Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
42Cost Reduction Technology for Airframe Maintenance

TAKASHI YARI*1NOZOMI SAITO*2

KAZUO HOTATE*3SATOSHI MATSUURA*4ENOMOTO KIYOSHI*5

Recently, aircraft structures adopt a damage tolerance design and its airworthiness ismaintained through the periodical inspections specified in the maintenance plan before it becomesfatal. The establishment of health monitoring technology to continuously monitor the condition andairworthiness of an aircraft structure will hopefully lengthen the present inspection intervals andreduce maintenance costs, thereby improving the efficiency of aircraft operation. The authors
developed an optical fiber sensor-based system for wide-area diagnosis of aircraft structures andevaluated its applicability to adopt structure condition monitoring through flight demonstration
testing. It was found that the developed system can monitor the structural condition during flightoperation.

|1. Introduction
In aircraft operation, shorter aircraft inspection intervals and lower monitoring costs areamong the matters required for higher managerial efficiency. Accordingly, there is hope for thecommercialization of a structural health monitoring (hereinafter abbreviated as SHM) system thatcan be substituted for the structural inspection of aircraft during operations.
A variety of approaches have been adopted to implement such a system and many
researchers and engineers are striving for its commercialization. Among such efforts, the authors
are engaged in R&D for the commercialization of the SHM system through the application ofBrillouin optical correlation domain analysis (hereinafter abbreviated to BOCDA) technology tomake wide-area diagnosis available with an optical fiber sensor that is light-weight, causes noelectromagnetic interference, and is capable of being embedded into composite material.
BOCDA can characteristically measure distributed strain along the entire length of an opticalfiber and dynamic strain at arbitrary points. Figure 1 shows a conceptual image of its application to
the SHM system based on such characteristics. The evaluation of optical fiber sensors placed acrossthe airframe structure is now designed to identify the fatigue damage/structural life through themonitoring of damage occurrence due to changes of distributed strain or the measurement ofdynamic strain.

Figure 1

SHM System Application Concept*1

Chief Staff Manager, Nagoya Research & Development Center, Technology & Innovation Headquarters*2

Nagoya Research & Development Center, Technology & Innovation Headquarters*3

University of Tokyo*4

Yokogawa Electric Corporation*5

The Materials Process Technology Center (Sokeizai Center)Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
43

Using distributed optical fiber sensors, technology has so far been developed to detectde-lamination and other damage in the aircraft composite structure.1 In this R&D, a BOCDA
measurement system that can be mounted on an aircraft was developed/prototyped and, in tests forflight demonstration, its applicability to damage monitoring during operations was checked.

|2. Development of BOCDA-SHM system
One of the features of BOCDA is the ability to measure distributed strain along the entire
length of an optical fiber and dynamic strain at arbitrary points. The following is a summary
concerning the principle of BOCDA measurement and the development of a BOCDA measurement
system that can be mounted on an aircraft.

2.1

BOCDA measurement principle

BOCDA is technology to measure the optical fiber’s strain/temperature in light of theBrillouin scattering phenomenon that occurs within the fiber. Two opposite laser light inputs, with
one referred to as a pump light and the other a probe light, can acquire Brillouin scattering light atarbitrary (correlation) points of an optical fiber through frequency modulation under certain
respective conditions.2
Since the Brillouin scattering light frequency changes in proportion to strain and temperature,
it is necessary in measuring the strain of an aircraft structure to measure the structure’s temperature
at the same time. Here, however, it becomes possible to simultaneously measure the optical fiber’s
strain and temperature if polarization maintaining fiber (PMF) is used as the optical fiber andchanges in not only PMF birefringence, but also Brillouin scattering light frequency, aremeasured.3 Figure 2 diagrams the measurement principle. Pump light and probe light are admitted
into the X axis of a polarization maintaining optical fiber to acquire the Brillouin scattering lightfrequency, and read-out light is admitted into the Y axis of the fiber to acquire BDG (BrillouinDynamic Grating) frequencies that change in proportion to the birefringence of the optical fiber.
Since Brillouin scattering light frequency changes and birefringence changes are different physicalphenomena, strain and temperature can be calculated from these changes.

