Beating the Cold: Understanding how cold weather affects an aircraft engine

Understanding how cold weather affects an aircraft engine By Gary Schmidt Probably no single law of physics has a broader and more powerful impact on the operation of mechanical apparatus than temperature. For those responsible for keeping...

Engine used for testing cold weather implications on aircraft engines
A cut away engine to offer access to
internal engine components was provided by the A&P school at Lake Area Technical Institute to Tanis who used the engine for testing cold weather implications on aircraft engines.

Preheating process affects TBO
So why all the fuss? None of this is new. We have adapted and overcome cold. Aircraft have operated even in very severe weather for decades. The key question is how and at what price?

Preheating has been a common practice since pilots first tried starting aircraft in cold weather and were left sitting on the ramp with a cold engine. However, preheat processes vary considerably because of facilities and equipment available as well as operator and maintenance preferences. The factors that vary are the source of the heat, what is heated, and how much heat is applied. Often neither mechanics or pilots understand the significance of preheating practices, how they solve (or fail to solve) the cold-impact issues we just mentioned, nor how they directly affect TBO.

Again lets go back to basic physics. Heat is energy. The rate of transfer of energy or heat is largely a factor of the amount of heat applied and both the thermal conductivity and the mass of what is being heated. Metal has reasonably good thermal conductivity but it has significant mass. Anyone who has used an acetelene welder knows that you can heat one end of a piece of metal to melting before the other end gets too hot to touch. Eventually however, the heat will get to the other end. The mass also allows the metal to stay hot for a significant period of time.

Preheating by hot air was the only solution a half a century ago and is still being used today. Used properly hot air preheaters work but there are noteworthy risks. Some of these are outlined in manufacturer service bulletins. Specifically Teledyne Continental Service Bulletin SIL 03-1 states, "excessively hot air can damage nonmetallic components such as seals, hoses, and drive(s) belts,—" Because of the mass of the engine, the hot air heating process takes time. This leads to the temptation to prematurely discontinue the heating before the engine is thoroughly heated or heat soaked.

The efficiency of hot air heaters may be a factor for some. Many of the BTUs of energy that heated the air escape with the warm air leaving the engine compartment.

Nevertheless, for those without access to AC current, hot air preheaters are the only choice.

Internal engine-mounted AC-powered preheaters are the most widely used today. Internal preheaters use heat sources that are mechanically affixed to the engine by glue or bolts. These are generally AC current powered. Internal preheaters can be categorized in two general types: those that heat only the oil and those that apply heat to the cylinder as well as the oil. There are two primary differences. First is the amount of heat created and second, cost. Oil sump heaters are widely used, especially in milder climates. They must be limited to about 100 watts of energy, however. Putting more heat on the oil can potentially oxidize the oil and in some cases, where thermostats fail, can boil the oil, totally eliminating its lubricating value.

Proponents of oil sump heaters indicate that it only requires 100 watts to heat an engine. This is true under two conditions: First, there is absolutely no air moving over the engine (it is securely covered with an insulated cover) and second, the heat is left on long enough. "Oil sump only" detractors, however, point out that putting heat on only the oil can force moisture in the oil out of the oil and that moisture will migrate to colder engine parts where it condenses. This naturally leads to corrosion. Although it is proven that getting oil above 200 F will force out moisture, there has been no scientific evidence produced to date that this leads directly to corrosion.

If the ultimate goal is to heat the entire engine, then preheaters such as the system offered by Tanis Aircraft, accomplish this goal. The basic system puts about 50 watts on each cylinder and 50 watts on the oil for a total of 250 watts on a four-cylinder engine. It accomplishes this with either a probe element that goes into the CHT port or a bolt that replaces an intake manifold bolt. The newer intake manifold bolts are more popular because of the growing use of engine analyzers.

Internal preheaters are also efficient. Heat applied directly to the cylinder head will migrate to other internal components quite rapidly. While the cylinder head will rise 90 F above ambient in about two hours, the valve guide will be 60 F above ambient and the wrist pin is nearly 40 degrees above ambient.

One additional point regarding internal preheaters as a group is the issue of corrosion. Many engine overhaulers have accused internal preheaters of causing engine corrosion when they are left on for extended periods. This is a controversial issue. On one hand, engine overhaulers give anecdotal evidence of engines that have preheaters and have corrosion. And preheater manufacturers point to many examples of aircraft owners who leave heaters on throughout the winter and when reaching TBO show no engine corrosion. They point to frequent instances of corrosion on engines that do not have preheaters. They conclude that the corrosion is a result of environmental conditions and other factors.

The most effective internal preheat application can be summed up by saying apply the heat to parts that need the heat as precisely as possible.

The discussion here is by no means exhaustive. More can be said about the impact on engines as well as the preheat solutions. It is clearly a much more complex matter than just "getting it started." To adapt an old proverb: just enough heat to get an airplane started is just enough to be dangerous. Not just dangerous to the longevity of the engine but even to the pilot. On the subject of adequate preheat, the same Contentinal Service Bulletin mentioned earlier states, "The engine may start and appear to run satisfactorily, but can be damaged from lack of lubrication due to the congealed oil blocking the proper oil flow through the engine . . . the engine may be severely damaged and may fail shortly following application of high power."

Cold is no trivial matter. That is why it behooves all of us to know more about cold and how to beat it.

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