ATR uses accelerometers for its in-development propeller vibration-monitoring system. ATR

Wireless Sensors To Increase Aircraft System Monitoring Possibilities

Increasing use of sensors is about to give maintenance providers an exhaustive view of an aircraft’s health

The ever-growing power of data processing is converging with another, less well-known trend to improve reliability and make predictive maintenance a reality: the increasing use of sensors, which are becoming cheaper or much smarter for the same cost. With a greater number of sensors or more intensive use of them, operators and manufacturers are better monitoring and understanding the behavior of many aircraft systems. Now the advent of wireless sensors is creating a new range of possibilities.

A combined enhancement in sensing and software solutions for the permanent assessment of systems is on the way to make aircraft more intelligent—and maintenance department managers will see it first-hand. “We are seeing an increased volume of new sensors available and more applications, which is starting to bring prices down,” says Bjorn Stickling, manager of diagnostics, prognostics and health management at Pratt & Whitney Canada. Meanwhile, the materials and components that enable modern sensors continue to drop in price, notes Thomas Wiegele, senior fellow in intelligent systems at UTC Aerospace Systems (UTAS). Taking  a life-cycle perspective, sensor costs include not only acquisition but also installation and maintenance. Reliability improvements on sensors are significantly reducing these costs, notes Johann Bordais, Embraer’s vice president for services and support. But tempering the overall impression of progress toward lower sensor costs is Fabien Darsonval, ATR’s head of propulsion systems, who says he does not yet see any indication that sensors are becoming less expensive.

Still, what most users perceive as a tendency toward cheaper components is not so straightforward. “Sensors are becoming more intelligent through added software, computation and connectivity functionality; this functionality is providing great value to intelligent aircraft systems, but not without a cost,” says UTAS’s Wiegele. In other words, manufacturers can expect either a lower price or better value for their money. Nevertheless, “price is still a challenge on smaller aircraft,” Stickling says.

Conventional wired sensors have been used mainly for control and fault monitoring; now they are increasingly being adopted for use on other systems, such as integrated health management. For example, Embraer is developing a scheduled structural health monitoring (S-SHM) solution. “It can replace complex and time-consuming inspections with simple and fast automatic evaluations of structural condition,” says Bordais. “Our S-SHM solution is already flying with one of our E-Jets customers.”

ATR and Meggitt Sensing Systems are developing a propeller-balance trending system. Vibration is monitored continuously, as is already the case for engines. Instead of having to arrange regular ground testing or putting maintenance personnel on revenue flights, the operator can receive much more frequent, automated reports. The bottom line is improved reliability, as an unbalanced propeller has an impact on engine accessories, ATR’s Darsonval says. Maintenance costs are therefore predicted to decrease.

Among the factors enabling the design of such systems is the recent improvement of data-acquisition units, which are now more compact. ATR hopes to offer a retrofittable option.

Pratt & Whitney Canada is developing a similar system. “We expect to go into production in June on ATR aircraft, and we have just installed the first system on the Bombardier Q400,” Stickling says. “It will allow airlines to put propeller-balancing maintenance activities into ‘on-condition’ mode, for a truly optimized environment.”

The next step for sensors is wireless communications, which is certain to be a far-reaching technology. For two years, ATR has been using wireless sensors for flight-testing purposes. Engine data are concentrated at the nacelle level and then sent to a central system. The experience gained in flight testing could be used to extend the capability to certified aircraft.

“We expect non-safety-related systems to be the first adopters of wireless,” says UTAS’s Wiegele, citing examples such as climate control. Next in line may be “smoke detectors, emergency lighting, cabin-pressure sensing, engine sensing and eventually flight-control actuation systems.”

Wireless sensors currently certified for civil aviation offer higher memory capacity, improved wireless offload capability and longer battery life. As a result, they can record a greater volume of high-quality data in terms of sensitivity and frequency, Stickling says.

Embraer’s Bordais is more circumspect. “We expect improvements on sensor resilience to interference such that new implementation possibilities are created,” he says. The World Radiocommunication Conference in 2015 agreed on spectrum for wireless avionics intra-communications (WAIC). Standardization bodies RTCA and Eurocae are targeting the first half of 2019 to issue minimum operational performance standards for WAIC.

The potential benefits of wireless sensors are immense. The first is design weight. On a large aircraft, with 11,000-13,000 lb. of wiring, they could mean a weight reduction of up to 3,300-4,000 lb., Wiegele suggests.


ATR uses accelerometers for its in-development propeller vibration-monitoring system.  Credit: ATR

For maintenance, oil condition monitoring could enable the detection of precursors of problems, such as metal chips. Wireless technologies can provide sensing access to hard-to-reach locations on the aircraft, which could include rotating elements. Temporary installation also becomes possible.

“Maintenance functions become more precise and timely, engineering models and designs benefit from more knowledge about product performance, and higher-level analytics are enabled by the additional sources of information,” says Wiegele. This could improve testing procedures and provide new prognostics and health-monitoring capabilities, concurs Embraer’s Bordais.

Wireless technology allows operators to connect and certify a sensor separately from the engine-certification process. That is helpful, as it is typically expensive to certify a wired sensor in a new engine configuration. With the ability to more easily introduce sensors, engineers can optimize where, when and how data are recorded. Since new-generation sensors have their own storage capability, data can now be downloaded when and where it is really needed,  via a smartphone or tablet.

Engines have led the way in the use of sensors for operations and maintenance and may make the most of wireless sensor technology in future.

Pratt & Whitney Canada is exploring new sensor technologies to monitor a range of parameters —from vibration data to temperature and pressure data—on the engine and the nacelle as well as the oil system. Since wireless sensors used for maintenance purposes are not part of the core engine bill of materials, sensor reliability does not affect dispatch reliability or availability of the engine. As a result, it is easier to make the case to add more sensors.

“There is still a large amount of untapped information in the engine control systems that we, as manufacturers, are well-positioned to leverage for analytics,” says Stickling, who sees the potential to greatly enhance the amount of data available for analysis. This will enable improved proactive, preventive and on-condition maintenance.

But with the significant increase in the number of sensors and data transmitted and stored on aircraft, new challenges arise. The hardware perspective should not supersede the software perspective—the need for pattern recognition in massive amounts of data and greater parallel computing requirements, Bordais points out. ATR is concerned that data ownership and utilization could be disputed between the operator and the service provider, which could be the engine maker. 

From Identification To Sensing

Radio-frequency identification (RFID) of parts is already helping to improve logistics. As repeated labeling of part numbers by hand is eliminated, “no error can be pushed into the maintenance system,” says Trevor Stone, Airbus’s head of value chain visibility and mobile solutions. In a cabin, using RFID to check the expiration date of life-vest containers takes a matter of minutes, instead of hours with conventional methods. Airbus refers to this process as Auto-ID, combining UHF communications and a barcode.

What if Auto-ID-enabled items could connect wirelessly to the aircraft automatically? Such a sensing infrastructure might first become a reality in the cabin. For example, it could help the crew ensure overhead bins are locked and everything is in the right place before takeoff. In the future, the technology could also help capture humidity or temperature data, among other parameters. 

Harvesting Energy

Going wireless also implies that electric power can no longer be brought to the sensor from elsewhere. Various means of developing self-powering sensors are being tested, but few details are available. Energy could be “harvested” locally from vibration, thermal gradient, sunlight or airframe stress. 

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