How do Boeing 737 fuel gauges and tank indications work?
Learn how Boeing 737 fuel gauges and tanks work, including capacitance probes, units, centre-tank feed logic, alerts and simulator faults.
On a Boeing 737, the fuel indications show calculated fuel mass in tank 1, the centre tank and tank 2, plus a total where fitted. Multiple capacitance tank units sense fuel level and properties; electronics convert those signals into kilograms or pounds. Pump-pressure lights and fuel alerts report separate conditions.
For our Aviation & Real-World Flying readers, this covers both the real aircraft and its representation in flight simulators. Display layout, alert logic and exact thresholds vary across the 737 Original, Classic, Next Generation, MAX and specially modified aircraft.
What do TANK 1, CTR, TANK 2 and TOTAL mean?
The three primary Boeing 737 fuel indications represent separate tanks; CTR means centre tank, not reserve fuel.
| Indication | Tank and location | Normal feed role |
|---|---|---|
| TANK 1 | Left main tank in the aircraft’s left wing | Normally supplies engine 1 through the left fuel manifold. |
| CTR | Centre tank in the wing centre section | Can supply both engine feed manifolds while its pumps are operating and producing pressure. |
| TANK 2 | Right main tank in the aircraft’s right wing | Normally supplies engine 2 through the right fuel manifold. |
| TOTAL | Sum of the indicated tanks | This is a calculated total, not another physical tank. |
Tank 1 and tank 2 identify the left and right sides; the numbers do not specify which tank must be consumed first. Crossfeed configuration, pump pressure and centre-tank operation can change which tank supplies an engine.
The figures are fuel mass, not volume. Check both the unit legend and any multiplier: on a display explicitly marked KGS X 1000, for example, a value of 5.20 represents 5,200 kg. Pounds, kilograms, litres and US gallons cannot be compared directly without applying the correct fuel-density conversion.
A last-digit difference between TOTAL and the manually added tank figures can result from each value being rounded separately. A large or persistent mismatch is not explained by rounding. Older 737s may use dedicated gauges, while later flight decks place the quantities on an engine or system display.
Where are the Boeing 737 fuel tanks located?
A standard airline 737 has two integral wing main tanks and a centre tank within the wing centre section.
| Area | What is there | What the crew sees |
|---|---|---|
| Left wing | Main tank 1 | TANK 1 quantity |
| Between the wing roots | Centre tank | CTR quantity |
| Right wing | Main tank 2 | TANK 2 quantity |
| Outboard wing areas | Surge and vent spaces | Not a normal selectable supply-tank indication |
For readers searching for a 位置圖, meaning a location diagram, the simple left-to-right arrangement is tank 1, centre tank, then tank 2. Actual maintenance drawings show several tank units distributed through each irregularly shaped tank rather than one sensor in the middle.
Business-jet and other special configurations can have auxiliary fuel systems. Those installations may transfer fuel into the standard system, so a three-tank diagram must not be used to identify components on a modified aircraft.
How is fuel measured in aircraft and on the 737?
The 737 measures fuel quantity with multiple capacitance tank units, often called capacitance probes, rather than relying on a single float.
A probe acts as a capacitor. Air and fuel have different dielectric properties, so its capacitance changes as more or less of the sensing element is immersed. The fuel quantity indication system combines the probe signals, applies calibration for the tank’s shape and compensates for fuel properties before presenting an indicated mass.
The exact electronics differ by generation and installation. A system may include tank units, a compensating device, associated wiring, signal-processing equipment and the cockpit display. Our fuller explanation of how aircraft capacitance probes produce a fuel indication covers the underlying measurement process.
Multiple probes reduce errors caused by the shape of the wings and changes in aircraft attitude. Signal averaging and display damping also limit rapid movement from fuel slosh, although a reading may take time to stabilise after refuelling or aircraft movement.
An indicated zero does not prove that the tank is physically dry. Residual or unusable fuel can remain below the point represented as usable quantity, and fuel trapped away from a pump inlet may not be available to an engine.
What does “fuel quantity probe” or “fuel quantity tank unit” mean?
A fuel quantity tank unit is an in-tank sensing component, normally one element of the capacitance measurement network. It is not the fuel pump, cockpit gauge or complete fuel quantity system.
One cockpit tank value usually represents the processed output from several tank units. A probe’s position and calibration matter, so two units that appear physically similar are not automatically interchangeable.
What do EA18B-692, EA379A-1440 and Avien 165-022-318 or 165-045-484 mean?
EA18B-692, EA379A-1440, Avien 165-022-318 and Avien 165-045-484 should be treated as component-identification references associated with fuel quantity tank units, not as fuel readings or cockpit fault codes.
A bare identifier does not prove that a unit fits a Boeing 737, identify which tank or probe position it occupies, or establish interchangeability. The complete dash number can represent a materially different configuration, and applicability may depend on 737 series, aircraft serial-number effectivity and incorporated modifications.
- For identification: Record the complete manufacturer, part number, serial number and modification status from the data plate rather than relying on one stamped number.
- For aircraft and location applicability: Check the effectivity and illustrated location in the aircraft’s approved Illustrated Parts Catalogue under ATA 28.
- For testing or repair: Use the applicable Component Maintenance Manual; an aircraft location drawing does not provide bench-test limits.
