Now that the cold weather is upon us, I have received questions about pre-heating and pre-oiling for aircraft engines.
Aircraft engines are pre-heated not just to thin out the oil, but also to expand the main engine crankcase.
The coefficient of thermal expansion for aluminum is greater than that for cast iron. Therefore, when the temperatures dip down well below freezing, the aluminum crankcase shrinks around the iron crankcase and the bearing clearance that is set at normal ambient temperature is reduced. This means that the oil flow and hydro-dynamic lubrication is limited right after startup. This means that the pre-heat process needs to heat the complete engine assembly and not just the oil.
If an external pre-heater is used, it is important that the heat is applied long enough to properly expand the engine assembly. This is especially important if the ambient temperature is below 0°F. The colder the temperature, the longer the pre-heat is needed.
The expansion of the engine assembly is especially important for pilots who use electric plug-in pre-heaters. A lot of these pre-heat systems use just a few heat pads on the oil pan.
These systems have two problems. First they do not properly heat the crankcase. This is fine if you are starting at an ambient temperature of around 20°F, but is not good if the ambient temperature is below 0°F.
The second problem is that many of these units put a lot of heat into a small area. This results in the surface temperature of the pan reaching above 300°F, which can cause coking and degradation of the oil.
The good plug-in pre-heaters have thermostatically controlled heat strips on the oil pan, plus heat elements on each cylinder. The cylinder element can be mounted in the thermocouple site or to a band around the cylinder. These systems work well especially with a cowl blanket to heat the whole assembly.
Another concern with plug-in systems is that many pilots like to leave their pre-heat system plugged in at all times so their plane is ready whenever they want it.
The problem here is that if the plane is at ambient temperature, the engine and oil will cool down at night and heat up during the day. If there is moisture in the oil, which is quite common in winter operations, then the heat on the oil pan will vaporize the moisture. This will rise until it hits the colder camshaft at the top of your Lycoming engine since the cold air enters the cowl at the top of the engine. This will greatly increase the rusting activity of the critical cam and lifters.
If you have a plug-in system with cylinder and oil pan heaters and you have cowl plugs and maybe a blanket, then I don’t see any problem.
A few months ago, my friend Paul McBride had a nice column about pre-oilers and their limited value in Lycoming engines. However, in Continental engines the cam is below the crankshaft, so if you have a pre-oiler, the oil from the crank bearings will usually drip down onto the critical cam and lifter interface. This is also true for many of the older radial engines.
In addition, I have seen ads for drip rails that can be installed in your Lycoming engine that will drip oil on the cam when pre-oiling. This may be an option at your next overhaul if your plane isn’t flown a lot.
Anytime it is below freezing I preheat. All I have is a red dragon. I run it for 30 minutes and that gets the oil warm and the engine about 40F. Start then wait until I have 300F on cylinders and 100F on oil before putting any power to it.
In WWII they had problems starting the old radials in cold weather. They started putting av gas in the oil to thin it out. Made the engines easier to start. My uncle was a mechanic in the army air corps and started the practice. They busted him to private when they found out but then did a study and found that the gas evaporated by the time the engine warmed up so they gave him a promotion and made it standard practice. I wouldn’t recommend it now. Synthetic oil is good to way below zero. I just wanted to pass on an interesting fact that’s been long forgotten, in my uncle’s honor. He was a mechanic all his life. Finally retired in his 70’s and didn’t last long after he retired. He died in the late 70’s.
I have a tannis heater on each cylender there’s a prob on Lycoming 540 that I leave plugged in all the time from about Oct to April & I live in Gulf Shores AL. I was told it wont wear out and you don’t want to plug it on & off w/temp fluctuations cus it causes condensation. It keeps the whole engine toasty & some thru the firewall to the panel. LongBayAviation,LLC
A blanket on the cowl and 150 watt incandescent bulb in lower cowl will result in almost 100°f
Forgot, this is overnight not just couple hours.
I know a maintenance facility who won’t start your engine if the ambient temp is below 45dF. They will also leave your airplane inside their heated hangar overnight if you’re scheduled to retrieve your airplane the next morning. They get it.
My 185 has a Tanis heater, an engine cover and a Honda 1000 generator for those cold, non-hangared nights. When in the hangar, a timer initiates heat four hours before the planned flight. We don’t like cold starts.
How about a staggered heat start up? For instance, start the upper cylinder heaters, say 20 – 40 minutes prior to the pan heater depending on ambient temperature.
This should nullify any condensation, or severely mitigate it when the oil pan heater is started.
An automated system could be done using a simple humidity sensor to start the lower heater once the upper area temperature was above the condensation temperature.
The system could shut down once the minimum engine temperature was reached per the manufactures recommendation plus 10% for margin.