Airframe and Powerplant

Winter Prep

January 1, 1992

Rules for the cool season

Winter has arrived, and that means not only preparing yourself for the cold, but helping your airplane adapt, too. Like humans, airplanes aren't particularly fond of very cold temperatures and require a bit of extra preparation to get going on those days when the mercury just can't seem to get off the bottom of the thermometer. A few simple steps can help make weathering winter a less difficult task for you and your airplane.

Let's begin with the engine. Cold weather is a wonderful thing for producing power in normally aspirated engines, and installations that typically run hot will breathe a mechanical sigh of relief at winter's onset. But some precautions are necessary so that your engine makes it through the cold season without damage.

The majority of wear in an engine occurs during the first few minutes of operation. This is true when the engine is warm, but particularly fearsome when it is cold. Put simply, cold starts are murder on air-cooled aircraft engines. Just what constitutes a cold start? The engine manufacturers say that's anytime the ambient temperature drops below 20 degrees Fahrenheit, but many others, including oil experts and engine overhaulers, think that cutoff should be between 40 and 60 degrees F. (Also, some specific engines, like the Lycoming O-320-H2AD, need preheat at higher temperatures to ward off valve-train troubles.) While this might seem awfully conservative, they have a point: Consider that an engine whose core temperature is 200 will see cylinder head temperatures of 200 degrees or more within minutes of startup. Such a rapid change in temperature is what the engine doesn't like.

Then there's the oil. Parts protected by oil must rely upon a very thin film of the stuff to prevent metal-to-metal contact. Plain bearings won't last long without that oil film, and during startup, the longer it takes the oil system to pressurize and provide lubrication, the greater chance that damage could occur. (That's why you hear of the popular Lycomings, like the aforementioned -H2AD, having problems with the valve train related to oil starvation. The camshaft's location at the top of the engine means it is the last to receive oil; Continental camshafts, living under the crankshaft, often have a better time of it.)

So when the temperature drops, it is an excellent idea to, at the very least, switch to a lighter viscosity oil. Better yet, change to a multigrade oil. You can find multigrades in petroleum-based form, as a synthetic, or as a semi-synthetic. Advantages of lighter weight oil and especially a multigrade include lower pumping resistance and better lubrication at start-up; some straight-grade oils have very poor pumpability at low temperatures. The sooner you can get that oil moving around the engine and lubricating properly, the better the engine's chances of living a long and happy life.

In many respects, the only reasonable course of action when it's cold outside is to preheat the airplane.

There are many ways to do this, including use of gas-fired preheaters or electric elements in the oil sump and having the airplane moved into a heated hangar. For those who normally base their airplanes indoors, here's an inexpensive way to preheat. Wrap the cowling in heavy blankets, and place two or more mechanic's drop lights in the forward openings or stuff them into the open cowl flaps. A pair of 75-watt bulbs will keep the engine compartment 20 to 40 degrees F above ambient temperature, for very little money. just be sure that the light isn't touching anything that might catch fire, and keep the area near the lights free of fuel and oil. One advantage to this method is that the entire engine compartment gets a good heating.

Having the right oil in the crankcase and ensuring the airplane gets a thorough preheat will make your battery's life easier, too. When the ambient temperature is 20 degrees F, the battery doesn't have some 50 percent of its normal, standard-day power, and gooey oil inside the powerplant won't make its job any easier.

Preheating the engine will also ease the fuel system's chores and perhaps make it possible to start the engine in the first place; some engines simply cannot be started without preheat on a sub-20 degrees F day. At low temperatures, fuel atomization is more difficult to attain in the induction system — carburetors have a particularly difficult time of it — and proper priming is essential to getting the engine going. Also, take extra care to make sure that the fuel tanks and gascolator are free of ice and water.

For example, one 30 degrees F morning, at an airport with an elevation of 6,000 feet, a Turbo Arrow I was flying absolutely refused to start, even taking into account the procedures recommended by the handbook. (For T-Arrow owners, this one had the optional electric primer, too.) Finally, about the time I was ready to call for external power, a mechanic suggested that I try turning the high boost pump on to all but drench the engine in fuel. Finally, the engine caught, first on one cylinder, and then, slowly, the other five came on line. Apparently, the combination of high altitude, the low compression of the Arrow's TSIO-360, and cold temperature required extraordinary starting procedures. The point is that I had nearly run the battery down before I discovered what worked.

Another item to consider is ignition timing. Have the magnetos checked for proper timing and general condition before the onset of winter, and suspect them should hard starting persist. Remember that some installations use an impulse coupling on one mag to both retard the timing and to boost spark output and that, when the starter is cranking, the other mag should be off line. (This coupling contains a spring-and-clutch mechanism that allows the magneto to wind up at starting speeds. After a slight amount of crankshaft rotation, the coupling releases and spins the magneto faster than the cranking speed, which provides a greater spark and retards its timing. Both actions help starting.) Another scheme induces a "shower of sparks" to the spark plug during start, and failure of the vibrator mechanism will render the airplane difficult, even impossible, to start. Also, spark-plug condition will contribute to how well the engine starts in the cold.

Finally, don't forget to lean the engine during taxi and at the power settings recommended by the engine manufacturer-the scavenging agents in avgas require some heat, usually around 1,2000F, to keep the lead from depositing itself in the combustion chamber. Just because the air is cold and dense doesn't mean you shouldn't lean; at idle and taxiing speeds, a full-rich mixture provides more than enough fuel.

