Definition
Fuel inside a fuel injection or carburetor system that is upstream of the metering unit — meaning it has not yet had its flow rate controlled or measured by the fuel metering device. It is at full pump pressure and full available volume, before being regulated for delivery to the engine.
Plain English
Fuel that is in the system but has not yet been measured out for the engine. It is sitting at pump pressure, waiting to be regulated.
Context Anchor
Seen in fuel system descriptions, engine diagrams, and maintenance discussions about where fuel is before or after the fuel-measuring part of the system.
Derivation
‘Metered’ comes from the Greek metron, meaning ‘measure.’ Unmetered simply means ‘not yet measured.’ In a fuel system, the fuel becomes ‘metered’ once the metering unit sets its flow rate; before that point, it is ‘unmetered.’
Why Pilots Care
Knowing unmetered fuel pressure helps confirm the pump is delivering adequate supply and isolates problems in filters, lines, or the metering unit before they affect engine power.
Analogy
Think of pouring fuel into a measuring cup before adding it to a machine. Before it reaches the measuring cup, it is available but not yet measured; after it passes through, the amount has been controlled.
Intuition Check
Unmetered does not mean unknown fuel, unusable fuel, or extra fuel. It means the fuel has not yet passed through the part of the system that measures how much goes to the engine.
Example Sentence 1
The fuel pump delivers unmetered fuel to the fuel control unit, where its flow is regulated before it reaches the injector nozzles.
Example Sentence 2
A drop in unmetered fuel pressure during run-up can indicate a weak pump or a clogged inlet filter.