Fuel level sensing for UAVs
Combustion-powered UAVs have to balance fuel, payload and endurance within a very tight mass budget. Fuel-level sensing therefore has to provide useful information without becoming another compromise in the airframe.
A combustion-powered UAV has a simple calculation to make: how much fuel remains, how much flight time does that represent, and when does the aircraft need to turn back? The calculation is only as useful as the fuel measurement behind it.
Unlike a road vehicle, a UAV has very little tolerance for unnecessary mass. Every component competes with payload and endurance. That makes fuel-level sensing an unusually demanding engineering problem. The sensor has to fit the tank. It has to measure the fuel actually being used. It has to tolerate the aircraft’s environment. It has to communicate with the flight or engine-management system. And it has to earn the mass and complexity it adds to the aircraft.
This is why adapting a conventional automotive fuel sensor can be less straightforward than it first appears.
Fuel is payload, endurance and return margin in one number
Electric propulsion removes the need to measure liquid fuel. Combustion propulsion does not. For UAVs using an internal combustion engine, fuel is one of the aircraft’s most important finite resources. The flight-management system and operator may use fuel information when assessing endurance, payload, reserve and return-to-base decisions.
The measurement becomes particularly important when payload changes between missions. A different payload changes aircraft mass. That can change fuel consumption. The available fuel may therefore need to be measured rather than inferred from a fixed assumption.
The fuel number is not just a tank measurement. It can become an operational input.
A tank has height. The aircraft needs volume.
A simple level measurement tells you where the liquid surface is. The aircraft may need to know how much usable fuel that level represents. Those are not always the same thing.
In a simple vertical tank, the relationship between height and volume may be relatively straightforward. In a shaped wing tank or irregular UAV tank, it may not. This is where tank profiling becomes important. The sensor can be configured so that its output reflects the tank’s geometry rather than simply reporting raw liquid height.
The accuracy of the resulting volume estimate depends on the tank geometry, calibration process, fuel properties and installation. Profiling improves the relationship between level and volume. It does not make any tank measurement exact.
Every gram has to earn its place in the airframe
On an aircraft, component mass is never free. A heavier sensor can reduce available payload, reduce endurance or require changes elsewhere in the design. That means the fuel sensor has to be evaluated on more than its purchase price. The engineering question is what measurement capability the sensor provides for the mass, space, power and integration burden it introduces.
Measure the sensor’s mass before you measure its price
For UAV design, the relevant cost is not simply the purchase price. The trade-off includes:
- Sensor mass
- Mounting hardware
- Cable and connector
- Interface electronics
- Tank modification
- Installation time
- Power requirement
- Calibration and maintenance access
- Measurement reliability
A sensor that saves mass but produces unreliable fuel information is not a saving. A sensor that provides useful information but requires unnecessary installation hardware may also fail the airframe’s weight-to-benefit test. The design objective is the complete sensing arrangement.
A car tank and a UAV tank are not the same measurement problem
Automotive fuel-level sensors are designed around automotive tanks, vehicle packaging, operating temperatures, vibration environments and electrical architectures. A UAV may impose a very different set of constraints. The tank may be unusually shallow, irregularly shaped, integrated into a wing, angled, space constrained, subject to significant vibration or exposed to different thermal conditions. The sensor may also have to meet a much tighter mass budget.
This does not make automotive sensing inherently unsuitable. It means that adapting an automotive component can introduce compromises that need to be assessed rather than assumed away. Gill’s UAV application experience identifies unreliable or inaccurate fuel-level information as a potential consequence when automotive fuel sensors are adapted to this environment.
The question is not “can it measure fuel?” It is “can it measure this tank?”
Most fuel-level sensing technologies can detect the presence or level of liquid. The difficult part is producing a useful measurement from the actual aircraft tank. The engineer needs to understand tank shape, sensor length, mounting position, fuel properties, aircraft attitude, vibration, usable volume, electrical interface and required output. This is why sensor geometry and tank geometry should be considered together.