Other cold-weather caveats include: Don't consider taking off until the oil temperature has stabilized at least at the bottom of the green. Don't try to expedite the warming of the engine by closing the cowl flaps, either; airflow over the cylinders during ground operation is not sufficient that way, and you'll only end up with lukewarm oil and hot heads. (And even if the CHT measurements look low, remember that the low volume of air flowing through the cowling on the ground can contribute to localized hot spots on the cylinders.) However, you should consider closing the cowl flaps in climb if the CHT hasn't reached its normal operating range. You can do nearly as much damage by running an engine too cool as you can running it too hot, which is a prime reason for installing cold-weather baffling on some airplanes. Usually, this means blocking off the oil-cooler inlet; check your pilot's operating handbook or service manual for details.

Pilots of turbocharged aircraft have additional cold-weather considerations. In automatic turbo systems, the wastegate usually is manipulated by engine oil. Typically, there will not be much oil flow during ground operation, and it's possible that the oil in the wastegate controller and related systems might be too cold to flow properly. This is why you should carefully monitor manifold pressure during the takeoff run to make sure it doesn't exceed limits. For pilots of manual systems, make sure you take into account the outside temperature for power setting calculations; thanks to the low intake-air temperature (and its consequent high density), you might be extracting more power from the engine than you believe.

Airframes, too, need attention during the winter. The most obvious tip here is to make sure the airplane is free of ice and frost. Use a piece of soft canvas or burlap to help remove the frost, and don't be shy about using deicing fluid. Remember that any ice or frost that you melt from the airframe could return to a frozen state once airborne. Frozen control hinges are not an uncommon result.

Before flight, make sure that the water drain holes in the fuselage, wings, and controls are clear. Water that enters and becomes trapped easily could freeze in position, dramatically affecting not just the all-up weight of the airplane, but, more important, the weight and balance of the control surfaces. Use a soft pipe cleaner to make sure these holes are clear. (For the Piper drivers: Don't tie the control wheel back with the seat belt. This leaves the drain holes in the stabilator at the high point, not the low point as they are intended to be.)

Before that first chilly flight is the time to check the heater components, too. You don't want to find out that the heater tubing has gone off to Jamaica as you climb into sub-zero air. A thorough check of the exhaust system before winter and installation of a carbon monoxide detector in the cabin are also highly desirable.

To help the heater in its endeavor, you might want to install cold-air-inlet covers or block them for the winter. Cessnas, in particular those with the overhead outlets that never seem to stay put, are prime candidates; screw-in covers are available for most high-wing Cessnas. (One caveat, though: You shouldn't block all the fresh-air vents in case you need to evacuate the cabin of smoke or carbon monoxide.)

So take a few minutes and think about what the cold weather is doing to your airplane, dress warmly, and try not to dwell upon the daunting task of preflighting on a frozen ramp. Then go out and see the wonders of winter from the air-a pleasure more often than not worth the effort of getting there in the first place.


Pickling

Some airplanes just don't fly in the winter. There are a few simple steps to take that help preserve the airplane — primarily the engine — during this time in inactivity. Three types of storage are considered typical by the engine manufacturers: flyable (from 7 to 30 days), temporary (30 to 90 days), and indefinite storage. Unless the airplane is down for a refurbishing project, either flyable or temporary storage should fit your needs.

In general, the airframe manufacturers recommend that for airplanes placed in storage-any of the above-mentioned types-the pitot tube and static ports should be covered. You should place covers over the windows or place shields inside and install cowl plugs. Also, the manufacturers say that the fuel tanks should be topped and drained of any water at the time of storage. Sealing the fuel caps isn't a bad idea, either, especially for certain Cessna models prone to water ingestion through the fuel fillers. (The so-called umbrella-type caps are excellent here.)

Engine preparation varies, depending upon the type of storage. Flyable storage calls for servicing the engine as recommended by the manual-i.e., the usual oil-change intervals and the like-and then rotating the prop by hand every seven days. For non-geared engines, you should stop the prop 45 or 90 degrees from the original location-, pulling through 10 to 20 blades is sufficient. (Moving the propeller helps redistribute oil on the cylinder walls, preventing corrosion.) In addition, you should fly the airplane at the end of the 30-day period. Merely running the engine on the ground isn't enough.

For temporary storage, you should remove the top spark plugs and spray the cylinder walls with a protective lubricant — Continental recommends a MIL-L-46002 type; ask your A&P for a good brand name-and run the prop through by hand. Make sure you stop the prop so that no pistons are at top-dead-center (the uppermost part of the compression stroke; put your finger over the spark plug hole and feel for the blast of air-that's the compression stroke.) Re-spray the cylinders, and then spray the protective lubricant into the oil filler tube, too. Reinstall the top spark plugs, and seal the airbox, fuel-injection servo or carburetor opening, and the exhaust system. Finally, attach a "Don't turn prop, engine preserved" sign to the prop.

Indefinite storage includes all of the steps from the temporary storage syllabus and adds installing dehydrator spark plugs in the top holes and filling the crankcase with a 3:1 mixture of fresh oil and Cosmoline (a commercially available preservative). Also, place desiccant in the exhaust pipes and oil breather tube, and then seal the whole mess up.

Returning an engine to service from either the indefinite or temporary storage is simple. Remove the plugs and desiccant bags, remove the bottom spark plugs and pump out the excess preservative oil, and change the oil and filter to your usual brand. Run the prop through several blades by hand to ensure that all the excess oil has been pumped out of the cylinders, thus eliminating the possibility of a hydraulic lock-and start the engine. After a brief ground run and thorough cowling-off inspection, go flying. Keep a close eye on temperatures and performance, and revel in the fact that your airplane has emerged from its deep slumber.