Section I: Equipment and Environment
Part IV, Chapter 12: Preheating Equipment and Methods
Aviation fuel is distilled with a much lower vapor pressure than automobile fuel so that during the climb to altitude engine failure will not be experienced due to vapor lock. What this means from an operational point of view is that once temperatures get down to about 15 degrees F/-9 degrees C, the engine will require preheating so the fuel can properly vaporize for ignition.
Compounding the problem, at about -5 degrees F/-21 degrees C most oils (the exceptions being synthetics like the discontinued Mobil AV-1) become too thick to lubricate the cylinders and bearings properly, and if you could manage to start your engine at this temperature (by heavy priming, or a bit of mild preheating), the wear that would take place until the oil began to flow would probably be equivalent to more than 50 hours of normal operation.
(Mike Busch, the Cessna Pilots Association's engine specialist, claims that one cold start at the lower temperature extremes can cause as much wear as 500 hours of normal cruise operation. This takes into consideration the metal-to-metal contact between the piston and cylinder walls caused by the uneven expansion of the aluminum pistons and steel cylinders as the engine goes rapidly from very cold to very hot.)
If you have a turbocharged engine you are well advised, even when using multi-grade oils, to preheat below 30 degrees F/-1 degree C: the turbocharger is very oil-critical. As for non-turbocharged equipment, preheating is mandatory for Continental and Lycoming piston engines below 10 degrees F/-12 degrees C. I preheat all my engines, turbocharged or not, below 30 degrees F/-1 degree C: I like to get my full TBO.
There are no convenient ways around these problems, and, like the FAA and taxes, they are a fact of aviation life. In this chapter we will take a look at the best methods available to both the remote-area bush pilot and the city-based airport pilot when operating under daily conditions that require preheating. However, before doing so, let's continue our examination of a piece of equipment (mentioned in the previous chapter) that is necessary to both areas: the insulated engine cover.
Now made out of nylon like our wing and windshield covers, engine covers are padded with about an inch of synthetic insulation and are designed to fit snugly over the engine cowling, fastening with Velcro strips, nylon ties, and bungee cords (see illustrations 28 and 29). Used properly, they also serve a number of related uses during the cold, stormy months of winter.
During periods of deep cold (-20 degrees F/-29 degrees C and below), they keep the engine warm between flights, so preheating does not have to be repeated throughout the day. If placed on the cowling as soon as the engine is shut down, oil temperatures will remain in the green for about three to six hours, depending on whether you are using a thin cover like the Tanair, or a thick cover like those manufactured by Alaska Wing Covers. In those rare emergencies where you are forced to RON someplace where preheating facilities are not available and the portable preheating equipment that should be in the airplane's survival kit has been left behind, the running of the engine for a few minutes every few hours will keep it warm enough to remain operational. The only problem you need worry about here -- aside from lack of sleep -- is the chance of running low on fuel if emergency conditions, usually weather, continue too long.
At urban bases or public airports where electricity is available, the covers are used in combination with an electric heater to maintain engine heat for 'round-the-clock availability.
At base camp, in the bush, or on airports without available electricity, engine covers are used in combination with combustion heaters to focus and maintain heat flow (see illustration 28).
And, as mentioned in the previous chapter, they are used to keep the engine compartment free of blowing snow during wind storms.
In "Ye olden days" bush pilots often used little 750 watt "car warmer" heaters that cost about $20. These heaters typically measured about 4"x6"x7" and contained a metal heating element, a temperature-controlled safety switch, and a small fan. One or two placed in the engine compartment after the last flight of the day, with the cowling then covered by an engine cover, did a fine job, and they were the standard until the fine Tanis preheat system came on the market. Now the car warmers are mostly relegated to the cockpit, where they help keep it and the instruments warm.
The Tanis preheating system is a light-weight (one to two pounds, depending on application) system of thermal pads that attach to the crankcase and oil pan, along with heater elements that attach to the cylinders. In the better installations, a flush-mounted power receptacle is installed in a non-structural skin area (such as an engine baffle) on airplanes like the Super Cub, Cherokee Six, or Bonanza that have cowlings that open easily for pre-flight inspections (see illustration 30). On airplanes like the C-180/185/206 where you have to use a screwdriver to get the cowling open, a flush plug door, similar to the ground service plug receptacle for an external power source, is installed. The standard Tanis TAS-100 system is good down to about -30 degrees F/-34 degrees C; the Tanis "Super System," which I use, is good down to -65 degrees F/-54 degrees C. Recommended highly.
An emergency technique
Sometimes a pilot will get caught away from home base without his combustion preheating equipment, but will have electricity available. Here the thing to do is borrow a droplight (using a 100 or 150 watt bulb) from the local maintenance shop and place it in the engine compartment. With the engine cover in place, this will keep the engine warm indefinitely as long as the temperature does not drop too far.