Capacitive sensing turns fuel level into an electrical measurement
A capacitive fuel-level sensor uses the electrical properties of the fuel and the sensor geometry to determine liquid level. As the amount of sensor immersed in fuel changes, the effective capacitance changes. The electronics convert that change into a usable output.
Because the measurement depends on the electrical properties of the liquid, fuel type matters. Different fuels can have different dielectric properties. The sensor must therefore be calibrated or configured for the fuel that the aircraft will actually use.
Temperature matters for the same reason. The dielectric constant of a fuel changes as its temperature changes, which is why certified aircraft capacitance fuel-gauging systems carry a compensator unit low in the tank. Where a UAV works across a wide temperature range, agree with the system designer whether that variation needs compensating in the receiving system.
Why capacitive sensing suits the UAV constraint
Capacitive sensing is useful here because it can provide continuous liquid-level measurement without a mechanical float or moving linkage. That can simplify the sensing mechanism and allows the sensor to be packaged around the tank geometry. The advantage is not simply the absence of moving parts. It is the combination of continuous measurement, configurable sensing length, tank profiling, compact integration, sealed construction and a choice of electrical interfaces. Those characteristics are stated here as they apply to LevelLite.
Specify the fuel system, not just the sensor
- The sensing length needs to correspond to the actual tank geometry. A fixed catalogue length can create unusable measurement zones. A user-defined length allows the sensor to be designed around the tank rather than forcing the tank around the sensor. LevelLite can be specified to around 750 mm.
- Capacitive measurement depends on the electrical characteristics of the liquid. Specify the actual fuel. Do not assume that a calibration for one fuel automatically transfers to another.
- If the tank is irregular, the relationship between level and usable volume may not be linear. Where supported, the sensor can be programmed around the tank profile so that its output corresponds more closely to the required volume representation.
- The sensor has to communicate with the flight-management or engine-management system. LevelLite gives voltage and current outputs, with RS232 for configuration. CAN is available on bespoke sensors. The correct interface depends on the aircraft architecture rather than on a general preference.
- Specify temperature, vibration, fuel exposure, moisture, pressure where relevant and installation location. IP68 addresses dust and water ingress, and under IEC 60529 the conditions for the continuous immersion test are agreed between manufacturer and user rather than fixed by the standard. The IP code also states plainly what it does not cover, including mechanical impact, corrosive solvents, condensation and explosive atmospheres. It does not by itself establish fuel compatibility, vibration tolerance or temperature capability.
Five questions before specifying fuel-level sensing
| Question | Why it matters |
|---|---|
| What fuel will the aircraft use? | Capacitive measurement depends on liquid properties |
| What is the tank geometry? | Determines the relationship between level and volume |
| What measurement length is required? | Prevents unnecessary dead zones or unsuitable sensor length |
| What output does the aircraft need? | Determines electrical integration |
| What does the sensor actually experience? | Defines temperature, vibration, moisture and environmental requirements |
For an irregular tank, add a sixth: does the system need to report level, estimated volume or usable fuel?
The installation is part of the measurement
A fuel-level sensor cannot correct for a tank design it does not understand. The installation needs to preserve the relationship between fuel, sensor, tank geometry and aircraft system.
Calibrate for the fuel
Capacitive measurement depends on the electrical properties of the liquid. The sensor should therefore be configured or calibrated for the fuel used in the application. If the fuel changes, the effect on the measurement should be assessed.
Profile the tank
A shaped UAV tank can have a very different relationship between fuel height and usable volume. Where the sensor supports tank profiling, configure the response around the actual tank geometry. The objective is to give the aircraft a useful representation of fuel quantity rather than simply liquid height.
Match the output
Define the receiving system before selecting the output. LevelLite gives voltage and current outputs, with RS232 for configuration and CAN on bespoke sensors.
Control the mechanical installation
Sensor position, orientation, mounting and tank geometry all affect the measurement. The installation must keep the sensor correctly positioned and account for aircraft vibration and movement.
