure occurs rather quickly. . . not after three years. Even at room temperature, creep can be caused by excessive torque applied during the installation of a bolt. This applies only to a limited degree, but it must be applied for an extended period of time.
Failure analysis reports from real-world applications and academic research not only imply but also demonstrate that this is something that can occur with titanium bolts.
There is a wealth of knowledge available, and it is imperative that engineers make the effort to search for it.
After all, there is no such thing as a brand-new problem!
The application of sound engineering principles can actually result in cost and time savings.
When data and comprehensive justification are presented, a warranty claim submitted to the maintenance organization has the potential to have very good chances of success.
Titanium bolts are expensive, especially when the same problem has already affected a significant portion of your fleet and it is likely to affect the rest of your fleet. . . if the same organization and maintainers continued to use the same method of excessive torque application.
In the long run, adopting a more focused and analytical point of view will not only help you save money and time, but it will almost certainly lead to safer aircraft operations as well.
Have you encountered any issues with titanium bolts that are comparable? Share your experience with us in the comments section below.