FLIGHT TEST WEEK 'OPEN CONFERENCE' - hosted by Society of Flight Test Engineers (SFTE)

Running in Auditorium 3 on the Exhibits floor) on 18, 19, 20 Nov 2008


Strain Gage Installation – Quality, Reliability & Performance
Wednesday 19th November 2008
11.00 - 11.30
Auditorium 3

Statement of Problem: Edwards Simmons and Arthur Ruge were co-credited with inventing the modern strain gage in the late thirties, while it’s no longer a cutting edge technology, manufacturing processes have increased quality, precision of these engineering aids while simultaneously lowering the cost of strain gages.  Strain gages have been in use by the engineering community for decades, and still prove to be an accurate and economical method for measuring stress and strains for the hundreds of measurements required on airframes and associated structures.  Many of our loads measurements are buried in the airframe and inaccessible once the aircraft is assembled, so it requires an extreme level of quality control since there’s one opportunity to do it right. It was recognized very earlier on by Fort worth flight test that in order to insure the availability of actuate and dependable data, the strain gage discipline would need to be controlled and nurtured into what many people consider an art form as much as skill.  At LM Aeronautics in Fort Worth, the strain gage lab approach is that the best technical processes and procedure are worthless unless you have a system that insures that they are being used and followed. These installations are inspected by a quality inspector, but prior to being presented to inspection, engineering has monitored every detail of the mechanics installation to make sure every installation was accomplished per the installation procedure and engineering instructions.  As flight test requirements and test vehicles increase in complexity, there is pressure within the unique strain gage installation community to develop new, improve old, and react real time to problems encountered.  A network of coordinated, trained and informed community of stain gage installation specialists must answer to these critical issues.


Objectives:  The results achieved by the Forth Worth strain gage lab have been unquestionable; we currently have (5) F-16 loads aircraft that have been in service 20 to 25 years since there original instrumentation was installed. F-16A-271 is still being used for ground test using strain gage installation dating from 1979.  How is this quality and sustainability achieved?  It is not necessarily the processes or procedures, but more so a collaborative effort that engineering set up between itself and the installation mechanics, where engineering accepts the everyday hands on responsibility for the product.


Approach: Over the years when dealing with other manufacturers and vendors, there have been many approaches to develop strain gage installation into a discipline that produces high quality installations while assuring that the process skills are maintained and nurtured.   But the approaches vary from the casual approach of picking it up through reading a book, to approaches that imbed the process into the manufacturing/production world where the installation expertise is quickly learned to get the job done and then lost.  As program team make ups change develop and change again, and aircraft come together from sections fabricated on a global landscape, it becomes essential that sharing of process technology, lessons learned and developments within the industry merge together into a community that iteratively communicates on such issues as installation and process knowledge.


Results: The Fort Worth strain gage lab has maintained that in order for their mechanics to be proficient they must have constant training and repetition.  Instead of 100 part time mechanics with questionable skills, due to the importance of this data it is preferable to have 10 highly skilled mechanics. Unfortunately, we’ve had to justify the way the strain gage lab does business at the start of every program and it seems to become more difficult each time.


Conclusions:  If there is enough understanding of the uniqueness of this specialized niche and what it take to achieve and maintain it in today’s test world versus say today’s manufacturing world we feel that this skill can survive and produce the quality of data necessary to verify aerospace products now and in the future.


Lessons Learned: Lessons learned if properly managed within a globally coordinated stain gage community is one of the avenues that can facilitate better development and fabrication of the flight test vehicle.  A perfect product will produce perfect results which sometimes may make or break further development of a particular aerospace vehicle.