Fuel does not stay level when the aircraft does not
A level sensor measures the fuel distribution around its sensing geometry. An aircraft in climb, descent, bank or acceleration can change the local fuel distribution inside the tank. Tank geometry, sensor position, baffling and system calibration therefore matter. Where fuel quantity must remain meaningful under dynamic flight conditions, the tank design and measurement architecture need to be considered together.
The sensor flies inside the machine
The sensor experiences the aircraft’s environment rather than sitting in a benign laboratory tank. Engine vibration, structural vibration, thermal cycling, fuel exposure, moisture, connector integrity, tank movement and installation loads all apply. The sensor specification should be assessed alongside the actual UAV environment. IP68 does not cover vibration, temperature or fuel compatibility, which are separate requirements.
The fuel measurement has to reach the flight system
Fuel tank → LevelLite → electrical output → flight or engine-management system → fuel estimate → operational decision
The sensor provides a measurement. The receiving system determines how that measurement is interpreted and used. A technically good sensor cannot compensate for an incorrect tank profile, unsuitable calibration, incompatible interface or incorrect system scaling.
A plausible fuel number is not always an accurate one
- Fuel calibration mismatch. The fuel’s electrical properties differ from the calibration assumption.
- Tank profile mismatch. The sensor measures level correctly but the system converts level to volume incorrectly.
- Unrepresentative installation. Sensor position or orientation does not represent the usable tank volume.
- Interface scaling error. The sensor output is interpreted incorrectly by the receiving system.
- Dynamic fuel movement. An aircraft manoeuvre changes the local fuel distribution.
The sensor is only one part of the measurement chain.
How we apply the principle at Gill
Gill’s fuel-level sensing technology has roots in motorsport, where fuel measurement is also constrained by mass, tank geometry, vibration and the consequences of an inaccurate remaining-fuel calculation. That experience has been applied to UAV applications where the same engineering constraints appear in a different form.
LevelLite is a lightweight, sealed capacitive fuel-level sensor designed around applications where packaging, environmental protection and electrical integration matter. It uses an aluminium body and sealed electronics, with IP68 protection and a +5 to +32 V supply range. Outputs are voltage and current, with RS232 for configuration and CAN on bespoke sensors. The measurement length is user-defined, to around 750 mm according to the supplied product information.
The value is not simply the sensor body. The sensor can be configured around the tank and control-system requirements. That includes defining the measurement length and, where appropriate, profiling the response for an irregular tank.
LevelLite at a glance
| Specification | LevelLite |
|---|---|
| Sensing technology | Capacitive |
| Body | Aluminium |
| Electronics | Sealed |
| Protection | IP68 |
| Supply | +5 to +32 V |
| Outputs | Voltage and current; RS232 for configuration, CAN on bespoke sensors |
| Measurement length | User-defined, to around 750 mm |
| Application | Fuel and liquid level measurement |
The technology came from a place where fuel measurement already had to earn its keep
Motorsport provides a useful engineering heritage for this type of sensing. Fuel measurement has to operate within tight packaging constraints while coping with vibration, temperature variation and demanding performance requirements. Gill level sensors have been used in motorsport fuel cells and in UAV fuel tanks. That is application heritage, not aviation certification. The environment changes. The engineering discipline does not.
The tank changes. The sensing principle stays useful.
- UAV wing tank. A long, narrow tank can require a sensor length and installation geometry that differs substantially from a conventional vehicle tank.
- Autonomous helicopter. An angled or unusually shaped tank can require the sensor and activator geometry to be designed around the installation. Angled installations have been built for autonomous helicopter tanks.
- Motorsport fuel cell. A motorsport fuel cell places the same pressure on mass, packaging and reliable fuel measurement. The environments are not identical. The engineering constraints are shared.
The measurement has to fit the aircraft, not just the tank
Fuel-level sensing for a UAV is an integration problem. The sensor has to fit the tank geometry. The measurement has to correspond to the fuel actually being used. The output has to reach the aircraft’s systems. The complete installation has to survive vibration, temperature and fuel exposure. And every gram has to justify its place in the airframe.
