New system by Saab Avionics uses fiber optic sensors for engine and bleed air temperature measurements.
By Glenn Connor
ATP. Cessna 425 President, Discover Technology Intl
A key to knowing your airplane is monitoring how your engines are doing, especially when they get hot. For small single-engine aircraft, the status of just a few things tells you what the engine is up to – literally – in terms of running and exhaust gas temperatures.
The development of engine efficiency has led to higher operating temperatures, and engine installations now often include some complexity due to the use of composite materials.
There is a physical challenge of having enough strategically-placed engine and overheat temperature sensors to help detect bleed air loss, which is a serious concern in more modern aircraft designs.
Saab Avionics’ timely announcement at NBAA-BACE 2019 unveiled a solution – the comapny’s new fiber optic-based Overheat Detection System (OHDS).
This is a significantly lighter system with fewer components than conventional setups, thus saving weight and providing a new way of delivering large amounts of information to feed the analytics for maintenance forecasting.
Origins of Saab’s OHDS
Saab began developing OHDS in 2013 as part of the Gripen program. The Gripen is Saab’s most recent fighter aircraft. It incorporates a number of Saab innovations, including flight controls, structures, and avionics.
There was a need for monitoring bleed air ducting, especially in areas where bleed air and composites were used.
Rather than using a conventional approach that would include numerous components, thus adding too much weight, Saab decided to use what are called Fiber Bragg Grating (FBG) sensors – a string of temperature sensors engraved along a fiber optic line, coupled to a unique optical digital processor.
Operation of the FBG sensors allows changes in temperature to be measured in wavelength and correlated with temperature.
The lightweight nature of the Saab design also enables more of the components spread around the entire aircraft to cover and monitor more areas of the engine and bleed air compartments.
The OHDS solution reduces the total number of components by up to 90% compared to conventional systems, and reduces weight by 80% – both highly significant factors in aviation. Saab saw that OHDS was a perfect new technology that could transition to commercial aviation, and the company received its first order from Airbus for the A350.
The manufacturer was so pleased with the new system that it awarded Saab its prestigious Airbus Innovation Award for OHDS.
In any turbine aircraft, systems are installed for overheat sensing, rate of rise of extreme temperatures, and, of course, flame detection.
Furthermore, for each aircraft, there are several types of systems in different locations, which also consist of multiple detector sensors. These include what is called a continuous loop system, which works to provide a more complete area of coverage. Typically, modern bizjets have a dual loop fire detection system that is set to alarm based on a given temperature limit.
The bleed air systems are monitored by thermal switches for high temperatures. As is clear from a physical inspection beneath the skin of the aircraft, each of these systems requires many components, such as mechanical tubes, brackets, and other large elements. The actual temperature sensors for aircraft are tried and true devices, generally based on older foundations.
The most typical heat sensor for aircraft is a tube packed with thermally sensitive eutectic salt and a nickel wire center conductor. If high temperature or overheat are detected, the resistance of the eutectic salt drops, enabling flow of electrical current to provide the alarm.
Multiple temperature sensors can be connected to a controller which processes the alerts or alarms to be sent to the flightdeck. Thermocouples are another classic type of temperature sensor. If the temperature rises rapidly, the thermocouple produces a voltage back to the cockpit, activating a fire warning circuit.
The one common challenge faced by all of the conventional fire detection systems and OHDS is how to get sensors in as many places as possible, including the odd areas of the engine or bleed air systems, without the significant cost in wire and supporting components.
A solution by Saab
Saab’s OHDS is different. It uses a lightweight fiber optical line that has temperature sensors engraved at different lengths of the fiber optic cable. This strand of sensors can then be placed throughout the bleed air ducts and in and around composite structures, as well as in engine areas.
The magic of the Saab OHDS comes in 2 parts – its use of FBG sensors placed along the length of the fiber optic cable, and an electro-optical control unit for integrating and processing continuous temperature data. Connected to the optical fiber is an optical interrogator that produces and reads the optical signals over the entire length of the fiber line.
Optical interrogators are small electronics that manage the data moving across the fiber optic line for use by the processing hardware and software. This component is also where communications with the temperature data and the aircraft avionics are accomplished via digital databus.
The Saab OHDS power requirements are minimal, at 28 VDC. The installation or routing of sensor cables follows the bleed air duct routing. So, in practice, the use of the Saab OHDS provides the exact location of a bleed air overheat event, saving the detective work of guessing at the general location of the fault in the aircraft, which is what conventional single-point location sensors allow for.
In aviation, as technology evolves, we get exposed to new fancy-sounding words, and a Fiber Bragg Grating is one of these new terms. What an FBG does is actually simple – changes in temperature will change the reflected light moving along a fiber optic cable, which in turn can be correlated to a temperature.
With a control unit that pulses light continuously through the fiber, wavelengths of light can be converted to data that relates to temperature with a high degree of accuracy of both location and temperature measurement. What is also interesting is that these types of FBG sensor can also be used to measure pressure or strain.
In fact, FBG has been in use in extremely harsh environments such as seismology, and even as downhole sensors in oil and gas wells for measurement of the effects of external pressure and temperature.
Well received attributes
Additional new features of Saab’s OHDS include reliability, low-cost redundancy, ease of integration, and the ability to add numerous temperature sensors along the length of the fiber optic line.
Since the actual temperature data is gathered by light, the distance to connections does not impair the data, so future implementations may be very clever in nature all around the aircraft. The Saab OHDS is also designed to react to a set alarm threshold, and allows for fault location to a high degree of accuracy, as well as for tracing trends.
Another function of the new Saab OHDS is the ability to supply data logs regarding engine operation. The ability to track engine temperatures and excursions from the norm enables forecasting of both trends and preventative maintenance.
Chasing the flame
One of the most fearful things that can be said on a flightdeck is “Fire!” The results of a fire of any kind in a modern aircraft are so horrendous that fire and overheat detection systems are put everywhere from the wheel well to the cargo bay, and from the APU to bleed air systems, and, of course, all over the engine compartment.
The use of composite structures is a challenge to conventional fire and overheat detection systems, with sensor location mostly determined by an educated guess based on the nearest heat source – which is totally logical. But what if you miss a spot? For the most part, the technology of commercial aviation overheat and fire sensing has been advanced along the margins with no real breakthroughs – until now.
The use of modern fiber optic cable etched with FBG sensors all along its length takes science-fiction-sounding terminology and creates an award-winning new product by Saab Avionics. In any aircraft system, an 80% reduction in weight is stunning. In addition, when you can reduce your parts count by 90%, you have a more productive aircraft.
There is a tiny glass-like cable in the future of the modern airplane. Serpentine around the engines, nacelle, bleed air ducts, APU, and other places where its sensors will live inconspicuously, providing real-time data for the moment, and predictions of things to come.
And the light-pulsing device that is part of the OHDS will be gathering huge amounts of data that can be shipped out to forecast the future of your aircraft’s health, as well as yours.