Aircraft fuel sensors and fuel measurement electronics development

Why distinguish fuel level, quantity, flow and quality?

Level describes fuel height in a particular tank region. Volume relates to tank geometry, while mass also depends on density. Flow is fuel passing through a line per unit time. Consumption calculated from line flow is not an independent tank quantity measurement unless the initial amount and all relevant flows are correctly known.

Fuel quality is not one quantity. Water, contamination or another property requires a method suited to that target. Changes in a level probe signal may provide diagnostic information under some conditions, but do not automatically prove fuel usability or compliance with a standard. The interface separately names litres, kilograms, flow and faults; records preserve the origins of measured and calculated results.

How does a capacitive aircraft fuel sensor work?

In a capacitive probe, the dielectric properties of the medium between electrodes affect the measurement. Capacitance changes as the fuel-covered portion of the probe changes. FAA training material explains this principle; actual systems may contain several probes and additional sensing elements. Capacitance is not directly the litres or kilograms in the complete tank.

Electronics read the probe through defined excitation and measurement methods. Cable capacitance, leakage, contamination and connections are also part of the measurement. Laboratory work can first test the reading chain using equivalent electrical loads, then examine the actual probe in a suitable setup. Electrical energy limits inside the tank are not selected solely for sensitivity; protection and fault-analysis requirements must be design inputs.

Tank geometry, probe position and fuel density

Level and volume may be easily related in a uniform laboratory vessel, but not in an irregular wing tank. Probe placement and the tank model are evaluated together. Crane explains that fuel quantity systems may pass probe information through an electronics unit or data concentrator to calculate total fuel. Architecture varies by platform.

Density matters in mass calculation. Fuel type and temperature-dependent properties can limit fixed-coefficient approaches. Collins Aerospace provides an example of this need through direct-density measurement solutions; this is not a VOMRO performance claim. The project defines fuel, mixture limits, temperature and expected density behaviour. Tank models, calculation coefficients and test data can share version control.

How are attitude, sloshing and data validity handled?

When the tank tilts or fuel moves, the same total quantity can produce different levels in different regions. Testing is therefore not limited to a level, stationary tank. Required accuracy at different attitudes follows platform use. Filtering can steady the display, but is not assumed to eliminate physical uncertainty.

An example development study may log raw channel data, calculated quantity, attitude inputs and validity together. Filter delay, startup and missing-channel behaviour are measured. Old values remaining on screen are distinguished from new valid measurements. Independence of additional functions such as low-level warnings is not accepted merely because a second software threshold exists; it must be justified by architecture and fault assessment.

Project scope for measurement electronics and embedded software

Custom electronics development defines probe excitation, capacitance reading, temperature channels, power protection and communication as a whole. Interfaces follow the actual connected system's electrical and data definitions. Listing a protocol name does not establish compatibility with a particular aircraft connection. Message scales, update times and fault states are examined separately.

Firmware requirements may include unit conversions, coefficient versions, range checks, channel diagnostics and record formats. Software developed for a test rig is not assumed to have the same approval scope as flight software. VOMRO quotations define schematic/PCB, software and mechanical sources, integration responsibilities and validation tools as separate deliverables. Documentation access and usage terms for third-party probes and software are also evaluated.

Embedded software development to project requirements →

Fuel tank safety and limitations on aircraft use

FAA AC 25.981-1D provides guidance on demonstrating compliance for preventing non-lightning ignition sources in transport-category aircraft tanks. Its scope does not apply uniformly to every aircraft. The platform's certification basis, electrical fault conditions and environmental requirements must be established with the responsible parties. Low board power consumption in normal operation alone is not proof of tank suitability.

Protection is assessed alongside potential faults, wiring, connectors and effects from the system. A working prototype, a passed environmental test or a component certificate does not constitute aircraft installation approval. Responsibilities for ground trials, development prototypes and final installations requiring approval are clearly distinguished. The contract specifies VOMRO's technical data deliverables and external assessments.

What information is needed for calibration and project startup?

The electronic reading chain is validated with known electrical references, while quantity calculations use defined tank and filling conditions. Records retain the tank geometry model, filling steps, reference quantity method, temperature and attitude. Acceptance targets specify whether error is a percentage of full scale, actual quantity or a combination.

Start with the platform type, research or intended use, tank drawing, fuel definition, operating temperature, quantity range and target interface. For an existing probe, share the model code and technical documents you are permitted to provide. The first phase may be method feasibility, an electrical simulator, a tank trial or a comprehensive electronics prototype. Success conditions and decisions on progressing are made concrete without creating an expectation of a ready-made flight product.

Separate requirements for fuel line pressure measurement →