Gill’s LevelLite approach combines capacitive sensing with configurable measurement length, tank profiling and multiple output options, so the fuel measurement can be designed around the aircraft rather than the aircraft around a generic automotive component.
Four things to remember
- Fuel level is an operational input. On a combustion-powered UAV, fuel information can contribute to endurance, reserve and mission decisions.
- The tank matters as much as the sensor. Level and usable volume are not necessarily the same measurement, particularly in irregular tanks.
- Every gram has a consequence. Evaluate the complete sensor, mounting, wiring, power and integration burden.
- Calibrate and configure around the aircraft. Fuel type, tank geometry, output and installation all affect the usefulness of the measurement.
Frequently asked questions
Fuel level in a combustion-powered UAV can be measured using technologies including capacitive sensing. A capacitive fuel-level sensor detects changes associated with the amount of fuel around the sensing element and converts that into an electrical output. The sensor must be configured around the fuel type, tank geometry and required aircraft-system interface.
UAV fuel-level systems can use several sensing technologies, but capacitive sensing is useful where low mechanical complexity, continuous measurement and flexible tank integration are important. The correct technology depends on the aircraft, fuel, tank geometry, environmental conditions, mass constraints and electrical architecture.
Fuel level can provide an important input to decisions about endurance, reserve and mission management. The value of the measurement depends on how accurately the sensor represents the fuel quantity available to the aircraft and how that information is integrated into the flight or engine-management system.
Automotive fuel-level sensors are designed around vehicle tanks, packaging, vibration, temperature and electrical architectures that may differ significantly from those of a UAV. Adapting an automotive sensor can therefore introduce compromises in weight, tank geometry, installation or measurement behaviour. The suitability of a particular automotive sensor must be assessed against the UAV application rather than assumed.
A capacitive fuel-level sensor uses changes in electrical capacitance associated with the liquid around the sensing element to determine fuel level. Because the measurement depends on the electrical properties of the liquid, the fuel type and sensor configuration are important parts of the application specification.
The sensor forms a capacitance whose electrical characteristics change as the proportion of sensing element surrounded by fuel changes. Electronics detect that change and convert it into a usable level output. The exact measurement architecture and calibration depend on the individual sensor.
Capacitive sensing can measure liquid level without a mechanical float or contacting linkage. This removes the moving mechanical element associated with some conventional fuel-level sensors. The complete installation still includes mechanical mounting, connectors and the surrounding tank structure.
Capacitive measurement depends on the electrical properties of the liquid. Different fuels can therefore produce different sensor responses. The sensor should be calibrated or configured for the fuel used in the application, and any change of fuel should be assessed rather than assumed to have no effect.
It can provide an output that represents volume when the relationship between sensor level and tank volume is known and configured appropriately. In an irregular tank, the level-to-volume relationship may not be linear. Tank profiling can therefore be used where the sensor and system support it.
Tank geometry determines how liquid height relates to fuel volume. A simple tank may have a relatively predictable relationship, while an irregular or wing-integrated tank may not. The sensor length, mounting position and any tank-profile configuration therefore need to be designed around the actual tank.
Yes, where the sensor and configuration support the required geometry. Gill's LevelLite can be configured for user-defined measurement length and can be programmed around an irregular tank profile where required. The resulting measurement still depends on the tank design, fuel properties and installation.
Some fuel-level installations can use angled sensor arrangements where the tank geometry requires it. The supplied Gill application material identifies angled installations for autonomous helicopter tanks. The sensor orientation and tank profile need to be designed together for the actual application.
Climb, descent, banking and acceleration can redistribute fuel within the tank. A local level measurement may therefore change even when the total amount of fuel has not. Tank geometry, sensor location, baffling, calibration and the way the aircraft system interprets the measurement all need to be considered where fuel quantity must remain meaningful during dynamic flight.
The required accuracy depends on the aircraft's fuel capacity, consumption rate, reserve policy, mission duration and consequences of measurement error. The specification should define the usable measurement range and the operational decision the measurement supports rather than selecting accuracy as an isolated number.