- For installation or purchase: Require approved interchangeability data, serviceability and traceability. A matching appearance or partial number is insufficient.
We cannot responsibly assign any of these exact identifiers to tank 1, the centre tank or tank 2 without the aircraft-specific parts data. That is especially important for removed or surplus units, where a web description may omit effectivity and modification details.
Is an uneven or changing fuel indication normal?
Это нормально? means “Is this normal?” In a 737, some movement and small differences are expected, but a frozen tank value, impossible total or sustained unexplained imbalance needs investigation.
| Observation | Likely explanation | Assessment |
|---|---|---|
| Centre quantity decreases before the main tanks | Centre pumps normally produce higher feed pressure. | Normal with the required pump configuration. |
| TOTAL differs by the final displayed digit | Individual and total values may be rounded separately. | Usually normal. |
| Small left-to-right difference | Loading tolerance, consumption, attitude or display resolution. | Can be normal; compare it with the limit for the exact aircraft. |
| Reading settles after refuelling or movement | Fuel slosh, damping or simulated loading logic. | A brief change can be normal. |
| One value remains frozen while the others change | Tank-unit channel, wiring, processing or simulator-state fault. | Not normal. |
| Blank indications in a cold-and-dark simulator aircraft | The display or indication system may not yet have electrical power. | Can be normal until the correct buses are powered. |
| LOW PRESSURE with substantial fuel in the tank | The pump may be off, failed or not producing sufficient pressure. | Fuel quantity alone does not explain the light. |
Why does the centre tank quantity decrease first?
The centre tank normally empties first because its pumps supply the engine feed manifolds at a higher pressure than the main-tank pumps.
When centre-tank pressure is available, that pressure takes priority at the feed system. Once centre pressure is removed, the main tanks resume supplying their respective sides. This is feed sequencing by pump pressure; the centre fuel is not first transferred into the wing tanks.
The crossfeed valve connects the left and right feed manifolds but does not, by itself, pump fuel from one main tank into the other. Pump and crossfeed selection can make both engines consume from one side, as explained in our guide to 737 fuel feed, crossfeed and balancing behaviour.
Do LOW PRESSURE lights mean a tank is empty?
No. A fuel-pump LOW PRESSURE light means that the monitored pump output pressure is inadequate under the applicable switch and system logic; it is not a direct quantity measurement.
- LOW fuel quantity: On common 737NG installations, an amber LOW indication appears when either main tank falls below approximately 2,000 lb or 907 kg.
- LOW PRESSURE: This may result from depletion, an uncovered pump inlet, an unpowered or failed pump, or another supply problem. A tank can contain fuel while its pump produces low pressure.
- FUEL CONFIG: Common NG logic includes a warning when more than roughly 1,600 lb or 726 kg remains in the centre tank with both centre pumps off, or when the main-tank imbalance exceeds roughly 1,000 lb or 453 kg.
Those figures describe commonly published NG logic, not every 737. Engine-running conditions, timing, ground or air logic, display software and aircraft series can affect the alerts. Real crews follow the aircraft-specific flight crew manual and non-normal checklist rather than treating generic thresholds as operating instructions.
Why do simulator fuel gauges look wrong?
Simulator discrepancies most often come from mixed units, incorrect tank distribution, missing electrical power or two fuel-loading systems overwriting each other.
- Match the units and scale. Confirm whether the cockpit, simulator fuel menu and aircraft loader use kilograms, pounds, litres or US gallons. Also check for an
X 1000display multiplier. - Use one loading method. If a detailed add-on provides an electronic flight bag or dedicated loader, use the method specified by that aircraft. Editing the simulator’s native fuel menu afterwards may overwrite the add-on, or the add-on may overwrite the native values during initialisation.
- Inspect all three tanks. A plausible total can hide an impossible distribution. The main tanks should normally be balanced, while centre fuel must follow the loading rules modelled for that variant.
- Supply the required electrical power. Battery power alone may not energise every display or processing channel. Follow the aircraft’s power-up procedure; our 737 cold-and-dark simulator procedure provides the relevant panel context.
- Allow the aircraft to initialise. Park level, complete refuelling and let a detailed aircraft model finish loading its saved state. Some basic models update instantly, while more complex ones simulate delays.
- Separate pressure from quantity. A centre-pump LOW PRESSURE light after centre fuel has been consumed does not mean the quantity gauge is faulty.
- Check failures and persistent state. A deliberately failed probe, saved maintenance fault or corrupted aircraft state can freeze one indication. If the same tank remains wrong after a clean reload using one loader, the problem is more likely an add-on fault than normal fuel behaviour.
A mistake we see constantly is comparing a cockpit value in kilograms with a loading screen in pounds or gallons. Compare like with like before changing fuel quantities or diagnosing a failed gauge.
Can a Boeing 737 fuel gauge show flying time remaining?
No. The gauges show fuel mass; they do not directly show endurance, range, reserve compliance or fuel expected at the destination.
A rough endurance calculation divides usable fuel above the required reserve by combined fuel flow, but that ignores climb, descent, wind, routing and changing thrust. A correctly programmed flight-management prediction is more useful, while our analysis of the variables that determine 737 fuel consumption per hour explains why one fixed burn figure is unreliable.