Aircraft maintenance: Proper engine preheating

March 10, 2014

In our last preheating segment, we covered the real reasons that preheating is so important for aircraft engines. This time, we’ll talk about the different methods of preheating and how to make the most of them.

Jeff SimonMost experts recommend preheating anytime that temperatures drop near or below freezing. However, 32 degrees Fahrenheit is only a general guideline. There are more critical temperatures below freezing where preheating becomes far more important, as well as temperatures above freezing where preheating still has some benefits, especially for other components under the cowl such as batteries, hoses, etc. According to research cited by Peter Tanis (of Tanis Aircraft Services, Inc.), 20 degrees Fahrenheit should be the minimum engine temperature to prevent engine damage at startup. Keeping the engine above 40 degrees Fahrenheit is easier on the battery during starts, reducing startup times and the associated wear. And, if you try to keep your engine above 60 degrees Fahrenheit before starting, you will generally see additional benefits in reduced engine stress and cylinder wear, and more efficient run-up times.

As we discussed in our last segment, preheating is about far more than just oil temperature. Proper preheating involves heating the entire engine, so that all critical engine parts can be brought into the ‘safe’ temperature range. In an ideal world, you would heat the entire aircraft to reduce wear in everything from pulleys to gear components, avionics to gyroscopic instruments. But, the world isn’t ideal, especially with the rising costs of “everything aviation.”  I spent the past 16 years digging my airplane out of every snow storm before moving to a hangar this winter. I consider myself very fortunate, and I’m keenly aware that hangars are a luxury we can’t all afford.

That leaves two basic preheating options for most of us: installed preheaters and portable preheaters.

Installed preheaters

The most common preheaters are built-in, electric preheating systems. The most common systems are produced by Tanis Aircraft and Reiff Preheat Systems. Basic preheaters consist of a small electric pad that is bonded to the oil sump. However, it really pays to make the (small) investment in a more extensive preheating system that includes heating elements for other parts of the engine, including the case and cylinders. Tanis uses a variety of component options to heat the different parts of the engine, including heated intake tube bolts and valve cover bolts. Reiff uses heated bands that wrap around the base of each cylinder. Both are excellent solutions. I have personally been using a Tanis system with oil sump heater, case heater, and heated valve cover bolts for many years with excellent results. With either system, a well-insulated cowl cover is strongly recommended to ensure that the entire engine compartment is kept warm.

While you can certainly go with the minimum installation of only a sump heater pad, I strongly recommend paying a little more for a complete system. The main problem with the sump-only preheating systems is that they don’t always address the critical clearance issues that we described earlier. Your oil may be warm, but if your cylinders are cold, you can still have expansion issues and excessive wear at startup. In addition, heating just the oil sump for long periods of time can do more damage than good. The problem lies in condensation.

Condensation occurs anytime warm, moist air flows over a surface colder than the dewpoint. In the case of electric oil sump heaters, the warm air above the oil can condense on the cold parts of the engine, such as the cylinders and camshaft. Since water is a key ingredient for corrosion, leaving only an oil sump heater plugged in for extended periods of time can lead to premature cylinder and camshaft wear. However, if a complete engine heating system is used in conjunction with an insulated cover, corrosion concerns can be largely eliminated. I have personally tested this theory using a humidity gauge while tied down outside in the cold, wet weather. Using only the sump with no blanket yielded high humidity levels. Adding either a cover or a complete engine heating system yielded much less humidity, and the combination of a high-quality insulated cover along with a complete engine preheating system produced an environment in the cowl that was exceptionally warm and dry. I found that even residual moisture in the cowl was gone within hours and my warm-up times before takeoff were virtually eliminated by the time I reached the runway. I later added a thermostat control to maintain a constant temperature in the cowl.

Leaving your preheater on for extended periods of time is controversial, mainly due to the potential for increased corrosion; a valid concern. Different situations dictate different recommendations. Above all, regular flying of the aircraft, frequent oil changes, and the use of anti-corrosion additives such as CamGuard will have the greatest impact on reducing your exposure to corrosion. If you adhere to these first, you should be able to keep your engine compartment constantly preheated as noted above with no negative results (other than wasted energy). This complete setup is about as close to a heated hangar as you can get while still outside. If, however, you expect weeks to pass between flights, then it makes more sense to switch to preheating before each flight and storage at low winter temperatures where the temperature itself will slow the corrosive process. There are a variety of remote-controlled switches available now that make it easier than ever to get your aircraft heating before you even venture out to the airport.

Portable engine preheaters

If you don’t have electricity available at your tie down, or if you want to avoid the cost of an installed system, you will have to rely on some form of portable, forced hot-air preheating. This is the most common form of rapid engine heating used by FBOs and flight schools. These systems usually require both electricity and propane to create a powerful flow of hot air into the engine compartment. The air is either blown into the bottom of the cowl at the exhaust opening, or through the front of the cowl at the air inlets.  The Red Dragon brand of portable pre-heaters is the most common, but if access to electricity is an issue, Northern Companion makes a portable preheater that requires only fuel and accepts a wide range of fuels at that (100LL, Jet-A, Kerosene, etc.). With either solution, the key is to provide enough time to get the entire engine up to a reasonable temperature before attempting a start.

The bottom line is that, when the mercury drops, preheating can have a huge impact on the longevity of your aircraft.  At a minimum, try to get the entire engine above 40 degrees Fahrenheit to provide the best environment for your engine’s long-term health. And, if you have an option to get some heat into the cabin to warm the avionics and gyros before they spool up, even better.

Whichever method of preheating you choose, remember that the best way to keep your aircraft in top condition is to fly it often. Winter may mean cold temperatures, but it can also mean smooth air, impressive climb, great visibility, and a wonderful opportunity to get out of the house and see the winter wonderland from a pilot’s perspective. Happy flying!

Social FlightJeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the free mobile app and website that maps over 10,000 aviation events. Free apps available for iPhone, iPad and Android, and on the Web at www.SocialFlight.com.