40% lighter than traditional aluminum components, carbon fiber composite tubes for aircraft fuel gauge inspection probes

40% lighter than traditional aluminum components, 

carbon fiber composite tubes for aircraft fuel gauge inspection probes

Founded in 1989, Fairfax, Vermont-based Liquid Measurement Systems (LMS) is a company that specializes in the design and manufacture of fuel quantity gauges on rotary and fixed-wing aircraft. Its product offering includes a series of capacitive fuel metering probes, and each probe consists of two concentric carbon fiber composite tubes that act as two conductive plates between which dielectric currents flow.

Traditional capacitive fuel metering systems consist of two separate pieces of hardware connected by wires: a sensor (also called a probe) and a signal conditioning unit (SCU); where the probe is a passive device that is excited and measured by the SCU to monitor and report the fuel level in the tank.

And the SCU conditions the signals from all probes in the tank, along with factors such as probe position, tank geometry, attitude (aircraft orientation), fuel temperature, and other variables to calculate the total amount of fuel in the tank - expressed in weight rather than volume. embedded software in the SCU converts all the data received into the number of pounds of fuel in each tank into pounds of fuel and transmits this information to the display unit in the cockpit or on the refueling panel.

While aluminum is the traditional material used for metering probes, LMS has been using carbon fiber-reinforced polymer (CFRP) composites as the primary structural component of its fuel metering probes since its inception in 1989.

The company manufactures all of its fuel gauge tubes in the field using a winding process in which layers of prepreg are rolled and glued to the mandrel by hand and then cured in an oven. Prior to assembly, the tubes are uncoiled, cut to length and machined to uniform diameters and wall thicknesses. lMS says that for these tubes, it selected carbon fiber prepregs with proprietary resin systems for their excellent mechanical and electrical properties, as well as their resistance to the corrosive and oxidizing effects of jet fuel.

Compared to aluminum, LMS says it is 40 percent lighter to manufacture these parts using carbon fiber composites. In addition, the composites resist indentations, scratches, cracks and corrosion and do not absorb hand oils, which can affect performance. Aluminum tubes also require riveted connections to the probe's electronics, which can wear out over time due to friction, while composite tubes can be bonded to the electrical components.

LMS' carbon fiber fuel metering probes offer options for a range of aircraft fuel tanks. Recently, the company decided to develop a product specifically for smaller aircraft, aiming for greater safety, accuracy and reliability in smaller fuel tanks.

The resulting onboard processing probe (OPP), available in the summer of 2022, features the same carbon fiber composite dual-tube design as other LMS products and is an all-in-one system that combines a level sensing probe with an SCU, eliminating the need for two pieces of hardware or complex wiring.

The system is designed to work independently on single-tank aircraft with simple tank geometries, or as one of multiple independent units to measure the amount of fuel in one or more tanks, with each unit returning its respective fuel level to a designated aircraft data destination.