Combustion heaters (in the city)
Combustion heaters range from the large Herman Nelson and space heaters used by FBOs for transient aircraft (for prices ranging from $25.00 to $40.00 per engine) to the small portable units used by the bush pilots. All these unites need to be used wisely, for applying just enough heat to get an engine started once it has been cold-soaked overnight is a fine way to damage it, and bring on an early overhaul.
Herman Nelson. These large and efficient heaters, usually seen on wheeled carts, were originally designed for DC-3 size aircraft and are used extensively all over the North by FBOs and charter outfits for the preheating of transient aircraft, company Twin Otters, Beavers, etc. They are combustion heaters which use a forced-air fan operated by either an electric or gasoline motor to drive heated air through long ducts into the engine compartment, battery compartment, and cabin of the airplane.
When used on light aircraft under conditions of deep cold (-20 degrees F/-29 degrees C and below), these heaters will easily get your engine warm enough to start before the oil has an opportunity to completely thaw within the crankcase. This can present serious problems if, when you are in the air, a mass of half-frozen oil blocks the oil pump. In fact, as the old saying goes, it could ruin your whole day
If circumstances do not allow you to use the Herman Nelson longer than the bare minimum required to start the engine, the following emergency procedure should be followed:
Start the engine and, with a careful eye on the oil pressure gauge, warm it up until the cylinder head temperatures reach about 3/4 into the green.
Shut the engine down and cover it with the engine cover.
Wait about 40 minutes until the engine's heat can transfer to the oil, thawing it.
One final point. When an airplane has been preheated, the oil in the dipstick area seems to thaw first, so it is a mistake to check the oil, note that it runs free on the dipstick, and therefore assume that it has been adequately thawed. If you doubt this, a little test will demonstrate the principle involved. Take a can of oil that has been left outside in -40 degree F/-40 degree C temperatures and bring it in to where the wood or oil stove is. Punch a hole in the top of the can and place it on the hot stove. Let it sit there until the outside of the can is too hot to touch and a nail dipped inside the can shows free-flowing oil. Then pour the oil from the can into another can. Depending on timing, there will be a large or small lump of still-frozen oil in the can; this is a sample of what the oil inside the engine is like.
Space heaters. These electric/combustion heaters, refugees from the construction industry, are also used by FBOs and charter outfits, mostly in Alaska, for company and transient aircraft. They are placed on the ground under the engine compartment, which is then draped with a blanket or piece of canvas.
While these heaters are hardly in the Herman Nelson class, they do a good job when suddenly called upon to heat an engine, and all you need remember is the caveat concerning the frozen oil.
Combustion heaters (in the bush)
Preheating an engine in a wilderness environment demands that your combustion heater be dependable, durable, and portable.
For dependability, avoid anything too fancy or complicated, including those cute (and expensive) little blower heaters operated by a 12 or 24 volt DC motor that are designed to be hooked up to the airplane's electrical system. In conditions of deep cold, these will often not work due to a cold-soaked battery.
For durability, avoid most of the camp stoves now on the market. These are designed for the weekend camper; they usually lack quality construction and are not something you would wish to stake your life on.
For portability, choose a heat source that is small and light enough to be carried in the airplane's winter survival kit (see Chapter 24). This is important, for the professional bush pilot will every now and then find himself stuck overnight someplace due to a fast-moving weather system and will require the services of his engine preheater once the front moves through.
The best bush preheating rig is one that uses a simple old-fashioned gasoline blowtorch that can be operated on avgas taken directly from the airplane's fuel sumps. Since this is a systems approach, if you wish to use a different heat source you will find little difficulty in adapting this technique to your purpose. For the convenience of those who are unable to locate gasoline-powered blowtorches -- they are hard to find these modern days -- several alternative heat sources have been listed, though they all fall far below the level of utility that the blowtorches achieve.
Blowtorch and stovepipe method. The equipment list for this system consists of a small fire extinguisher, one or two gasoline blowtorches, three lengths of 3" stovepipe, an insulated engine cover, and a two- or five-gallon metal gasoline can. Here is how the system works:
After the last flight of the day, the oil is drained from the engine into the gasoline can. Quick-drains should be installed on all airplanes in the fleet to facilitate this daily chore.
The can of oil is taken inside your office or home, where it is placed near the stove so it can be kept warm until needed again.
When the airplane is to be called into service, the can of oil, with the lid removed, is placed on top of the stove and heated to about 122 degrees F/50 degrees C.
While the oil is warming, take the three lengths of 3" stovepipe and fasten them together. Place one end inside the engine compartment and temporarily rest the other end on the ground. The best place for the pipe to go into the Super Cub's engine compartment is on the right side by the brace to the exhaust stack. With the Cessna 180/185, it is best to drop the right cowl-flap and place the pipe there (see illustrations 28 and 31).
Carefully light the blowtorch away from the airplane, and when it is operating properly place the nozzle into the lower end of the stovepipe.
