Modern aviation owes much of its success to gas turbine engines that convert fuel into massive thrust, enabling large passenger aircraft to travel long distances at high speeds. Jet engines, however, require large fuel loads, most of which is carried in fuel tanks that span both of an aircraft’s main wings. The Airbus A380, for example, has a maximum fuel capacity of over 85,000 gallons. To ensure the safety of flight, the commanding pilot is required to accurately monitor and log fuel consumption rates to properly manage the plane’s weight and balance.
Understanding the critical importance of in-flight fuel monitoring and continuously striving to advance aerospace technology, the Fluid Systems Division of Parker Aerospace has teamed with Oxsensis Ltd., a manufacturer of optical instrumentation used in harsh environments, to develop a unique optical-based system that will transform and simplify the next generation of aircraft fuel measurement systems. This new technology is expected to reduce installation time and maintenance costs with fewer components.
Current fuel measurement systems use capacitance-based technology
The current capacitance-based technology used on most aircraft to measure fuel capacity is proven, safe, and reliable. Its architecture incorporates an array of approximately 20 to 30 capacitance probes that are mounted vertically throughout the fuel tanks.
The probes act as variable capacitors, with the fuel acting as the dielectric. As the fuel level near the probe changes, changing the ratio of fuel-to-air in contact with the probe, the capacitance of the probe changes, allowing the system to measure the “wetted length” of the probe ‒ an empty tank has lower values of probe wetted length than a full tank. These measurements are made by a dedicated electronics box outside the fuel tank, by injecting a tiny time-varying electrical current via electrical wiring to the probes in the fuel tank.
This information is transmitted back to fuel gauging computers elsewhere on the aircraft, where the data is combined with other measurements such as fuel temperature and density, to compute the fuel quantity on board the aircraft. This data is then relayed to the pilot in the cockpit, and to other aircraft systems.
While highly reliable, capacitance-based fuel measurement systems can be complex and require a great deal of time and expense during the installation stage. Typically, a total of 20 to 30 probes need to be installed throughout each of the wing tanks in most large commercial transports, in addition to corresponding electrical wiring needed to relay the probes’ data. This electrical wiring, combined with the increased trend in the aviation industry toward the use of composite materials, particularly in wing design, means measures must be taken to ensure that lightning strikes do not move through the wiring system, causing fuel tank combustion. A great deal of engineering and cost go into mitigating these risks.
A better way to measure aircraft fuel is on the horizon: SOHAPS
Parker is collaborating with Oxsensis to develop silica-based optical high-accuracy pressure sensor (SOHAPS) systems that will replace the 20 to 30 capacitance probes plus additional fuel property sensors. Instead, each tank would need only three
electromagnetic interference (EMI)-immune, non-electrical, optical sensors to measure fuel system pressure.
Slated to be fully tested and ready for entry into service by the mid-2020s, SOHAPS will provide the next generation of business jets and commercial aircraft with a next-generation solution for fuel measurement. This system is a major advancement because it is as accurate as today’s capacitance system, but is far simpler in design, requires fewer sensors, is intrinsically safe, is immune to EMI, will require far less time to install in the aircraft, and is expected to reduce maintenance costs.
As part of the joint development of the innovative new system, Parker and Oxensis are actively engaged with a major aircraft manufacturer in preliminary manufacturability and integration activities.
This proven and innovative technology has demonstrated endurance success in land-based gas turbine applications for several years. The sensor technology developed in the SOHAPS program has the potential to be applied to multiple aerospace applications including hydraulic systems, lubrication pumps, landing gear, and other major systems. Parker is investigating these future possibilities as part of our ongoing commitment to ensure our customers’ success.
How SOHAPS fuel measurement systems are different
The joint development of the SOHAPS system combines Parker’s significant fuel system engineering, integration, and qualification abilities with Oxsensis’ unique optical pressure sensor capabilities. Unlike capacitance systems that measure the capacitance of the fuel in a system to compute remaining fuel volume, SOHAPS systems measure the pressure within the fuel tank and compute fuel volume. SOHAPS system data is transmitted to a single onboard computer via fiber optic cable, where the data is interpreted and relayed to the cockpit as remaining fuel information.
The sensor has been shown to provide more precision than currently available electrical pressure sensors, while also being suitable for low-temperature and high-vibration conditions. The use of fiber optics is expected to deliver a sensor that is immune to electromagnetic interference and completely free of any metallic or electrically conductive material. It also means that, by eliminating metallic wiring in the fuel tanks, the risk associated with the conduction of electricity due to lightning strikes is eliminated as well.
Simply less complex
The innovative SOHAPS system will greatly simplify aircraft fuel measurement systems.
By needing as few as three sensors for an aircraft fuel tank, SOHAPS systems will require far less time to install during manufacturing, reducing installation cost, part numbers, and complexity.
Elimination of metallic wiring will also reduce manufacturing time as well as the risk of transmission of electrical charges as well as EMI interference and electrical discharge in the tanks.
SOHAPS systems will not require a relay electronics device to collect data for transmission to an onboard computer for data interpretation. All collected data goes directly to the onboard computer via optical cable.
In addition to measuring fuel quantity, other measurement capabilities are possible using the same set of onboard sensors, further reducing weight on the aircraft. These additional measuring capabilities include fuel temperature and fuel properties, eliminating the need for other types of sensors in the fuel system, and making the system equally capable of operating with today’s standard fuels and future fuel types
Due to the wide availability of optical data transmission equipment from mature industries such as the telecom industry, SOHAPS systems can take advantage of significant cost savings by using robust off-the-shelf components that can meet aircraft qualification standards.
Lower maintenance costs are predicted
Due to far fewer parts in a SOHAPS system, maintenance costs are expected to be lower than current systems. And unlike capacitance systems, it is anticipated that optical-sensor-based systems will be capable of operating with any type of fuel, meaning that aircraft operators will no longer need to go to the trouble and expense of recalibrating their systems as they introduce new fuel types and fuel additives for their aircraft.
Drawing on the expertise of Parker Aerospace and Oxsensis, silica-based optical high-accuracy pressure sensor technology is coming closer to market with advantages that include comparable or better accuracy than current capacitance systems, simpler design, the need for fewer sensors, the ability to operate with future fuel types, immunity to EMI, less time to install in the aircraft, and an expected reduction in maintenance costs. SOHAPS is the next generation of fuel measurement systems for major aircraft manufacturers to watch.
For additional information on Parker Aerospace systems and capabilities, please visit our website.
This post was contributed by Principal Investigator, Fuel Gauging & Sensors Dr. Lewis Boyd of the Parker Aerospace Fluid Systems Division.