A primer draws fuel from the tanks and injects the vaporized fuel directly into one or more of the engine's cylinders or the induction manifold to aid in starting. Typically situated near the throttle and mixture controls, the primer knob is located on the instrument panel, and two pumps of the primer are usually enough to prime the engine before the first start of the day.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a primer in aircraft?
A primer in aircraft is a manual fuel primer that injects vaporized fuel directly into one or more of the engine's cylinders or the induction manifold. Typically, two pumps of the primer are used to prime the engine before the first start of the day, but fuel-injected engines do not require primer. Some POHs recommend against priming an already warm engine.
The primer knob is typically located on the instrument panel near mixture controls, and the primer lines are ordered or installed by the airframe manufacturer. The fuel primer is used to vaporize fuel directly into cylinders; fuel is pumped to intake ports located upstream of inlet valves, and additional fuel enriches mixture during start. Many aircraft have a dedicated engine primer, yet some aircraft are produced with only one cylinder primed, so the number of cylinders that are primed must be contemplated since the total fuel delivered by the primer will be divided and sent to these cylinders. Engines with only one cylinder being primed are extremely hard to start in cold weather. A carbureted airplane engine usually needs two or three strokes of prime in the summer and more strokes when the air and engine are really cold. Before winter, maintenance is needed to have primer lines checked, and the pilot must confirm the primer is locked by pulling on it to make sure it does not move; the primer knob is locked in the closed position after use.
What does the fuel primer do in aviation?
The fuel primer is used to draw fuel from the tanks and vaporize it directly into the cylinders prior to starting the engine. During cold weather, engines are difficult to start because there is not enough heat available to vaporize the fuel in the carburetor. The primer helps by admitting an extra slug of fuel to the cylinders without using the carburetor. When you pull the primer's plunger on the instrument panel, fuel is drawn from the lines usually just beyond the gascolator and is then pumped straight to the intake ports of the engine just upstream of the inlet valves. Small fuel lines and special nozzles create a fine mist as the fuel is injected directly into the engine cylinders or the induction manifold. This priming system ensures easier starts and reduces the chance of an engine compartment fire.
