The Hot and Cold of It

Tips for safe and efficient preheating

December 1, 1995

Of all the old wives' tales and conflicting reports that pilots hear about how to operate their airplanes, one common bit of advice prevails — a thorough preheat can go a long way to protect engine and instruments from damage and premature replacement.

Consider your engine the morning after a night in subzero temperatures. The oil becomes a syrup so thick that it cannot circulate through the engine to lubricate the cylinders and valve train. The main bearings have contracted tightly against the crankshaft, squeezing out most of the protective layer of oil and allowing metal-to-metal contact.

To make matters worse, the various metals that make up an engine (steel, aluminum, copper, nickel, etc.) have different rates of expansion after startup. Steel expands and contracts more slowly than aluminum. Therefore, the aluminum pistons will expand quickly within the steel cylinders, setting the stage for lots of wear — which is further aggravated by the poor lubrication of the syrupy oil. These seconds (or minutes) of unlubricated metal-to-metal contact cause the most damaging wear that an engine may ever experience.

Like the engine, instruments benefit greatly from a preheat, too. Gyros, which normally spin at 30,000 rpm or more, experience the same problems as an engine during a cold start. Gyros, however, aren't the only instruments that have many intricate and expensive spinning components. Tachometers spin quite fast and will likely show their age on a cold day by not working until takeoff, if at all. To save the engine, instruments, and your wallet from unnecessary wear, a good preheat is in order.

In extremely cold climates, preheats are a fact of life — if you don't preheat your engine, it will not run. Some operators go so far as to drain their engine oil into a container after flight and bring it home with them. Before the next flight, the oil can be heated on the stove or in an oven and quickly brought out to the airport. A combination of warm oil, a propane-fueled preheater, blankets, and plenty of patience can bring life to a frozen engine in the bush.

For those fortunate enough to have electricity available at their hangar or tiedown, several systems can be used to heat the engine and panel. These systems can be as simple as a light bulb or as elaborate as a multi-probe engine preheater. Advantages of each system are roughly analogous to their cost.

The simplest and least expensive preheat is often a shop light placed in the cowl or underneath the panel to warm the components for their cold wake-up. This system works well but might be frowned upon by those in the insurance business. One drop of cold fuel or oil that drips on the hot bulb could shatter the bulb and ignite a fire. If a light is used under the instrument panel, be sure to place it under or near the spinning instruments (gyros, tach) but away from wet-line instruments like the fuel-flow (or pressure) and oil pressure gauges.

A safer, yet still economical preheater is a small electric space heater. These little cubes measure about six inches square and put out far more heat than a light bulb. For preheating the engine, aircraft-heater or clothes-dryer ductwork can be affixed to the heater and fed through the bottom of the cowl or nacelle. This setup has proven safer and more effective than light bulbs, but there are some drawbacks.

Even with cowl plugs in the air intakes and a blanket over the cowl, this application must begin several hours in advance of your flight. But don't leave the heater plugged in at all times. Besides using a lot of electricity, continuous preheating can create lots of condensation that can rust and corrode your engine from the inside out. If you don't believe it, look at the amount of condensation on the oil dipstick after a long preheat. Consider using a timer to start the heater the night before your flight.

If you use this method to preheat, elevate the heater off the floor of your hangar (if applicable) in case the floor of the hangar floods or fuel spills. It may also be a good idea to utilize a circuit breaker to protect against a runaway heater. Also, a tip switch should be utilized to shut the unit off in case it is knocked over. Many hardware stores and catalogs have these heaters with circuit breakers and tip switches already installed.

If your airplane winters outside, the space heater setup might not be practical because of the attention it can draw from vandals or thieves jealous of your creativity. However, many loosely-cowled engines may allow the entire heater inside the cowl. With a little modification to the heater, you may be able to install the whole assembly inside the cowl. Since most space heaters have a thermostat, they will maintain a set temperature inside the cowl or in the cockpit — not bad for $35 spent at a local hardware store.

Your choice of a preheating system will depend a lot upon which type of engine your airplane uses. Carbureted engines are generally tough to start in temperatures below 20 degrees Fahrenheit because of poor fuel atomization and the trickle-down of fuel in the induction system. Fuel-injected engines, which have fuel sprayed directly into the cylinders' intake ports to force atomization, generally suffer fewer cold-starting problems.

Given the cold-start habits of a carbureted engine, you should opt for an all-over engine preheater like those discussed earlier, or one like the Tanis system. Considered by many to be the Cadillac of preheat systems, the Tanis uses multiple probes and a pad to heat the oil and cylinder heads simultaneously. What sets the Tanis apart from others is that it is a permanent fixture on the aircraft. If there is electricity available, you can get a thorough preheat. This will eliminate the need to carry your space heaters, light bulbs, and other paraphernalia with you on trips.