With the Cessna 180/185 and 206 you should use two blowtorches, and these must be placed in the same pipe (see illustration 31). Never use two pipes, one going to the right side cowl-flap and the other going to the left side cowl-flap, because of the danger that one of the blowtorches might go out, while still spraying fuel into the stovepipe. The second torch could then ignite the fuel inside the engine compartment. With both torches in the same stovepipe this danger is averted.
Adjust the insulated engine cover over the top and sides of the engine compartment so the right cooling air-inlet, behind the prop, is covered and the left air-inlet is open (see illustration 28). This is done to allow the heated air to pass over the engine and out the far side, taking the cold with it. Remember, heat-circulation is the name of the game. When using combustion heaters, if both air-inlets are blocked, it will take longer for the engine to reach starting temperatures.
When the engine is warm enough that the prop moves freely and the carburetor or fuel-injector is able to vaporize fuel correctly (about 45 minutes at -4 degrees F/-20 degrees C; 1+20 at -40 degrees F/-40 degrees C), the blowtorch(es) can be turned off and set aside to cool. At this point the hot oil is poured into the engine, three or four shots of priming are given, and the engine is turned over four to six times by hand. It is now ready to go, and should start on the first turn of the prop.
Propane torch method. Some pilots use a propane torch of the "weed-burner" type at base camp -- they are not suitable for the survival kit. These torches (sold by Aircraft Components, Inc., as well as in hardware stores) have a long handle, about ten feet of hose, and a needle-valve. They attach to regular refillable propane tanks (five-gallon or larger), which are available from local fuel suppliers. Costing between $50.00 and $60.00 (exclusive of the tank), they do the job, though you must be careful when using them not to apply too much heat to the engine compartment. Naturally, the fuel tank must be kept inside a heated building when not in use; otherwise, the cold will freeze the propane, preventing it from flowing properly.
Coleman camp stoves. Along with the old-fashioned plumber's pots, these are sometimes used, but are not all that good.
The last combustion preheater I will mention is the Kennon SureStart II. This little unit comes in a toolbox and weighs 22 pounds. It has an electric blower that attaches to your airplane's battery, uses 14.1 oz. disposable propane cylinders, and costs about $500.00. As part of our throwaway culture, it is unquestionably cute, and I know of a flying school that uses them to warm their Cessna 152s on cold mornings. While not the sort of thing a bush pilot would want -- consider being tied to city stores and their supply of little disposable cylinders -- it is quite popular with private pilots who do not fly regularly in the winter, but need to occasionally heat their airplane either in a no-electricity hangar, or out at the tiedown. In these circumstances it does a good job.
While the engine compartment is the primary area we have to preheat -- for VFR flight it is the only area that is truly necessary -- we do have to consider the cockpit. Besides comfort, the reasons for this are:
In airplanes that are to be operated at night or in IMC, including whiteouts, radios and gyros must be at or above their minimum operating temperatures. With panel-mounted radios and indicators -- HSI, RMI, VOR, etc. -- this is usually -4 degrees F/-20 degrees C; with pneumatic gyros, -22 degrees F/-30 degrees C. (The slaved remote DG and flux detector that drive the King KI 525 HSI and KI 229 RMI are certified to -66 degrees F/-54 degrees C).
Windshield condensation can often be quite severe at very low temperatures, when defroster efficiency is impaired for the first few minutes of the flight. A little heat, just enough to remove cold-soaking, will go a long way towards resolving this problem.
I use one or two 1500 watt electric "car warmer" heaters in the cockpit. In the city, where electricity is available, I plug them in the night before, at the same time I plug in the engine preheater. In the bush, at my base of operations, I use a small electric generator to operate the heaters, turning them on at the same time I start preheating the engine with blowtorches.
In this way, by the time the plane is ready to go, the cockpit and instruments are warm enough to be operational.
(Tanis manufactures a panel instrument heater system, which I have never tried because I prefer heating the whole cockpit with a car warmer, but I imagine it works as well as the rest of their equipment.)
Away from my base, in the bush where no power is available, I have to content myself with VFR until the cabin heater has warmed the radios, indicators, and gyros to their operating temperatures. This usually works out okay, for low temperatures in the bush, away from the cities and their dense blankets of ice fog, usually mean VFR weather.
Cold also affects the battery; at seriously low temperatures it will not have enough power to start the engine. In the city, when electricity is available, many pilots with C-180/185s and Super Cubs will use a simple battery heater, such as the one sold by Tanis, or put a small "car-warmer" heater in the battery compartment. Pilots with airplanes like the C-206 do not need to bother, for the battery is in the engine compartment and will be kept warm by the engine heater as long as the engine cover is in place. In the bush, however, things are different, and you must either remove the battery and take it inside for the night so it can be kept warm, or you must handprop the airplane for the first flight of the following day.
[Please Note: The next Chapter, Chapter 13: Cold Weather Engine Operations, is unavailable on the web. The online edition continues with Section I, Part IV: Introduction: Navigation Tips (Silk Scarf to High Tech).]