Monitoring Aircraft Structural
Health Using Optical FiberSensors
TAKASHI YARI*1

MASAHITO ISHIOKA*1

KANEHIRO NAGAI*1MIKISHIGE IBARAGI*1

KAZUO HOTATE*2

YASUHIRO KOSHIOKA*3

We have developed a system for monitoring aircraft structural health that uses optical fiber sensors. Our prototype device
for Brillouin optical correlation domain analysis (BOCDA) uses an optical ﬁber as a sensor, and we veriﬁed that this device issufﬁciently durable for installation on board aircraft. It functioned effectively as an aircraft structural health monitoring system
in a ﬂight demonstration test using a business jet. As the next development step, we plan to improve the reliability of this system forpractical application as a product monitoring service.

I t i s n e c e s s a r y t o r e duc e t he o p e r a t i n g c o s t s ofcommercial aircraft. Reducing the cost of inspecting theairframe and the maintenance downtime this requires would
further this goal. A structural health monitoring (SHM)
system that can evaluate the health of the airframe structure
while the plane is in ﬂight and on the ground would provide
a practical application that would satisfy this requirement.
Mitsubishi Heavy Industries, Ltd. (MHI) is developing justsuch an aircraft SHM technology that applies Brillion opticalcorrelation analysis (BOCDA), which can measure the strain
distributed along the total length of an optical fiber sensor
and the dynamic strain at arbitrary points along the opticalfiber sensor. Consequently, it will be possible to measure
the strain distribution, monitor deformation, and diagnose
damage from the load history in a continuous section of theairframe structure during operation.
This report introduces the development of an aircraft
structural health monitoring technology that uses BOCDA.

optical fiber. Since the frequency shift of the stimulated
Brillouin scat tering light is propor tional to the strain
generated in the optical fiber, the axial strain along theoptical ﬁber can be assessed by measuring this frequency.
This shift is approximately 50 kHz/με for the optical ﬁbers
used for telecommunication.
Figure 1 shows the BOCDA measurement principle.
W hen t he f requencies of t he pu mp a nd probe l ight s
are modulated at the same period, the point where thecorrelation between these lights is always constant (thecorrelation peak point) appears in the optical fiber. As
the correlation between the pump and probe lights is notFrequency

1. Introduction

Pump light

Probe light
Optical fiber sensor
Correlation peak point

2. Development of an onboard measuring device
2.1 Measuring technique
Optical fiber is a suitable sensor for SHM because ofits light weight, electromagnetic non-inductiveness, anddurability. Of the measuring methods using optical fiber
as a sensor, BOCDA, which can measure the distributed
strain along the total length of an optical ﬁber sensor or thedynamic strain at arbitrary points on the optical ﬁber sensor,
is an excellent technique for detecting a wide range andvariety of damage.1 BOCDA utilizes stimulated Brillouinscattering generated by the interaction between the pumplight and the probe light launched from opposite ends of an*1 Nagoya Aerospace Systems Works*2 Faculty of Engineering, The University of Tokyo*3 R&D Institute of Metals and Composites for Future Industries (RIMCOF)
5 5Applied strain
Optical fiber sensor
Pump light
Scattered lightintensity

Stimulated Brillouin scattering
Probe light
Distance

Scattered light frequency

StrainFig. 1 BOCDA measurement principles

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 4 (Dec. 2008)

constant except at the correlation peak point, the amount ofstimulated Brillouin scattering light power is very low, andthe scattered light generated at the correlation peak point
can be measured selectively. In addition, this correlation
peak point can be moved by changing the modulation periodsof the pump and probe lights, and the distributed strain canbe measured along the total length of the optical ﬁber sensor.
A prototype BOCDA measuring device was manufactured
in cooperation with Yokogawa Electric Corporation. To
improve its accuracy and increase the measurement range,pola r i zat ion diversit y 2 and temporal gat ing 3 schemes
were adopted. The specifications of the measuring device
included a spatial resolution of 30 mm (the minimum unit
length for which strain can be measured), a measuring rangeof 500 m, a sampling time of 1/60 s, and a strain accuracyof ± 13 με. These substantially exceed the performance of
conventional measuring devices utilizing Brillouin scattering
light. With these speciﬁcations, the device can be applied tolarge equipment and structures other than airframes. The
prototype of the BOCDA measuring device is 430 mm wide,
550 mm deep, and weighs 45 kg (Fig. 2).

