Primed for Start

Engine start should be such a simple thing. Why then do so many pilots scramble to get the prop turning?


by Ray Leis

We have a pilots meeting in West San Antonio once a month. Before the infamous 9/11 we were a large instructor pilots group, but now, due to the heavy shrinkage of students and active CFIs, we have become a welcome all pilots group.

Several subjects are tossed around for comment at each meeting, along with all of the warnings about active construction projects on the municipal airport and the ever-increasing MOAs developed by nearby military operations.

Recently, I was surprised to hear the heated discussions, caused by a casual question from a private pilot, on the use of the engine priming system. From the 20 pilots there seemed to be 20 opinions. Most held for pumping the throttle, with others holding for turning on a boost pump. A staunch group of three said that you are better off not using any prime at all.

During numerous BFRs and instrument proficiency rides, I have observed some confusion (and delays) in the way various pilots start airplane engines. Poor technique can prevent the engine from starting or, even worse, lead to engine compartment fires.

Gentlemen, Start Your Engines
The automobile engine is certainly the easiest kind of reciprocating engine to start because the huge market of automobile owners demands it. Automobile engine starting procedures have evolved from an early nightmare of hand cranks, chokes, manual throttles and instruction books that werent much help. But now it works.

No matter what the weather or the temperature, simply turn the ignition key and the computer-controlled electronic fuel injection does the rest instantly.

Mostly because of the need for simple design and the burden of cost that results from low production, the light plane aircraft engine is dated. As pilots we have to deal with shaky fuel delivery systems, magnetos that may or may not deliver the required sparks, fixed timing and a bulky pre-start list of items.

Carbureted aircraft engines have primers because the typical carburetor is the updraft type, mounted below the engine. Any fuel that collects in the carburetor when the engine is stopped drains out by gravity onto the ground or the bottom of the engine cowling.

The priming system aims to get fuel to the cylinders quickly to ensure easier starts. Yet the primer is not the weapon of choice many pilots use when doing battle with a balky engine.

Pumping the throttle, when the carburetor is not equipped with an accelerator pump, puts the starting fuel in the air box or on the ground, where it cant reach the cylinders. Carburetors that are equipped with accelerator pumps are mechanically linked inside the carburetor to a unit that will spray extra fuel into the carburetor throat when the throttle is opened.

The function of the accelerator pump is to increase the normal fuel output during engine acceleration to prevent the engine misfiring. This smoothes out the engine while airflow and fuel flow are getting balanced.

With the accelerator pump-equipped carburetor, it is possible to pump the throttle, delivering raw fuel to the carburetor throat, prior to starting, even though there is no airflow through the carburetor. This does not, however, guarantee that the fuel gets to a place and quantity that does the most good for starting. Where you want the fuel to go for starting is straight to the intake ports of the engine, just upstream of the inlet valves. Thats the job of the primer.

Different Primers
By using the primer to get a sufficiently rich fuel/air mixture to the combustion chambers, the engine usually starts when it is first turned over.

Carbureted engines typically have one of three different kinds of primer systems. Some primer systems feed fuel directly to only one or two cylinders. Others use individual fuel lines to deliver fuel to the cylinders. Still others deliver a spray of fuel to a point just downstream of the carburetor. Fuel injected engines typically use the fuel pump or a dedicated manual primer to send fuel to the injector body.

Cessnas switch from carbureted engines to fuel injection in 172s is said by some to be the biggest problem that pilots encounter when switching from older models to newer ones. The Pilots Operating Handbook or Airplane Flight Manual will tell you the kind of system involved and give some hints for getting the engine to fire, but thats far from foolproof.

The primers for carbureted engines are generally the manually operated piston/plunger (Kohler) types. The plunger draws fuel from the main fuel strainer, or a place downstream of the fuel selector valve, up to the plunger itself.

Its basically a syringe, sealed by rubber O-rings. The O-rings make a tight seal, preventing fuel from leaking around the plunger to the engine when the engine is shut down. The seals also prevent fuel from coming in to the cockpit. Check valves are part of the unit, to ensure that the primer plunger sends fuel in only one direction, from the strainer or main fuel line to the intake manifold.

Some airplanes, such as many carbureted Pipers, use the electric fuel pump as a primer pump as well. Some have a manual pump and an electric boost pump.

A boost pump is not needed to start a carbureted engine. Its a good precaution to turn it on before starting, to purge vapor or air in the fuel lines. It is also a good time to see if the boost pump and the fuel-pressure gauge are working.

Priming a fuel injected engine generally involves using an electric boost pump to send fuel to the cylinders. Light aircraft use continuous-flow port fuel injection systems, which are actually finely calibrated full-time primer systems.

Priming for starting is simple. Put mixture full forward, activate the fuel pump for 5 to 10 seconds, or until a positive indication is shown on the fuel flow indicator. Verify throttle is about three-quarters. Priming is complete. Ready to start.

Dearth of Info
Surprisingly, most pilot operating handbooks and engine operators manuals contain only sketchy information regarding use of the primer system. Some advise to use 1 to 3 shots when the engine is cold, some even less than that. For example, a Piper Warrior II POH contains this illuminating instruction for using a primer: An optional engine priming system is available to facilitate starting.

Even the Lycoming Operators Manual provides little insight, saying simply, prime with 1 to 3 strokes of manual priming pump…

In may be little wonder, then, that some pilots get frustrated with what they see as uneven results and fall back on pumping the throttle to get the engine to fire. And fire it may.

Engines of more than 115 hp generally are equipped with accelerator pumps, which give an extra shot of fuel if the throttle is moved briskly. The pump puts fuel directly into the carburetor throat, allowing an extra rich mixture to get sucked into the cylinders.

However, updraft carburetors may allow fuel deposited by an accelerator pump to pool under the engine, creating the danger of a fire in the engine compartment. By contrast, the primer pumps fuel right to the cylinders, so the chance of an engine compartment fire is much reduced.

If you have become one of the many pilots who dont use the primer to start your engine – opting to pump the throttle instead – you may want to reconsider your technique. You may be able to get better starts. The POH might give you a number of primer plunger strokes to use under normal conditions, but it may also leave you to your own devices.

Make sure your primer system is not clogged and working properly. The primer nozzles may be clogged. The nozzles can carbon up, being close to the intake valve. The nozzles may not carbon up the same amount, so that one cylinder could get all of the prime. Primer nozzles need to be cleaned at least once a year.

Talk to owners of similar aircraft. Experiment on your own. Eventually you will hit on the magic combination that fires your engine the most reliably. For some engine/airframe combinations, that may even include pumping the throttle, but work to minimize your chances of unintended consequences if you take this route.

The primer system can be a great aid in engine starting, if youll take the time to give it a little study.

Also With This Article
Click here to view “Using the Manual Primer.”
Click here to view “Hard Starting.”
Click here to view “Primer Problems.”

-Ray Leis is an ATP, CFII and Aviation Safey Counselor.


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