Fuel Quantity Indication: Float and Capacitance Gauging Systems

022-01-03Pablo Asensio Martínez2026-04-012 min

Accurate fuel quantity indication is vital for flight safety and efficiency. Systems aim to determine the mass of fuel on board, as this determines the energy available, unlike volume which changes with temperature.

Verification

After refueling, pilots must verify the fuel load:

  • Ensure Fuel On Board (FOB) matches the flight plan.
  • Check that any imbalance is within limits.
  • Compare planned vs. actual uplift (in kg or lbs) to identify errors or leaks. Discrepancies outside Aircraft Flight Manual (AFM) limits require maintenance.

Fuel Gauging Systems

Float-Type System

This is the most basic system, typically found in smaller aircraft.

  • Principle: A mechanical float sits on the fuel surface. As the level changes, it moves a mechanical arm connected to a variable resistor (DC circuit), changing the cockpit indication.
  • Limitation: It measures volume, not mass. Fuel expands when warm and contracts when cold. At high temperatures, the volume increases, causing the gauge to read higher even though the mass remains the same. It is also susceptible to errors during maneuvers and turbulence.

Capacitance-Type System

Modern aircraft use capacitance systems to measure fuel quantity with greater accuracy.

  • Principle: Measures the capacitance between two plates (tubes) in the tank. The capacitance depends on the dielectric material between them.
  • Dielectric: The system distinguishes between fuel (permittivity ~2.0) and air (permittivity ~1.0). As fuel replaces air between the plates, capacitance increases (e.g., Full = 210pF vs Empty = 100pF).
  • Volume to Mass: A compensator unit (or densitometer) located at the bottom of the tank is always immersed in fuel. It measures the fuel's specific properties to account for density changes due to temperature. This allows the system to electronically convert the measured volume into a precise mass indication.

Units and Conversion

  • Mass: Kilograms (kg), Pounds (lb). ($1 \text{ kg} \approx 2.2 \text{ lb}$)
  • Volume: Liters (L), US Gallons (US Gal).
  • Conversion: $1 \text{ US Gal} \approx 3.785 \text{ Liters}$.
  • Example: To check uplift, if the gauge requires verifying 272 Liters against a US Gal dispenser: $272 / 3.785 \approx 72 \text{ US Gal}$.