BOCDA device

Shock direction

zy

Shock testing machine

Fig. 3 The shock test of the BOCDA system

Table 1 Tests of environmental durability
Environmental
test

Test condition

Test result

Temperaturetest

T = 0°C

Operates accurately

T = 40°C

Operates accurately-axis vibration

Operates accurately-axis vibration

Strain measurement accuracy
was reduced to ± 25.

6 G ( -axis)

Resisted the loadVibration test
Shock test

Optical unit

in Table 1. Although the stability of the BOCDA device
was reduced slightly, it was sufficient for monitoring thestructural health and estimate residual life of the airframe ina ﬂight test environment.

460 mmControl unitx

3. Demonstration test
3.1 Test method
We conducted a test in an actual flight environment toverify the effectiveness of the SHM system by BOCDA. For
the ﬂight test, we used a Mitsubishi MU-300 business jet thatwas 13.7 m wide and 14.7 m long as the ﬂight test bed (Fig. 4).
We measured the structural strain in a skin panel stringer inthe upper part of the fuselage. Generally, it is important toPower supply
430 mmFig. 2 The onboard BOCDA device

2.2 Performance of the onboard measuring device
For the aircraft SHM system under development, it isnecessary to mount the BOCDA device on an aircraft in order
to measure the strain generated in the airframe during ﬂight.
Any onboard measuring device must be durable and survivethe temperatures, vibration, and shock encountered during
the operation of the aircraft, while functioning accurately insuch an environment. Therefore, we conducted three tests
to verify the performance of our prototype BOCDA device(Fig. 3). 1. A temperature test to verify normal operation
over a temperature range from 0–40°C. 2. A vibration test toverify normal operating at a frequency range of 10–2 kHz andan acceleration power spectral density of 3.2×10 -6 to 3.4×10 -4
G2/Hz. And 3. a shock test to verify resistance of the device toshock at an amplitude of 6 G and a duration of 20 ms.
The results of the environmental tests are summarized

Strain measuring section

Fig. 4 Flight demonstration test bed (MU-300)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 4 (Dec. 2008)

6

Frame: BOCDA: Strain gauge/div.)

Skin

FWD

Strain (1005 mmStringer= 150 mm
0=040Time (s)6080(a) During pull-up operation: Optical fiber sensor: BOCDA: Strain gauge/div.): Dynamic strain measuring portion(installed on back side of strain gauge)

Strain (100Fig. 5 Schematic view of the optical ﬁber installation

FWD20Optical fiber sensor

02040Time (s)6080(b) During landing
Fig. 7 The strain history in the fuselage panel measured using
the BOCDA device
A comparison of the results using the BOCDA device and a strain
gauge

Strain gauge(back side)

motion during pull-up and pull-down, a tensile or compressive
load is generated in the panel in the upper surface of thefuselage due to bending. During landing, the point of supportof the airplane weight moves from the wings to the landinggear, which causes the load condition of the airframe toﬂuctuate. As Fig. 7 shows, excellent agreement was observed
between the BOCDA and strain gauge measurements during
both ﬂight and landing. Results indicated dispersion of thevalues measured using BOCDA during landing. This isthought to have arisen from vibrations transmitted to theBOCDA device when the landing gear was down and thef laps were moved to the landing position, which reduced
the measurement accuracy. Nevertheless, we showed thatthe load (strain) history generated in the airframe could bemeasured and demonstrated the effectiveness of monitoring
the aircraft structure.
Figure 8 shows the distributed strain measured duringlevel flight. The out-of-plane deformation due to the cabinpressure at the level flight altitude of the aircraft could be
detected as the distributed strain. The test results agreed
approximately with the values calculated from the cabinpressure, demonstrating that the onboard BOCDA device
can measure the distributed strain of the airframe in theflight environment. Consequently, it should be possible todetect damage from the ﬂuctuation of the distributed strain
measured using the BOCDA device at important sections of
the airframe structure, such as pressure bulkheads.

