Cold Weather Starting: Lighting It Slowly

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Starting a cold aircraft engine is almost an art form. Here's how to keep from damaging your engine on startup when the frost is on the pumpkin—and maybe the spark plugs.

Daylight Saving Time got switched off over the weekend, to the anguish of more than a few of my friends. Not only do they like sunlight of an evening, the return of Standard Time means they can no longer deny the nearly imminent arrival of winter and its challenges to aviating. It also seem to be when I hear numerous pilots start inquiring as to what is “cold” for a piston aircraft engine. At what temperature are you risking damage to your engine if you start it without some sort of preheating?

What’s good for those asking the question is that there is definitive guidance on the subject from the engine manufacturers — Continental Service Information Letter 03-1 and Lycoming Service Instruction 1505. If you want to get the straight story on what constitutes cold for your engine, as well as what to do about it, those publications will tell you in no uncertain terms.

In masterful understatement, the headline on Continental’s Letter pronounces: “Contains Useful Information Pertaining to Your Aircraft Engine.” You want to know when preheat is required? It tells you: “When the engine has been exposed to a temperature of at or below 20 degrees Fahrenheit (wind chill) for two hours or more.” As an aside, we do want to apply a dope slap to the writer who didn’t know that wind chill is not relevant to inanimate objects—moving air cannot cool an object to below ambient temperature, no matter how hard the wind is blowing. Despite that error, if your engine has been exposed to ambient air temps of 20 degrees F for two hours or more, you need to preheat the engine to protect it from damage on starting.

Lycoming’s Instruction states that preheating is required when the engine has dropped to a temperature of 10 degrees Fahrenheit or 20 degrees for its -76 engines.

Preheat Matters

Lycoming is as blunt as Continental about cold starting risks: “Improper cold weather starting can result in abnormal engine wear, reduced performance, shortened time between overhauls or failure for the engine to perform properly. “ We are of the opinion that the “or” in that sentence should be “and.”

Continental warns, “Failure to properly preheat a cold-soaked engine may result in oil congealing in the engine, oil hoses and oil cooler with subsequent loss of oil flow, possible internal damage to the engine, and subsequent engine failure.

Have they made themselves clear?

There are four generally accepted methods of getting a piston aircraft engine warmed up to a temperature at which it can be safely started and operated: 1. sticking the airplane into a heated hangar; 2. a high-volume hot air heater (preheater); 3. an engine-mounted electric heating system; 4. warm weather.

Item 4 is self-explanatory—pilots sometimes find themselves where there simply isn’t a method of safely heating the engine to above 20 degrees F. You may be able to start the engine, but that start could cost you a bunch of money downstream as the damage may not show up right away. “It’s a rental,” is not an excuse.

As pilots flying in mountainous areas often have to wait for the wind or temperatures to drop before they can fly, pilots at airports without preheaters or heated hangars need to wait for the temperature to warm up.

That also applies if you have single weight oil in the engine and it’s not the right viscosity for the cold temperature. Even though you may be able to preheat the engine, summer-weight oil may not provide adequate lubrication.

If nature’s warm air is not available, a heated hangar is second best—it warms up the entire airplane. Continental says to allow four hours to assure congealed oil is flowing.

From a technique standpoint, once the time in the heated hangar is drawing to a close, the pilot should have everything ready to go the moment the airplane is towed out—ideally with everyone in the airplane so the pilot can hit the starter as soon as the tug is clear. It helps to open the windows until the engine is running and the heater putting out warm air as windows have a tendency to fog and ice over quickly otherwise due to the respiration of the aircraft occupants.

Preheaters

Continental and Lycoming both provide guidance on the use of preheaters. Lycoming specifically disapproves the use of oil dipstick heaters because they don’t distribute the heat throughout the engine. We agree, they don’t cut it. Both manufacturers are explicit in calling for careful use of high-volume hot air preheaters to assure that all of the engine is heated—oil sump, external oil lines, cylinders, air intake, oil cooler and oil filter. Be careful not to damage non-metallic components such as seals, hoses, and drive belts. As someone who once unknowingly melted the sleeve of a down jacket using a forced-air preheater and then filled the cabin of the airplane with feathers on getting inside to start up, be careful where you point the heater and what you touch with hot parts of it.