Figure 2 Brillouin Scattering Light Measurement Principle

2.2

Airborne BOCDA system development

The BOCDA measurement system to measure distributed/dynamic strain and temperature
with an optical fiber sensor was prototyped as a device that can be mounted on an aircraft. Sincecompliance with Avionics Installation Standard ARINC600 makes it easier for it to be mounted on
business aircraft, an interfaced system installable on a rack conforming to ARINC600 wasmanufactured on a trial basis (Figure 3). The prototyped system, driven by a DC28V power
supply, is 388mm-wide, 193mm-tall and 412mm-deep, and weighs 14kg.
Figure 4 shows the result of measurements based on an optical fiber along the length ofwhich temperature increase sections of about 4m and 1.5m were arranged together with an applied
strain section of about 1m. Although strain/temperature measurement errors appear at both ends ofthe applied strain section, they are noise resulting from the difference in the measurement sectionof Brillouin scattering light from birefringence, and can be removed. In addition, tests for theevaluation of durability against the airborne environment were conducted, finding normal operation

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
44in such an environment. Table 1 relates the results of environmental durability evaluation tests andFiguer 5 shows how durability tests were conducted.

Figure 3 Outside Appearance ofSHM Device

Figure 4 BOCDA Measurement Separation ofStrain from Temperature

Figure 5 State of Environmental
Durability Tests
Table 1 Environmental Durability Evaluation Test Results
No.

Item1

Vibration environment

2

Shock environment

3EMI

Specifications
10―2000Hz, 0.74G(RTCA-DO-160 Category S2)
6G, 11ms(RTCA-DO-160 Category A)
Mutual influence with airborne flighttesting apparatus installed to be evaluatedResultNo equipment failure
No equipment failure
Normal operation|3. Flight Demonstration Tests
3.1

Test method

For flight demonstration tests, a business jet (an 11-seat MU-300 with a total length of 14.7m
and a total width of 13.7m) was used as the test bed. With an optical fiber sensor bonded to thevertical tail plane’s front spar of the test bed and connected to an airborne BOCDA measurement
system installed in the cabin, strain and temperature were measured during flight. Figure 6 shows
the installed states of the sensor and the BOCDA measurement system.

Figure 6

Installed State of Airborne SHM System

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
45

Strain/temperature measurements from the aircraft structure with a BOCDA measurement
system installed on the test bed were taken during take-off climbing, level flight, and turning.
Measurements were taken focusing on dynamic strain and temperature changes during take-off
climbing, distributed strain and temperature distribution during level flight, and strain changes
during turning.

3.2

Test results and discussions

Figures 7 through 9 show the result of measurements during take-off climbing, level flight,and turning. In the take-off climbing case (Fig. 7), it is found that a decrease in structural
temperature with climbing has been successfully measured. From around 700 seconds, where thetemperature sags below zero, however, a difference from conventional measurements starts tooccur. Due to the bonded optical fiber sensor’s tortuosity and for other reasons, it is possible thatcompressive structural strain may not be properly conveyed to the optical fiber. During level flight(Fig. 8), strain and temperature distributions present almost the same values as conventional
measurements. In the case of turning (Fig. 9), good agreement with conventional measurements
was achieved, and therefore, the measurement function is found to be sound in the actual
operational environment.

Figure 7

Result of Demonstration Tests (During Take-Off Climbing)

Figure 8

Result of Demonstration Tests (During Level Flight)

Figure 9

Demonstration Test Results (Turning Case)Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)46|4. Conclusion
As part of demonstration tests for the commercialization of an aircraft structure diagnostic
system, strain and temperature changes in the aircraft structure during aircraft operations weremeasured. The following results were obtained:y A BOCDA measurement system complying with the standard for installation on businessaircraft and able to endure the installed environment was prototyped.
y It was shown that strain and temperature of an aircraft structure during flight can bemonitored, but accuracy in the low temperature range needs to be improved.
Henceforth, development efforts are to be made for the commercialization of BOCDA
measuring equipment such as through the improvement of its measurement accuracy, an evaluationof its application to structural inspection for cost advantages, certification planning, and conformity
assessment. Finally, this work was conducted as a part of the “Aerospace Industry Innovation
Program – Advanced Materials & Process Development for Next-Generation Aircraft Structures”project under a contract with The Material Process Technology Center (SOKEIZAI Center),founded by the Ministry of Economy, Trade and Industry (METI) of Japan. We hereby express ourgratitude to everyone involved.

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