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

A pilot must make the decision whether to leave a plane's preheater plugged in all the time or not in the winter 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.
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