A Primer On Primers

We don’t give much thought to starting the engines in our cars, even in the dark depths of winter, thanks in part to microprocessor controlled electronic fuel injection. Starting up an airplane, however involves a bit more hands-on skill. With colder weather on the way (and, in some areas, well under way), this might be a good time to revisit the one critical component of the induction system that we usually lay our hands on first: the primer.

(Okay … Strictly speaking, some might argue that the first thing might really be pushing the mixture in to the full rich position. Picky, picky, picky…) One price we pay for the simplicity and reliability of even the most humble piston powered aircraft engine is a fuel delivery system that is primitive in comparison. Primitive or not however, in addition to delivering fuel to the cylinders in the right quantity and proportions for efficient operation, it must also supply fuel for initial starting.

First, we all know about carburetors. For carburetor-equipped aircraft, most carburetors are usually underneath the engine. For those carburetors with accelerator pumps (which reduce power lag by spraying extra fuel when extra muscle is called for), pumping the throttle in and out as you’re activating the starter motor only fills the carburetor air-box with raw fuel, which can be a fire hazard. That’s the wrong way to start a carbureted engine. Never pump the throttle of an engine having an updraft or side-draft carburetor unless the engine is actually being turned over by the starter motor. This draws the atomized fuel into the cylinders. Otherwise, the fuel mist that you inject into the intake manifold of a non-rotating engine can just drip out the carburetor and into the air box, which is when the excitement usually begins.

THE RIGHT WAY (after a proper preheat, if it’s at all cold) is to use the primer.

First, let’s do a little upstream navigation though the fuel system. Starting up at the fuel tanks — just for the sake of discussion, we’ll take a gravity feed system, say that of a Cessna 172. Fuel flows through a very coarse filter (a screen or strainer, only about 15 mesh/inch), across a fuel shut-off valve, and on down, often to the lowest point in the fuel system, which is the so-called gascolator. This is just a filter screen (about 60 mesh/inch, four times finer than the first one) and often a removable sediment bowl (which also functions as a water separator). Some have quick-drain valves for draining during preflight. Incidentally, gascolator bowl assemblies can potentially create fuel flow interruptions if the bowl leaks, or worse, falls off; if you have one, it’s a good thing to include in your preflight, as well as any associated drain.

After the gascolator comes the primer pump. When you pull that knob out from its knurled base, you hear the rising sound it makes as it sucks in fuel, downstream of the gascolator. (Pull s-l-o-w-l-y; if you pull too fast, less fuel will be drawn in.) Then, each time you push it back in, fuel is forced in another direction, not back into the gascolator, but onward into the cylinders. This trip is totally separate from (it bypasses) the carburetor. This is what you need to assure having a sufficiently rich fuel-air mixture for combustion and engine start when you crank the starter motor: raw fuel in carefully metered amounts, right into the intake ports.

A cold engine needs that much richer a mixture (though it’s just to get started). The colder the day, the more priming you need to do. How much is the right amount? Start with the POH, tempered by sage advice, your own ears, and experience. As a general rule, it’s best to prime conservatively less rather than more. One rule of thumb says to use no more than one stroke per primed cylinder. (It would be good to know just how many primed cylinders your engine may have. If you don’t know, ask your friendly A&P, crew chief, or someone who knows the airplane. For a Cessna 152, it could be three; for a Cessna 172, it may be just one; for a Super Cub, four; a 172RG, four as well.) Except for the risk of running the battery down, the next worst thing that under-priming gives you is practice at starting an engine (with perhaps a few character building moments of frustration as well).

Caution: Over-priming is not a good thing. In addition to flooding the engine and/or fouling the spark plugs, the cylinder walls and piston rings may be washed free of oil, which puts them at great risk of dying an early death as pistons grind up and down in the cylinders. Additionally, if you over-prime, liquid fuel may flow down through the intake manifold into the air-box, where it too may be ignited by a minor backfire during engine start (you’ll want to continue cranking to suck the flames back into the engine).

When it’s really cold, you can buy one last power play if you leave the primer knob out when you start cranking. Slowly pumping the primer (a darn sight smarter than pumping the throttle) admits extra fuel into the combustion chambers, which might be what you need to keep the engine going those few additional seconds. Fuel that has been atomized in the carburetor has to travel much further, against gravity, and at low temperatures; it can fall from suspension resulting in too lean a mixture before the engine has had that time it needs to warm up. Another tip in cold weather starting is to pull on the carburetor heat after the engine initially starts. This provides extra heat to help keep the fuel-air mixture atomized.

THE BOTTOM LINE: Like closing your flight plan though, once you’re done with the primer knob, be sure it’s latched back into its slot; if you leave it partly open, fuel can siphon from the primer into the intake, resulting in an overly-rich mixture. If it’s already warm, do we need to use the primer at all? If the engine was running an hour ago, or less, you probably don’t. Try it without priming, first. If it doesn’t catch after a few turns of the prop, stop and proceed with normal priming. (Just be mindful of the workload on the battery.) But remember that the way to sound your engine’s wake-up call is by using the primer, not the throttle!