Definition
In a turbine engine fuel system, the fuel that flows through the primary fuel nozzles or the primary slots of duplex nozzles during engine starting and low-power operation, producing a wide, well-atomized spray pattern at relatively low fuel flow rates.
Plain English
Primary fuel is the small, finely sprayed flow of fuel that a turbine engine uses for starting and low-power running. At higher power, a second, larger fuel flow joins it.
Context Anchor
Seen in turbine engine fuel system discussions, especially when describing duplex fuel nozzles, starting, idle operation, and the transition to higher power.
Derivation
From Latin primus, meaning 'first.' It is the 'first' fuel flow the engine uses — the one present from light-off through low power, before the secondary flow comes online.
Why Pilots Care
Knowing which fuel path is primary helps diagnose power loss and confirms the engine is receiving its normal operating fuel flow.
Intuition Check
Primary does not mean the main fuel tank or the best fuel. Here it means the first fuel flow path used in a turbine engine fuel nozzle or manifold.
Example Sentence 1
During engine start, only primary fuel is sprayed into the combustion chamber, producing a fine mist that lights off easily.
Example Sentence 2
At low power the engine used only the primary fuel nozzles; the secondary nozzles opened above 70 percent thrust.