Fuel Gauges and Flowmeters - ATPL 022 — Instrumentation

No cockpit measurement is planned around as carefully as fuel: it is the one quantity a crew computes before flight, checks after refuelling, and watches burn down against a plan for the rest of the day. This lesson covers the two instruments behind that discipline, the fuel-quantity gauge that reports what is in the tanks and the flowmeter that reports how fast it is leaving, together with the unit conversions and the post-refuelling check the exam likes to set as small calculations.

Volume or mass

Fuel quantity can be measured by volume or by mass, and the difference is not bookkeeping. A refuelling truck delivers litres, and a float in a tank senses litres. But the engines run on energy, and the energy in fuel travels with its mass: a kilogramme of kerosene releases the same heat warm or cold, while a litre of it does not, because density changes with temperature. Flight planning, gauge readouts on transport aircraft, and performance calculations therefore work in mass; the link between the two worlds is the fuel's density on the day, typically around 0.8 kg per litre for jet fuel.

The units, and converting between them

Mass: kilogramme (kg) and pound (lb) — 1 kg = 2.205 lb.
Volume: litre (L), US gallon and imperial gallon — 1 US gal = 3.785 L, 1 imp gal = 4.546 L, so 1 imp gal ≈ 1.2 US gal.
Volume to mass: multiply by density — mass = volume × density, with density in kg per litre.

What disturbs the measurement

Two families of effects stand between the fuel and an honest reading:

Temperature — changes the volume of a fixed mass of fuel: warm fuel expands, so the same kilogrammes fill more litres. A volume-based gauge therefore reads more on a hot day with nothing gained, while the mass, and with it the range, is unchanged.
Accelerations and attitudes — move the fuel itself: in a climb, a descent, a turn or during acceleration the fuel surface stays level (or tilts with the manoeuvre's apparent gravity) while the tank tilts around it, so the depth at any single point no longer represents the quantity.

The gauge design answers both. Against attitude and acceleration, the system uses several sensing points spread through the tank and averages them: what one probe over-reads, its partner under-reads. Against temperature, the capacitance system below senses a property of the fuel that itself varies with density, letting the electronics correct the reading toward true mass.

The gauge types

Float system — a float rides the fuel surface on a pivoted arm, and the arm's angle drives a transmitter (a variable resistance or similar) wired to the gauge. Simple and cheap, the light-aircraft standard, but it senses volume at one point: attitude, acceleration and temperature errors arrive undiminished, and the moving parts wear.
Capacitance system — vertical probes in the tank are built as capacitors, and the fuel itself sits between the plates. Fuel and air have different dielectric properties (how strongly the material between a capacitor's plates lets it store charge), so each probe's capacitance follows how deeply it is immersed. No moving parts, many probes averaged across the tank, and, because the fuel's dielectric constant rises and falls with its density, the system can correct toward mass: the transport-aircraft standard.
Ultrasound system — gauging by echo, timing sound pulses off the fuel surface. It exists in service but its learning objectives enter the syllabus at a later date; knowing the name is enough here.

The post-refuelling check

After refuelling, the crew closes the loop between what was ordered, what was delivered, and what the gauges claim. The procedure: record the volume filled from the refueller's meter; convert it with the day's density into the unit the aircraft's gauges use; add it to the fuel that was already aboard to get the calculated contents; and compare that with what the gauges now indicate. If the two disagree by more than the allowed tolerance, the discrepancy must be resolved before flight: the candidate explanations are an indication error, a wrong density used in the conversion, or fuel actually missing (a leak, or an uplift into the wrong tank), and each of them is a reason not to depart on the numbers as they stand. A dripless stick (a calibrated stick read from under the wing that samples the tank level without spilling fuel) or manual dip can arbitrate between the gauge and the paperwork.

Fuel flow and total consumption

Fuel flow is the rate at which fuel is being delivered to the engine, measured by a flowmeter in the fuel line to each engine, downstream of the tanks so it reports what the engine is actually receiving. Like quantity, flow may be measured by volume or by mass per unit of time. By mass: kilogrammes per hour (kg/h) and pounds per hour (lb/h). By volume: litres per hour, imperial gallons per hour and US gallons per hour. Transport aircraft indicate mass flow, for the same reason the quantity gauges do: the flight plan's numbers are mass.

Total fuel consumption is obtained by totalising the flow over time: the flowmeter's counter accumulates every kilogramme that passes, and the result is displayed as fuel used. Steady numbers multiply simply (a flow of 1,200 kg/h held for two hours is 2,400 kg used), and the totaliser does the same sum continuously when the flow varies. The pair of numbers it enables is the everyday cross-check of fuel management: fuel used plus fuel remaining should equal what the tanks started with, and a mismatch between the totaliser and the quantity gauges is investigated with the same seriousness as the post-refuelling discrepancy.

What the exam asks

Conversions and the post-refuelling arithmetic are the bank's favourite calculations here. The overview, following the learning objectives:

State that fuel quantity can be measured by volume or mass

List the fuel-quantity units and convert between them

Explain the parameters affecting the measurement and how the gauge design compensates

Describe and explain the float and capacitance fuel-gauge systems

Describe and complete the pilot's post-refuelling procedure

Define fuel flow, where it is measured, its units by mass and by volume

Explain how total fuel consumption is obtained