Fig. 6 The installed optical ﬁber

assess the ﬂight load or stress concentration sections suchas at the mountings of the wings and tail, the joints between
panels, and at holes. Our SHM technology should be usedin these areas. However, to apply it to an existing airframe
structure requires large-scale modification and approval
from the Civil Aviation Bureau. Therefore, for the purposeof the tests, we monitored the stringer of a fuselage skinpanel in a pressurized portion of the airframe. Figure 5
is a schematic of the area where the structural strain wasmeasured and the placement of the sensor. Figure 6 isa photograph of the sensor bonded on the stringer. The
optical fiber sensor was approximately 10 m long. For
comparison, data were obtained by installing a strain gauge
near the optical ﬁber sensor. The BOCDA system was bolted
to the cabin ﬂoor (shown in the photograph at the top of thispaper) and measured the dynamic and distributed strain ofthe test bed. The dynamic strain was measured during pullup operation and during landing sequence. The distributed
strain was measured during level ﬂight.
3.2 Test results
Figure 7 shows the dynamic strain of the test bed
measured during pull-up operation and during landing.
Since the vertical acceleration f luctuates with airframe

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 4 (Dec. 2008)7

on this development and to establish technologies for damage
evaluation and temperature compensation.
This research was conducted as part of a contract with R&D
Institute of Metals and Composites for Future Industries( R I MCOF ) as pa r t of t he Development of St r uct ura l
Health Monitoring Techniques component of the Advanced
Materials and Process Development for Next-Generation
Structures project, which is elemental technology researchin the Civil Aviation Fundamental Technology Program ofthe Ministry of Economy, Trade, and Industry.

Strain (100/div.)

Optical fiber sensor
FrameSkin
Stringer

References: BOCDA: Analysis value
00.2
0.4
0.6
0.8
Location of the optical fiber sensor, (m)1. Hot ate K ., Fiber Opt ic Ner ve Systems for St r uct ures a ndMaterials That Can Feel Pain, IEEE Newsletter Feb. 2006, pp.
5-10 (2006)
2. Hotate K., et al., BOCDA Fiber Optic Distributed Strain Sensing
System with a Polarization Diversity Scheme for Enlargement ofMeasurement Range, Proc. of SPIE vol.5855, pp. 591-594 (2005)
3. Kannou K., et al., Temporal Gating Scheme for Enlargement ofMeasurement Range of Brillouin Optical Correlation Domain
Analysis for Optical Fiber Distributed Strain Measurement, Proc.
in OFS-16 (2003)

1Fig. 8 BOCDA measurement of the fuselage panel
distributed strain

4. Conclusion
In this study, we manufactured a protot ype BOCDA
measuring device that is capable of monitoring the health ofan entire airframe and veriﬁed that it was sufﬁciently durable
to be installed on a ﬂight test bed by conducting a veriﬁcation
test for environmental durability. Then, we conducted aflight test and demonstrated that our device can measure
the load (strain) history generated in the airframe structurefrom takeoff, during ﬂight, and until landing. In addition, oursystem can detect damage from the distributed strain statechange during ﬂight.
As the next step, we plan to develop a durable, stable,
reliable aircraft structural health-monitoring system based

Takashi Yari

Masahito Ishioka

Kanehiro Nagai

Mikishige Ibaragi

Kazuo Hotate

Yasuhiro Koshioka

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 4 (Dec. 2008)8