The Big Debate: Can engine preheaters be plugged in all the time?

In the winter time, it can get down right cold outside. But is that going to stop pilots from flying? Of course not. In fact, that clear cold air makes for great flying. All the pilot needs to do is get the plane started. No problem. It has a preheater.

Oh, Oh. Is it plugged in? If not, how long do we need it plugged in before we can fly? These are the easy questions. The really BIG question is "Can I leave the preheater plugged in all the time?" It would save a lot of time and be much more convenient. That question, however, has sparked what could be called "The Big Debate."

Introduction of preheaters

This question surfaced not long after Peter Tanis invented the first internal engine preheater. Mr. Tanis designed a heating element probe that screwed into the CHT port and heated the entire cylinder. It worked great. He proceeded to get a patent and market this heater. Soon afterwards others marketed other types of engine preheaters. These preheaters became widely popular because they eliminated the fire risks, delays, and inconveniences of the hot air heaters affectionately known as "flame throwers."

Internal engine corrosion

Over time the aviation community began to discuss a problem - internal engine corrosion. Was this a coincidental occurrence or could it be a result of these new fangled engine preheaters? The big debate was started.

Who's to blame?

The engine preheater guys defended their preheaters. Some felt corrosion was always a problem but was never addressed before. They cite some old-timers who said the corrosion issue existed for years, especially before some of the newer, more effective oils came along. Others pointed to new fuel formulas and the increasing number of light aircraft. Some even placed the blame on the type of steel used by the engine manufacturers.

The engine manufacturers and mechanics pointed the finger in a somewhat "cause and effect" response to engine preheaters. They reasoned that warm air holds more moisture and heat hastens the corrosion process, thus increasing the potential for corrosion.

So after all these years of the big debate raging on, has anyone come to a conclusion? Do we have an answer? Yes. The answer is a definite "it depends."

As far as internal engine corrosion, there are three relevant factors to consider.

  • How often is the aircraft flown?

  • Where is the aircraft located?

  • What type of preheater does it have?

For those readers who have read far enough and want the simplest and easiest possible answer, the rule is this "Do not leave the heater plugged in all the time." Start the heater the required number of hours before you plan to fly.

However if you want to save the time and money of a lot of trips to the airport, read on.

First let's enumerate some basic facts relevant to this discussion:

  • There are many aircraft owners who keep their preheater plugged in all winter without adverse effects (in other words reaching full TBO).

  • Although many speak of preheaters generically, there are different types of engine preheaters and they relate to the corrosion issue in very different ways.

  • Corrosion is a complex chemical process. Water is essential to the process and it is accelerated by corrosive acids as well as heat. The primary source of this water is the combustion process some of which remains in the engine, held in the oil or air.

  • Most examples of corrosion discovered when engines are disassembled show some parts corroded and not others. Cam followers and lifters are the most common parts cited.

  • There are also many examples of engines that have suffered internal corrosion that did NOT have preheaters installed.

  • Heating air will lower relative humidity assuming no water is added to the air. However if liquid water is present, the heated air will absorb that moisture.

  • Moisture is always present in the crankcase of an engine. The relative humidity in a recently run engine could measure 94 percent.

  • Corrosive acids are also a byproduct of combustion and will always be present in used oil. The older the oil, the more acids.

  • A coating of oil will protect metal from corrosion by forming a barrier to the moisture. However this oil will drain from parts over time. Multi-viscosity oil will drain faster than standard weight oil and warm oil will drain faster than cool oil.

The conclusion of these facts is that if (and I emphasize "if") preheaters are a cause of corrosion, they are only one small aspect in a very complicated process.

Back to the three considerations relating to leaving the preheater plugged in continuously. Keep in mind the facts mentioned above.

How often is the aircraft flown?

If a pilot flies frequently (once a week or more and for at least 35 minutes and with the oil temp gauge reading 180 F or more), he will burn off some of the moisture in the oil and provide the internal parts with a coating of oil.

Where is the aircraft located?

If it is located outside where air movement can cause significant temperature changes in the engine which leads to condensation (i.e. liquid moisture on internal parts) you increase corrosion potential. Aircraft not located in a hangar are subject to wind or air movement and thus greater temperature changes. Aircraft located in hangars with covers on them are not as vulnerable. I should emphasize that even in hangars, the engine should be covered to prevent air movement through the engine.

The worst case scenario is if the preheater is controlled by a thermostat. This will cause the heater to turn on and off which is a condition you want to avoid.

What type of preheater does it have?

If the aircraft has an oil sump only heater (i.e. the preheater does not heat the cylinder heads), the heat on the oil can drive out moisture in the oil, which may condense on the colder parts in the cylinder area. Cylinders are, of course, air cooled and located some distance from the heat source. Oil sump only heaters cannot put excessive heat for fear of boiling the oil. Consequently that single source of heat requires a longer time for the heater to heat the entire engine. This creates the temptation to leave the preheater on continuously or at least for extended periods. Consequently many mechanics recommend Tanis preheaters because they put heat directly on the cylinder head allowing more heat applied evenly throughout the engine.

Remove the dipstick

A suggestion that many pilots who use Tanis preheaters follow is to remove the oil dipstick when the plane is in the hangar and the preheater is heating. This has been shown to reduce the humidity by up to 50 percent as the warm moist air rises out of the engine and is replaced with cooler, dryer air. Although the combustion process produces moisture in the crankcase, there is no effective way for that moisture to escape. This suggestion provides for that escape.

All this seems reasonably logical and straightforward. Yet, the big debate will undoubtedly continue. In spite of what the testing oil companies and preheater manufacturers do, we still can only speculate on exactly what is going on inside that engine. We merely can observe the end result when the engine is torn apart. It is true, a relatively small percentage of engines experience corrosion. Yet as long as corrosion remains an issue, aircraft owners want an answer to "Why?" and "What should I do?". In regards to the question "Why?" the preheater companies, the engine manufacturers, the oil and fuel suppliers all say "Don't blame me" and point in another direction. "What should I do?" Their answers vary based on their product.

It would be unfair to those who took the trouble to read this far not to attempt to at least answer the question "What should I do?" Based on the facts presented here, the recommendation is: "If a pilot flies about once a week or more (following the criteria as mentioned earlier) and has a heater that heats the entire engine, he or she can leave the heater plugged in while their aircraft is covered in a hangar. Otherwise, the preheater should be started a few hours before they intend to fly."

In the end, however, the practice that is followed is still up to the pilot. He or she must decide what to do. Hopefully this information will help you educate them when it comes time to make that decision. AMT