For testing purposes, we installed a Tanis heater on a 1975 Skyhawk. Although Tanis heaters vary greatly for different engines, most employ a pad for the oil pan plus the individual cylinder probes. In some cases, such as the Continental IO-520, Tanis installs an oilpan pad, a crankcase pad (installed on top of the crankcase), plus five or six probes. All told, the -520 installation draws 490 watts. The system for the 172's Lycoming O-320-E2D doesn't use a pad. Instead, a large probe screws directly into the sump in the suction oil screen area. The individual cylinder probes screw into the cylinder-head-temperature probe receptacles. If you already have CHT probes installed, the Tanis system can still heat individual cylinders by using a special probe, depending on the engine make and model.

For installations in Cessna singles, Tanis prefers to snake the plug alongside the dipstick tube, allowing access through the oil filler door. That's great if you have a hangar, but an open door on the cowl is an invitation for rain, snow, critters, or vandals. An option might be to relocate the plug in a less conspicuous place — just remember to unplug it.

With cowl plugs in the air intakes to help block the heat- robbing wind, the Tanis system did a good job of heating the Skyhawk engine on the coldest days in the Mid-Atlantic region. Tanis recommends the use of its customized blankets for optimum preheating; however, unless it is below 20 degrees with a stiff breeze, it probably will not be necesary. With engine blankets and higher-wattage choices, Tanis claims that the average preheat time is five to six hours in all temperatures. At $355 for the O-320 system, the Tanis is expensive, but it is a safe, effective system for those near an electrical outlet.

But do you really need the individual cylinder probes? Manufacturers of pad-only heaters say no. If you have a pad on the oil pan only, it will heat the entire engine — eventually. Through conduction, the oil pan will heat the case, cylinders, and so on, but the process takes several hours. While the oil pan may be heated to 140 degrees, the top of the crankcase may be only 40 degrees because of the heat lost during the conduction process.

However, pad heaters do minimize the time it takes to obtain adequate oil flow. On startup, oil pressure will rise rapidly and oil temperatures will already be in the green. This assures that the engine is adequately lubricated and avoids lengthy warm-ups. Plus, the circulation of the hot oil will quickly warm and — more important — lubricate the entire engine within a few minutes.

Reiff Corporation's Hotpadds come in a kit complete with all the necessary installation tools, including glue and a roller; and the installation is easy. The pads are equipped with two thermostats; one to maintain engine oil temperature at 130 to 150 degrees, and one to cut off power to the heater in the event of an overheat. For about one quarter the cost of a Tanis ($99.95), Reiff's Hotpadds do an adequate, if not admirable, job of preheating aircraft engines. Reiff also makes a similar heater to warm the battery in order to provide full cranking power to the starter on cold days.

For maximum engine protection, a preheater combined with a pre-oiler makes a nice addition to your cold-start arsenal. Oilamatic's pre-oiler uses a small electric motor to circulate the warm oil throughout the engine before it is started. When the pre- oiler is used in conjunction with pad-type oilpan heaters, you will pre-lubricate critical parts as well as heat them.

But what if there are no electrical outlets near your aircraft? No bones about it, you're simply going to have to get to the airport early to get a thorough preheat. A six-cylinder engine can take more than 30 minutes to heat thoroughly, even with a high-powered preheater. Too often, pilots reach in the cowl after 10 minutes of heating, feel warm cylinder cooling fins, and figure the engine is warmed up. Truth is, those cooling fins warmed up in a matter of seconds after applying heat. They're constantly doing the job they were designed to do — expel heat from the engine. Consider the cold- soaked steel crankshaft buried in the bowels of the engine and think about how long it takes the heat to penetrate from the outside in — it's quite a while.

Commercial preheaters, with BTU ratings measured in the hundreds of thousands, will preheat the engine fast, but care should be taken when installing their nozzles. If the cowl is composite, it can blister or even melt. FBO preheats are also expensive and can require a long wait if others are in front of you.

Purchasing your own portable propane-fired preheater may be a good idea if you often fly in cold weather. As in a gas grill, a bottle of liquid propane fuels the flame and the heated air is propelled by a battery-powered fan. Since flames and gas are present, it would be a good idea to chaperone the preheat process. Flame Engineering's Red Dragon preheater is powered by a car (or aircraft) battery and even has a cigarette lighter attachment. Red Dragon preheaters aren't convenient to take with you on a trip but are available on a stand- alone handcart for easy mobility in and around your tiedown or hangar.

Believe it or not, preheating on warmer days can also be advantageous. Any time you can bring the engine closer to operating temperature, the startup will be an easier one. But whether it's 60 degrees Fahrenheit or minus 20, it's refreshing to pull out the cowl plugs, feel warm cylinders, and see oil readily drip off the dipstick. What's more is the peace of mind in knowing that you will not grind your engine towards a premature overhaul every time you start it.

Flame Engineering, Post Office Box 577, La Crosse, Kansas 67548; 800/255-2469 or Sporty's Pilot Shop, 800/SPORTYS

Oilamatic Inc., Post Office Box 5284, Englewood, Colorado 80155; 800/343-7623

Reiff Corporation, S14 W31825 High Meadow Lane, Delafield, Wisconsin 53018; 414/968-2342

Tanis Aircraft Services Inc., Municipal Airport, Post Office Box 117, Glenwood, Minnesota 56334; 800/443-2136 or 612/634-4772 (in Minnesota, 800/862-2443)