Continental says to preheat for a minimum of 30 minutes. Lycoming says to apply heat in five to 10 minute intervals and then “feel the engine to be sure that it is retaining warmth.” It goes on to say that during the last five minutes, the heat should be directed to the top of the engine.

Once preheating is complete, both manufacturers call for starting the engine immediately. We agree—we’ve seen too many pilots finish preheating, then start setting up their iPads and plugging in the headsets over 10-15 minutes and then discover the engine won’t start because it’s gotten cold again. Have everything ready to go.

Engine-Mounted Heaters

Let the engine warm up to temps recommended by the manufacturer before bringing the power above idle while watching the oil pressure.

We like engine-mounted preheating systems. They have been reviewed periodically in our sister publication Aviation Consumer. Continental recommends a system that includes “individual cylinder head heater thermocouples, oil sump heater and crankcase heater pad.” Having owned three airplanes with engine-mounted preheaters, I have found them to be handy, especially when traveling as the combination of a blanket over the cowling and a long extension cord allows preheating at almost any airport. For more remote airports, I carried a generator to run the system for four or five hours before I wanted to start.

Continental warns against running engine-mounted preheaters continuously due to concerns with corrosion, however, so long as the entire engine can be kept above the dew point, there is no place for moisture to condense and cause corrosion. There are a number of devices that allow remote control of the engine-mounted heater via cell phone.  These were reviewed in the November 2017 issue of Aviation Consumer. Owner feedback has been positive.

Starting

Both Continental and Lycoming urge immediate starting after preheating is complete. They caution the pilot to assure that the start is made at low engine RPM, not more than 1000, due to risk of cylinder damage from lack of lubrication and to assure that oil pressure comes into the acceptable range soon after start.

Pumping the throttle before or during start is not a good idea. It creates a high risk of engine fire on a cold start. Pumping the throttle more than once usually does nothing but flood the carburetor.

For a carbureted engine, the proper procedure is to use the primer to put fuel directly into the cylinders. Many operators recommend leaving the primer out and letting it fill with fuel after the last pre-start priming shot. Then, as the engine is cranking and fires, give another shot or two of prime.

Continental goes into detail regarding post start procedures in cold weather. Briefly, it calls for frequently checking oil pressure to assure that there is not congealed oil somewhere in the system that can cause engine damage—it will manifest itself by high or low oil pressure indications. Do NOT let the RPM exceed 1000 until some oil temperature is indicated. This is important—we’ve all seen the pilots who start the engine at 1500 or 1700 RPM; they’re damaging the engine, hot or cold. When the checklist says “throttle cracked,” it means a small fraction of an inch, not half-way to the firewall.

Continental says that if the oil pressure cannot be maintained above 30 psi or below 100 psi, shut down and repeat the preheat process. It also says not to close the cowl flaps during engine warm up.

Once oil temperature is indicating, the engine may be operated as high as 1700 RPM, however, it should be approached gradually to make sure oil pressure does not exceed 100 psi. The runup can be conducted. Continental recommends cycling the propeller three or four times to move cold oil out of the propeller dome. On feathering propellers, do not let the RPM drop more than 300.

Only when oil temperature exceeds 100 degrees F and oil pressure does not exceed 60 psi at 2500 RPM, is the engine sufficiently warmed to accept full rated power.

I'll add the suggestion that it’s a good idea to take an absolute minimum of five seconds in going from idle to full power during a cold weather takeoff—at least 10 seconds is probably better. I’ve seen engines cut out with rapid throttle movement in cold weather.

When the temperature is above 20 degrees F and below 40 degrees F, Continental does not require the use of preheat, but it recommends that the post preheat engine starting procedures regarding RPM, oil pressure and oil temperature be followed.

Conclusion

For a piston aircraft engine, cold has a definition: 20 degrees F and below. Starting one below that temperature, without preheat, is almost guaranteeing internal damage.

Rick Durden is a CFII and ATP with type ratings in the Douglas DC-3 and Cessna Citation and is the author of The Thinking Pilot’s Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing It, Vols. 1 & 2.