Mass is an important consideration because every component contributes to the aircraft's total weight. The sensor should therefore be assessed alongside its mounting hardware, cabling, power requirements and interface electronics. The useful comparison is the mass and capability of the complete sensing arrangement.
The required output depends on the aircraft's electrical architecture. Gill's LevelLite gives voltage and current outputs, with RS232 for configuration and CAN on bespoke sensors. The exact output configuration should be matched to the receiving flight-management, engine-management or data-acquisition system.
LevelLite is Gill's capacitive liquid-level sensor with an aluminium body, sealed electronics and IP68 protection. It supports a +5 to +32 V supply and gives voltage and current outputs, with RS232 for configuration and CAN on bespoke sensors. Its measurement length is user-defined to around 750 mm according to the supplied product information.
Yes. The supplied Gill application material identifies LevelLite for UAV fuel-level measurement. The sensor can be configured around the required measurement length and, where appropriate, an irregular tank profile. The actual configuration needs to be matched to the UAV's fuel, tank geometry, environment and electrical interface.
LevelLite uses a user-defined measurement length, with the supplied product information identifying lengths to around 750 mm. The required length should be determined from the actual tank geometry rather than selected simply from a standard vehicle-tank dimension.
LevelLite is specified with IP68 protection. IP68 describes protection against dust and water ingress under defined test conditions. It does not by itself establish suitability for every fuel, temperature, vibration or aviation environment, which must be assessed separately.
The supplied LevelLite information specifies a +5 to +32 V supply range. The complete aircraft electrical architecture should be checked against the sensor's current requirements and the selected output configuration.
On bespoke sensors. CAN is available on bespoke LevelLite builds rather than as a standard output. The exact configuration should be selected according to the aircraft's electrical architecture and receiving system.
Yes. RS232 is one of the output options identified for LevelLite. Whether it is the appropriate interface depends on the aircraft's receiving system and wiring architecture.
The supplied application information states that capacitive measurement depends on the fuel's dielectric properties and recommends calibration for the fuel used in the aircraft. The exact calibration process and supported fuel types should be established for the specific application.
Yes, where the application and sensor configuration support it. The supplied information identifies programmable tank profiling so that the output can represent the required fuel quantity relationship rather than simply raw liquid height. The tank geometry and calibration data determine the resulting measurement.
LevelLite is identified in the supplied Gill material for UAV fuel-level applications. However, suitability for a particular aerospace programme depends on the required environmental, electrical, mechanical and certification requirements. The product should therefore be assessed against the aircraft specification rather than assuming that UAV application experience constitutes formal aviation certification.
Provide the tank dimensions and geometry, fuel type, required measurement range, installation orientation, expected temperature and vibration environment, available supply, required output interface and how the fuel measurement will be used by the aircraft system. These details determine whether the sensor architecture and configuration are appropriate.
Potential causes include incorrect fuel calibration, unsuitable tank profiling, incorrect sensor position, changes in sensing geometry, electrical scaling errors, fuel movement during flight and environmental effects outside the sensor specification. The sensor should therefore be considered as part of the complete fuel-measurement system.
It can provide a fuel-level measurement from which usable quantity may be estimated, but the result depends on tank geometry, calibration, sensor position, fuel movement and how the aircraft system interprets the measurement. A level sensor does not automatically know the amount of fuel that is physically usable under every flight condition.
Tank profiling matters because fuel height and fuel volume may not have a simple linear relationship. A shaped or irregular UAV tank can contain significantly different volumes at different heights. Profiling allows the measurement output to be configured around the actual tank geometry where the sensor supports that function.
The questions that usually follow
How does capacitive level sensing work?
The measurement principle in detail, including the electrodes, the dielectric properties of the liquid, calibration and the limits of the technique.
Read the answerLiquid level measurementCapacitive vs float level sensors
How continuous capacitive measurement and mechanical float switching differ in behaviour, installation, maintenance and whole-life cost.
Read the answerProductLevelLite
The full specification for the sealed capacitive level sensor, including outputs, supply range, protection rating and measurement length options.
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