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The next generation of aircraft may be equipped with structural sensors to monitor deformation and stress.
Modern commercial aircraft incorporate tens of thousands of sensors that monitor major systems. Most of these are internal sensors that communicate with each other, but others provide data that can be monitored in real time outside the aircraft to inform decision-making around flight operations and maintenance. Other sensors feed actionable data that is downloaded at the end of each flight.
Given that engines are the most valuable parts of aircraft, the most expensive to maintain and much of an airline’s fuel bill, they are the most sensor-rich systems on board. Historically, engine sensors provided critical measurements such as pressure, temperature and speed for engine control systems. Modern aircraft also use them for health monitoring to inform reactive, predictive or preventive maintenance, which has led to a significant increase in the number of sensors in modern engines. At the same time, modern engine sensors need to deliver greater accuracy while staying reliable in the harsher environment of the newest engines, which operate at higher temperatures and pressure ratios than their predecessors.
“The sensors themselves have evolved with technology to deliver higher performance and lower cost, weight or size, or some combination thereof,” says David Milne, senior director of program management for engines at Honeywell Aerospace Technologies. He points to precision pressure sensors, which have evolved from complex and expensive laser-welded quartz to simple and rugged piezoresistive elements. “We are also seeing additional or alternate sensors for difficult-to-measure parameters such as oil level, oil debris, noncontact position and fuel flow,” he adds.
Within the engine, the size and weight of certain sensors depend on the size of their protective packaging, which in turn has led to an increase in combination sensors, in which multiple sensor technologies are packaged into a single housing. For example, an engine gas-path-indicating sensor system might consist of a combination of temperature and pressure sensors.
“We have seen more combination sensors recently,” Milne says. “Our pressure sensors commonly also have integrated temperature sensors, either for calibration of the pressure measurement or for separate purposes. There are also examples of position sensors, either linear or rotary, combined with temperature sensors, such as in bleed valves or similar.”
Milne also points out efforts to make sensors and their housings more durable with materials like titanium or nickel-chromium superalloys. “For each sensor type, there are specific technologies and materials to improve durability,” he says. “For example, diaphragms in pressure sensors are exposed to sensed media, which can be hot, corrosive or contaminated, so special diaphragm coatings are used to maximize the durability of the sensor; typically these don’t affect weight, but could add cost.”
He adds that modern thermocouple and resistance temperature detector sensors are “very reliable,” with a mean time between failure of 1 million hr. or more, although these periods are much harder to achieve with complex items like active, digitally compensated pressure sensors.
SIGNAL TO NOISE
Signals from engine sensors are sent to the full-authority digital engine control (FADEC), or engine health monitoring system, where they are analyzed and their data is stored and utilized by software algorithms for engine control and monitoring. Some of these signals may be further transmitted from the FADEC to the flight deck to provide real-time engine operational displays to the pilots.
The FADEC protects the engine from excessive speed and temperature by monitoring these metrics and taking mitigative actions while also making adjustments to optimize performance. As engine technology has evolved, more sensor inputs have fed into the FADEC, and some of the newest engines, such as the CFM International Leap, have a second FADEC to monitor additional parameters.
Health monitoring systems are also evolving to act on wider sensor input. For example, Rolls-Royce’s engine vibration and health monitoring unit (EVHMU) provides instant access to around 10,000 engine performance and health parameters with what the company describes as “unprecedented levels of data quality.” The EVHMU monitors pressures, temperatures, vibrations and—in a new development—line replaceable units. “This will ultimately enable us to remove a unit before it ever causes a flight disruption,” says Nick Ward, vice president of digital systems.
“Rolls-Royce is developing improved EVHMU and engine monitoring units to allow for greater data gathering and easier transmission of data from all the latest sensors fitted to our engines,” Ward adds. “These improvements will come as we roll out these new components, and we will work with the aircraft OEMs to integrate where possible with their systems.”
The evolving capabilities of health monitoring systems and the machine learning algorithms that process their data mean more sensor inputs than ever before can be meaningfully analyzed. However, this does not necessarily mean that engines will pack in ever more sensors, says Milne, who notes that advances in software can in some instances reduce the need for hardware.
“In some cases, we have been able to take advantage of sophisticated algorithms and analytics to synthesize parameters rather than measuring them, thereby eliminating sensors,” he says.
This may help to mitigate dips in supply. Milne notes that sensors, like many other aircraft systems, have faced supply chain problems. “Sensors that have integrated electronics are exposed to the same challenges regarding availability of foreign-sourced semiconductors,” he says. “The CHIPS Act from the U.S. government is addressing this in a positive way and has helped Honeywell expand and modernize our electronics manufacturing capability. . . . Vertical integration gives us more control of our destiny.”
Software can also improve accuracy, Milne says, which is important given that it is difficult to achieve big gains in this respect from the engine sensors themselves, which are relatively mature and proven technologies.
“With the substantial computing power in our modern FADECs, we can perform sophisticated calibration and linearization to improve accuracy,” he says. “We can also employ sensor fusion of not only multiple sensors, but also synthetic parameters that are generated by onboard algorithms and models.”
NEW INPUTS
One relatively untapped use of sensors is embedding them in aircraft structures to monitor stress and deformation of materials like metal and carbon-fiber. Most of the major airframers have conducted extensive research and development of the necessary technologies, with many flight hours now logged on pilot programs such as Delta Air Lines’ Boeing 737-700s and Brazilian carrier Azul’s Embraer 190s.
President and CEO of Embraer Services & Support Carlos Naufel says that the manufacturer’s Aircraft Health Analysis and Diagnosis (AHEAD) platform “is equipped with capabilities to detect and alert operators of adverse exceedance events, ensuring timely maintenance interventions.”
The Structural Monitoring Systems sensors used in the Delta trials were based on comparative vacuum monitoring (CVM) technology, which applies small, self-adhesive elastomeric patches to the material being monitored. Cracks are then sensed if there is any change to the vacuum pressure applied to the sensor’s built-in rows of interconnected channels.
However, CVM sensors need to be positioned at the point where a crack occurs. Another maturing technology is the piezoelectric sensor, which can be used in ultrasonic or acoustic mode in a network design to monitor a larger structural area for damage.
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