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shot orientations.
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 Structural Health Monitoring is the continuous monitoring of
structures/components using integrated or applied sensors.
 Aimed at assuring structural integrity of the aircraft, replacing onevent
and periodic inspections to detect damages resulting from
fatigue, corrosion, excessive loads, impact ...
 Monitoring of structures does not necessarily mean knowing the
status of the structure in real-time.
 Structures are designed with acceptable margins such that, after
normal or exceptional events, maintenance tasks can be planned
at next appropriate inspection.
 Systems are available for aircraft condition monitoring - mostly for
loads (accelerations, flight parameters, etc.) and enable decisions
to be made based on actual flight load levels. Indirect surveillance
of the structure is not comprehensive or reliable enough to avoid
interval inspections.
What is SHM
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Conventional vs. Condition Based
Management
 Currently, NDT is applied starting with visual inspections followed by for more subtle or
hidden flaws, procedures are defined based on eddy currents, ultrasonics, x-rays, etc…
 Inspection intervals are usually based on knowledge of the structure residual strength,
operating environment, applied loads, damage growth rate and failure consequences.
 Of course, inspections result in downtime and inaccessible areas of structure often
require significant effort to remove equipment or strip protective coatings for access,
which then must be restored after the inspection.
 Monitoring activity comes at a considerable cost and accounts for an average of 44% of
all on-aircraft maintenance man-hours for commercial aircraft (Andresen, 2006). In
terms of life cycle cost, a US DoD study attributed 27% of the total cost of an aircraft
being maintenance related with structural inspection being a significant driver of this
cost (Kudva, et.al. 1999) suggesting that SHM could save up to 44% of current
inspection time on modern fighter aircraft.
 Ultimate concept imitates the human nervous system, though SHM will better since
structures are monitoring directly, measuring the effect of damage.
 Compared to conventional NDT, SHM has many advantages:
 No access to the inspection area necessary – fewer access panels & component removal
requirements
 No physical operation in the area - safe inspection of hazardous areas
 No use of scanners necessary – eliminating time consuming setup
 Sensors used in the inspection are integral to the structure
 Automated process - no human factors influence on inspection POD
 Interrogating many locations or wide field at once - significant time saving
H. Speckmann, Materials & Processes - Testing Technology, Airbus
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 Time spent inspecting the structure to assure continued airworthiness increases as
aircraft age.
 To allow for statistical variation of the real life of the structure, a safety factor is applied
to the demonstrated lives of components.
 To reduce the inspection burden, some industries have introduced automated on-line
structural health monitoring systems with maintenance only being carried out when the
health of the structure indicates a need for it.
 CBM approach to maintenance if applied to aerospace structures has the potential to not
only reduce the time spent inspecting these structures, but also improve airworthiness
by detecting damage at an earlier stage than possible during discrete periodic
inspections.
 For safe life components it would also be possible to detect early failures and withdraw
them from service ahead of their expected life, or for healthy components continue to
use them beyond the design life and only withdraw them when their health indicates a
requirement to do so.
 Potential to both improve airworthiness and gain economic benefits was originally
conceived within the context of a rotorcraft Health and Usage Monitoring System (HUMS)
but is applicable to any health or usage monitoring system.
CBM Approach – SHM Potential
T. Ewbank, Cranfield University,
Application Of Condition Based Maintenance On Aerospace Structures
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• Difficult to access inspections
• Hot spot monitoring
SHM Applications
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• Impact damage in composite
• Wireless/passive sensing
• Appropriate for in-situ embedded or attached
robust sensors during component manufacture
• Cost effective, lightweight sensors (optical,
　
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