Fuel Quantity Test Set Calibration

Fuel Quantity Test sets are self-contained, universal test sets primarily used to test the fuel quantity measurement systems and their line replaceable electrical components. These test sets allow aircraft owners to troubleshoot and isolate fuel system-related issues in advance by simulating the tank capacitance (AC/DC) for its entire fuel measurement range. These test sets can also be used for oil, water, or LOX quantity measuring systems operating on Capacitance based principle.

Aircraft Fuel Quantity Measurement Systems:

Two methods of fuel quality measurement are predominant on an aircraft: Capacitance based and Resistance based. Both these systems provide the quantity of fuel by varying a precisely controlled electrical signal. Based on the parameter changing the electrical signal, they are classified as Capacitance and Resistance based. Both measuring systems have their pros and cons. Capacitance-based aircraft fuel quantity measurement systems use a Capacitive element to vary the signal proportional to the fuel quantity in the tank. It consists of two conductive tubes separated by an air gap. By measuring the dielectric changes due to the fuel level in the tank, The varying AC or DC signal drives a pointer in the fuel quantity indicator to indicate the quantity. On the other hand, Resistance based fuel quantity measurement systems use a variable resistive element to vary the electrical DC voltage proportional to the fuel quantity and indicate the fuel quantity on a dashboard. Though Resistive based systems are reliable and accurate, Capacitance systems are considered better than resistance systems due to their higher precision and long-term reliability. Capacitance systems are often higher-priced and used as a standard measuring system for military and heavy aircraft. Resistance-based systems are typically low-cost systems found in low-performance aircraft.

Aircraft fuel systems operate due to the change in the capacitance of the fuel probe as the fuel level increases or decreases and air replaces the fuel in the tank. A signal conditioner measures this change in capacitance and sends the reading to the fuel indicator in the cockpit. In most cases, the change in capacitance is very small, under 50 picofarads so the wiring and fuel system components must be able to perform very accurately. For accurate fuel quantity measurement, multiple Capacitance or Resistance based probes are used on the tanks.

Fuel Quantity Test Set Operation Principle:

Fuel Quantity Test Sets contain the following components to test the aircraft fuel system at various conditions. Each component of the test kit performs a specific function:

• A Microamp (mA) current indicator to check various analog or digital current indicators.
• A low-voltage Megohmmeter to measure the insulation resistance of wiring, probes, or other system components.
• A capacitance simulator to output accurate capacitance values to represent empty or full fuel tank conditions.
• A capacitance bridge to measure the capacitance of individual fuel probes and interconnecting wiring.
• An internal battery to make them portable for ease when working in or around the aircraft.
• Fuel Quantity Test Sets also contain either special adapter modules or test cable harnesses for each specific aircraft type to connect the test set can be in line with the signal conditioner and the probe system to make the appropriate measurements and adjustments.

Selection of Fuel Quantity Test set:

Selection of test set depends on the fuel system existing on-board an aircraft, power requirements, and the range of measurement, as mentioned below:

1. On-board aircraft fuel system: The test set is selected based on the type of fuel quantity measurement system onboard the aircraft (Resistive or Capacitive based), AC or DC type Capacitance type measurement system, and guarded or unguarded systems.

2. Range of measurement: The test set’s measurement range should be more or equal to the Capacitance range of the aircraft tank. Modern test sets can measure the capacitance of up to 8000 pF, often found in large aircraft fuel quantity measuring systems.

3. Fixed or portable systems: Modern hand-held fuel test sets are portable and operate without an external power source. Fixed test sets require a continuous power supply for operation. They are commonly used to test Resistive type fuel quantity measuring systems, and portable sets are used for Capacitance type systems and are battery-powered.

Applications of Fuel Quantity Test Set:

In addition to testing the accuracy of fuel quantity measurement systems, most modern test sets are also capable of:

• Simulating capacitance or resistance values on fuel, oil, or water quantity measurement system on full or partially filled tank systems
• Simultaneous capacitance measurement of the individual or interconnected tank units or simulators
• Insulation strength of harness or electrical cables and connectors
• Test performance of indicators and signal conditioners or bridge amplifiers
• Measurement of AC or DC Voltage (potentiometer)
• DTF Measurements
• Log measurement and calibration data and alert owners on calibration due dates

Calibration of Fuel Quantity Test Sets – Why is it essential?

1. Stray Capacitance Compensation: Often, test sets contain a certain number of stray capacitances induced due to the exposure of unshielded lead wires at connecting points or capacitance between switch contacts. These stray capacitances induce errors while testing the fuel quantity systems. Calibration ensures that the stray capacitances (if any) are subtracted from the measured values.

2. Dry Tank test: The test is performed without fuel in the tanks to ensure the Test set’s empty level measurement (zero indication). A substituted capacitance value equal to the fuel (full) in the tank is set, and the indicator is then adjusted to display the full level. Differences in the actual and measured readings are determined, which are then added or subtracted to the final test set readings.

3. Wet Tank test: Wet Tank Calibration method is used to simulate values for when fuel is present in the tank. The trial commences by electrically removing the fuel quality measuring system onboard the aircraft. The test set values are correlated with the fuel quality in the tank to determine the test set accuracy by adjusting the simulated values.

Test readings errors can often result in adjusting the settings of fuel quantity systems and may lead to catastrophic failures, downtime, and maintenance losses. Following a scheduled preventive maintenance and calibration activity, avoid these errors, ensure the fuel quantity system and its components’ accuracy for sustained and repeated use.

e2b calibration offers industry-leading ISO-certified Fuel quantity test set maintenance and calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to calibrate your test sets. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.