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 machines running smoothly, "hot" is a very familiar subject. Heat comes from converting fuel to energy and powerplants deliver a lot of heat. We are required to design and build our machines to deal with very hot temperatures. Cold on the other hand doesn't get nearly as much attention. Obviously it doesn't affect a lot of the folks who live in areas where it never gets cold. But aircraft operations cannot be restricted to regions where cold temps are not a factor.
So what are the problems relating to cold, or in a more scientific sense, an absence of heat? For aircraft owners there are three primary issues. The most significant is getting the power plants up and running. The other two are the airframe and avionics. In the following discussion, we will focus on the powerplant.
Aluminum and steel expand and contract at different rates
Dealing with temperature changes is not a simple matter. Cold affects the various parts of an aircraft engine in different ways. Metals contract at different rates changing tolerances. Lubricants lose their viscosity creating friction and wear issues for moving parts. Plastics and rubber parts become brittle. Even the elements such as moisture contained in the air that surrounds all parts behave differently affecting the internal combustion process.
Understanding the physics of temperature and how it affects various elements is the first step in knowing the best way to deal with cold for an internal combustion engine. Tanis Aircraft Services has conducted ongoing testing in various areas concerning cold weather aircraft operations. The following information is based on that testing:
We all know that a drop in temperature causes most matter to contract. Naturally this causes a change in the tolerances of parts. The fact that an aircraft engine uses both aluminum and steel creates a huge problem because the co-efficient of thermal expansion between steel and aluminum is almost double. But how does that specifically affect an engine? Tests show a steel crankshaft within an aluminum crankcase at -14 F has a reduction in tolerance of more than .001. Minimum manufacturer's tolerances are as low as .0015. The result is a tolerance that may not allow enough room for cold oil to flow to lubricate the parts.
Wrist pins are another problem. A wrist pin floats in the piston. At normal temperatures the pin will float freely. Tests at -15 F show virtually no float. Interestingly, when the wrist pin was thoroughly cleaned it did move freely, indicating that the thin film of oil on the parts actually caused the problem. In any event, the consequence is that if the pin at the down stroke extends to the limit (i.e. cylinder wall) at the up stroke it will scuff the smaller diameter wall at the choke point. That fact is the major reason pilots must be reminded that hand proping a cold engine can cause damage.
Once combustion occurs, another unexpected process starts in the cold cylinder. Combustion produces a 1,200 to 1,400 F flame in direct contact with the aluminum piston, which produces exponential growth of the piston while the cylinder wall warms and grows more slowly. The result is potentially more piston and ring scuffing. Essentially it is the reverse of "shock cooling," commonly perceived as potentially damaging.
Oil is dramatically affected by cold
Automotive lubricants have of course received the major engineering focus in the world of engines and cold weather. The characteristics of oil and grease change dramatically in very cold temperatures. Years ago the commonly used single weight oils performed best at 50 F and warmer. However, when the temperatures dropped below 0 F, an engine needed to have heat applied to the oil. Then came multiviscosity oils that seemed to solve everything. Sure the transmission and bearings were a little stiff but the engine started and if it started you could force everything else to go.
In aviation, however, that is an extremely costly and dangerous operating practice. One 20W50 oil on the market has a "pour temp" of -29 F. That means you could tip a cup of oil upside down and it wouldn't fall out.
Another cold temp viscosity measurement offered by one oil company is the length of time it takes for the oil to travel from the sump to rockers and lifters. Its 15W50 oil touted a time of less than 19 seconds at -20 F. Considering that tests show that multiviscosity oils, when warm, are prone to drain off of surfaces leaving little oil for lubrication, the piston will have at least five strokes before oil reaches the part.
Tests conducted in the Tanis lab revealed that multiviscosity oil is subject to the same effects of cold as single weight oils, only not to the same degree. That means that 15W50 oil does not flow as freely when it's cold as one might expect. Here is a simple test that you could conduct in your own shop. Take about 5 ounces of oil, place it in a cup and pour into another container through a 1/4-inch hole. You will find a dramatic difference in the flow time and quantity when the oil was cooled. In the Tanis test, fresh oil at 80 F, 4 ounces of oil flowed in 37 seconds. At -12 F it took more than nine minutes for 4 ounces to pour from one container to another. Less than a quarter of an ounce poured through the hole in the first 30 seconds.
Another interesting result of the test was that used oil didn't flow as well as fresh oil. The assumption is that the impurities in the oil collect the congealed oil creating "globs" that do not flow smoothly through the 1/4-inch hole.
Other lubricating issues relate to heavier weight oils and greases used in gearboxes and transmissions in helicopters. For example, the oil in the Robinson 44 gearbox has the consistency of peanut butter at -20 F. Although the engine has enough power to rotate the rotor, significant stress is created on the shaft, bearings, and gears to create unnecessary wear.
Yet another issue is the combustion process. Water is a natural by-product of combustion. Burning a pound of fuel creates about a cup of water. If a spark plug fires once or twice it creates a small bit of moisture. In a cold soaked engine, that moisture will condense on the plug as frost and that spells the end of that plug firing again until the frost is gone.
Finally, there is the issue of priming. Because fuel does not vaporize well in cold temperatures, pilots have a tendency to overprime. This extra fuel can wash off any oil that hasn't already drained off the cylinder walls.
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.
About the author
Gray Schmidt is owner and president of Tanis Aircraft Services. You can contact Gary at (320) 634-4773 or by e-mail at Info@tanair.com.