CVM™ (Comparative Vacuum Monitoring) offers a novel method for in-situ, monitoring of crack initiation and/or propagation. CVM™ technology uses the principle that a vacuum maintained within a small volume is extremely sensitive to leakage. Essentially the CVM™ technology provides a measure of the differential pressure between alternating channels containing air at a partial vacuum pressure and channels containing air at atmospheric channels in a simple manifold.
The CVM™ principle relies on placing a sensor onto the surface of a component where damage is expected to occur. The sensor contains a manifold of fine channels that are open to the surface. Once the sensor has been installed on the surface, the channels form closed “galleries” to which a vacuum can be applied. It is important to note that the surface of the component forms part of the sensor system, with the crack itself providing the leakage path for air into the vacuum galleries.
If no flaw is present, the vacuum will remain at a stable level. When a crack develops, it forms a leakage path between the atmospheric and vacuum galleries, producing a measurable change in the vacuum level. This change is detected by the CVM™ system (PM200 device).
The PM200 is a microprocessor controlled handheld instrument powered internally from a rechargeable Lithium-Ion battery capable of interrogating groups of hard to access sensors from a suitable access point. The PM200 stores the location of inspection and interrogation pass / fail data for download by USB connection.
The sensitivity of the sensor is governed by the gallery spacing and the rate of air flow, this provides an indication of the size of the developing flaw. Sensors may either take the form of self-adhesive polymer sensors or may form part of the component. Since the sensor physics is based on pressure measurements, there is no electrical excitation involved.
The CVM™ sensor is manufactured from multiple layers of PTFE sheets, where 2 to 8 sheets are laminated together with an acrylic pressure-sensitive adhesive, the same adhesive is on the bottom layer and facilitates the adhesion to the aircraft.
The principle behind CVM™ is uniquely simple; a vacuum contained in a small volume is extremely sensitive to any leakages.
The CVM™ principle relies on placing a sensor onto the surface of a component where damage is expected to occur. The sensor contains fine channels in parallel which are open to the surface. Once the sensor has been installed on the surface, the channels form closed “galleries” to which a vacuum can be applied. It is important to note that the surface of the component forms part of the sensor system, with the crack itself providing the leakage path for air into the vacuum galleries (Figures 1,2,3).
The sensor is connected to a vacuum source with an accurate flow meter. Figures 4, 5 and 6 show a snippet of the equipment used in laboratory trials which allows continuous monitoring of a sensor, or several sensors connected in series or parallel (not shown). At the beginning of the test, a continuity test is completed to ensure galleries do not contain a blockage (Figure 4). If there is no damage on the component, then the vacuum in the sensor will be approximately the same as the vacuum source (Figure 5). If however a crack develops, a leakage path will exist and the vacuum level will be reduced in the sensor manifold (Figure 6.)
The PM200 being a portable stable vacuum source with an accurate flow meter allows continuous monitoring of a sensor, or several sensors connected in series (Figure 7).
It is permissible to install CVM™sensors in series to monitor multiple expected flaw locations from a single interrogation point. This reduces the time to inspect sensors by grouping together each test the PM200 completes. Total number of sensors that can be joined is proportional to sensor gallery and tube length. The PM200 records the location of sensors and detected value at interrogation completion (Figure 8).
Aircraft maintenance and repairs represent about a quarter of a commercial fleet’s operating costs. The application of Structural Health Monitoring (SHM) systems using distributed sensor networks can reduce these costs by facilitating rapid and global assessments of structural integrity. These systems also allow for condition-based maintenance practices to be substituted for the current time-based or cycle-based maintenance approach thus optimizing maintenance labor. Other advantages of on-board distributed sensor systems are that they can eliminate costly, and potentially damaging disassembly, improve sensitivity by producing optimum placement of sensors with minimized deployment concerns and decrease maintenance costs by eliminating more time-consuming manual inspections.Through the use of in-situ sensors, it is possible to quickly, routinely, and remotely monitor the integrity of a structure in service.
Current aircraft maintenance operations require personnel entry into normally inaccessible or hazardous areas to perform mandated, nondestructive inspections. To gain access for these inspections, structure must be removed, sealant must be removed and restored, fuel cells must be vented to a safe condition, or other disassembly processes must be completed. These processes are not only time-consuming, but they provide the opportunity to induce damage to the structure. The use of in-situ sensors, coupled with remote interrogation, can be employed to overcome multiple inspection impediments. Furthermore, prevention of unexpected flaw growth and structural failure can be improved if on-board health monitoring systems exist that could regularly assess structural integrity. Such systems would be able to detect incipient damage before catastrophic failures occur. The ease of monitoring an entire on-board network of distributed sensors means that structural health assessments can occur more often, allowing operators to be even more vigilant with respect to flaw onset.
The implementation of CVM™ technology will reduce the cost of mandatory structural integrity maintenance inspections and allow for condition-based maintenance practices to be substituted for the current time-based or cycle-based maintenance approach thus optimizing maintenance labor, increase aircraft performance, and provide an increase in the safety of air travel.
The fundamental simplicity of the CVM™ technology provides a unique set of competitive advantages and great flexibility in the design and application of structural monitoring systems. CVM™ systems are:
Operators organize their inspection and maintenance tasks in order to achieve compliance with regulations and OEM recommendations while maximizing aircraft availability. The various checks associated with general aircraft maintenance are as follows:
The intervals between services are dependent upon aircraft utilization, flight cycles, and required aircraft maintenance tasks. C-checks can take up to one week to complete while D-Checks require approximately one month to complete. Operators may choose to implement their maintenance activities in block, segmented, phased, or continuous maintenance visits. These options allow the various maintenance tasks to be broken into different intervals and completed in segments over the required interval.
Recently, the FAA has produced an Issue Paper (IP) in response to a formal request for use of CVM technology on structural inspections associated with the Wi-Fi installations . The IP represents the first formal set of guidelines from the FAA to produce the data necessary for certification of Structural Health Monitoring (SHM) systems in routine maintenance activities. The IP contains the general guidelines for producing SHM performance data to ensure that the proposed SHM system can adequately and reliably detect damage for compliance. Specifically, the IP addresses the use of “Comparative Vacuum Monitoring (CVM) for Damage Detection in Structure of Antenna Installations.”
SHM are relatively new technologies for conducting inspections. Therefore, applicants need to demonstrate that SHM systems effectively and reliably detect damage. Applicants need to show the proposed SHM system to be as good as the current inspection program that the SHM system is replacing. This IP specifies key elements and criteria the applicant must address to demonstrate that their proposed SHM system adequately replaces existing Instructions for Continued Airworthiness that are necessary for compliance
The elements and criteria identified in this IP (FAA Position) will guide the applicant’s comprehensive assessment of the functionality, reliability, durability, and maintainability of the proposed SHM system.
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