Fire and Ice

The potential for carburetor fires due to over-priming — especially in cold weather

Source: www.faa.gov/news/safety_briefing, By Susan Parsons

During an unusually cold and miserable east coast winter a few years ago, I got this email from a flying club member: “Carb fire. Aircraft grounded.”

Uh-oh. My plans for a cozy day by the fireplace went up in smoke as I scrambled to get information and take care of all the details that arise from even minor incidents. Though the damage was significant enough to require extensive and expensive repairs to the carburetor, there were no injuries, and the airplane was largely unscathed.

With winter weather fast approaching, we chose to focus this issue on helping you mitigate the hazards that winter poses for GA so you can better enjoy some of the benefits (e.g., better performance, clearer skies). My flying club’s experience illustrates one of those hazards. If you fly an airplane with a carburetor, you should always be mindful of the potential for carburetor fires due to over-priming — especially in cold weather.

Prime Conditions for Fire

Even with preheat — essential in some circumstances, and highly desirable in many others — GA airplane engines can be very hard to start in cold weather. Pilots quickly learn that judicious use of the fuel primer can help, and here’s why. In a carbureted aircraft, the primer delivers vaporized fuel directly to the cylinders. This “pre-vaporization” can help with cold-weather ignition because there is otherwise not enough heat already available to vaporize fuel in the carburetor and get the combustion process going in the engine, where it belongs.

The problem arises when too much time passes between priming and attempting to start the engine. Vaporized fuel doesn’t last very long in that state. It quickly pools at the intake manifold, and the excess fuel drips into the cowl, around the air intake, and onto the ground below. Now all it takes is a single spark to ignite the various puddles of fuel and start a fire that could cause major damage or injury.

An Ounce of Prevention

There are easy and obvious ways to reduce the risk of carburetor fires. First is to avoid the need (and the temptation) for extra priming by keeping the engine warm to begin with. Keeping the bird in a hangar is helpful. A friend takes it a step farther: his AMT installed a small oil heater that he can plug into a hangar wall outlet. He uses an “internet of things” style app to activate the gadget only when he plans to fly. Remotely commanding the heater to start before he leaves for the airport gets the airplane’s engine and vital fluids toasty enough for an easy start on cold mornings. (Yes, I am envious.)

If you are stuck with an outdoor tiedown space, it’s worth renting overnight hangar parking from the FBO, if such options are available. My club’s airplane has a rule requiring the pilot to arrange overnight hangar parking if temperatures fall below certain values.

The third option — using preheat devices — is better than attempting a stone-cold start, but it’s the least desirable because it is awkward and often inefficient. It’s also tough on the pilot and passengers, who get colder by the minute even as the engine is theoretically getting warmer.

Regardless of engine temperatures, the most important way to prevent carb fires is to avoid over-priming. Start with the smallest number of primer strokes, and increase up to the limit if the engine does not start. What’s the limit? Theories differ, but many pilots (and schools) limit themselves to four strokes of the primer. Even if the POH allows for up to six strokes — and some do — consider that conditions requiring such measures are telling you something (e.g., you need to preheat!). If the engine starts and then falters, use the same number of strokes of prime, and crank the engine as you push in the last priming stroke. Engine roughness and black smoke are clear signs of over-priming.

What If ...?

Even if you don’t think you are over-priming, you and your passengers should always be prepared for the possibility of a fire. Know the POH procedures by heart, and be prepared to execute them immediately if you see or suspect an engine fire. You and your passengers should also know the location and procedures for using the on-board fire extinguisher. I hope it goes without saying that you should not fly the airplane after any actual or suspected fire until your AMT inspects it for damage.

Keep the fire where it belongs, and enjoy the benefits of winter flying!

Susan Parson is editor of FAA Safety Briefing. She is an active general aviation pilot and flight instructor.