Quantity Indication - Fuel Systems - Airframes & Aircraft Systems #67

Summary

This lesson explores fuel quantity measurement in aircraft, contrasting volume-based methods using floats and variable resistors with modern mass-based systems employing variable capacitors. The script explains how traditional systems face issues like maneuvering errors and specific gravity changes, while capacitive systems offer a more accurate measurement by compensating for these variables. It also covers backup manual systems like dipsticks and magnetic level indicators, and digital fuel quantity indicating systems in modern aircraft, which provide comprehensive fuel information to pilots.

Takeaways

• 🔍 There are two primary methods for measuring fuel quantity: by volume (using a float and variable resistor) and by mass or weight (using variable capacitors).
• 🛩️ Volume measurement systems are typically found in light aircraft and use a float mechanism linked to a variable resistor, which is read by an ammeter calibrated in gallons or liters.
• ⚖️ Modern gas turbine engine aircraft prioritize measuring fuel by mass due to its impact on airframe performance and energy content, which is weight-dependent.
• 🔋 Capacitive fuel quantity systems use variable capacitors in fuel tanks, where the dielectric constant changes with the fuel level, allowing for a more accurate measurement of fuel volume.
• 🌡️ The capacitive system compensates for temperature changes in fuel by considering how specific gravity and dielectric constant vary with temperature, ensuring accurate mass measurement.
• ✈️ Attitude errors in fuel measurement are mitigated in capacitive systems by using multiple capacitor probes in the tank to average out fuel level readings.
• 🔧 Older aircraft with analog fuel quantity gauges have fail-safe circuits that alert pilots to system failures by moving the gauge pointer towards the empty position.
• 💻 Modern aircraft use digital gauges controlled by a Fuel Quantity Indicating System (FQIS) computer, which monitors the system and provides warnings for any failures.
• 🔧 In case of system failure, manual backup systems like dipsticks, drip sticks, or drop sticks are used to determine the fuel quantity on board.
• 📊 Large aircraft display fuel system information, including tank schematics, fuel mass, valve positions, pump operations, and total fuel on board, on electrically presented schematic displays.

Q & A

What are the two methods of measuring fuel quantity mentioned in the script?

The two methods of measuring fuel quantity are by measuring the volume of the fuel or by measuring the mass or weight of the fuel.

How does a volume measurement system for fuel work?

A volume measurement system uses a float in the fuel tank that is connected to a variable resistor. As the fuel level decreases, the resistance increases, leading to a reduction in current flow, which is calibrated to show fuel quantity on a cockpit gauge.

What are the two major drawbacks of the volume measurement system?

The two major drawbacks are maneuvering error, where the indicated fuel quantity changes with the aircraft's attitude, and the inability to compensate for changes in the specific gravity of the fuel, which affects the reading as the fuel temperature changes.

Why is it important to measure fuel in terms of weight or mass on modern gas turbine engine aircraft?

It is important to measure fuel in terms of weight or mass because the weight of the fuel affects airframe performance, and the energy in the fuel is measured by weight or mass, which the pilot needs to know for accurate fuel management.

How does the capacitive method measure the weight or mass of fuel?

The capacitive method uses variable capacitors in the fuel tanks. By supplying alternating current to the capacitor plates, the current flow depends on factors including the dielectric constant of the material separating the plates, which varies with the fuel level and its specific gravity, allowing the system to measure the mass of fuel.

How does the capacitive system compensate for changes in fuel temperature?

The capacitive system compensates for changes in fuel temperature by using the relationship between the dielectric constant of fuel and its specific gravity. As the temperature changes, the specific gravity changes, which in turn affects the dielectric constant, allowing the system to measure the mass rather than the volume of fuel.

What is the purpose of having multiple capacitor probes in the tank in the capacitive system?

Having multiple capacitor probes in the tank helps to average the measurement of fuel, compensating for changes in attitude. As the aircraft's attitude changes, one probe might sense an increase in fuel level while another senses a decrease, providing an accurate average reading.

How does the fail-safe circuit in older aircraft work when the capacitive gauging system fails?

The fail-safe circuit in older aircraft drives the gauge pointer slowly towards the empty position when the capacitive gauging system fails, ensuring that the indicator does not show more fuel than is actually present.

What manual backup systems are mentioned for determining fuel quantity on aircraft?

Manual backup systems mentioned include using a dipstick in the top of the tank, a drip stick which is a calibrated hollow tube, and a drop stick or magnetic level indicator which uses a magnet on a float to indicate fuel level.

How is the fuel system instrumentation displayed on modern aircraft?

Modern aircraft display fuel system instrumentation using electrically presented schematic displays, showing information such as fuel tank mass, temperature, valve positions, pump operations, and total fuel on board.