Airframe and Powerplant

Cold Weather Whys and Hows

December 1, 2000

The 'whistle slot' should be checked before winter operations

In an ideal world of winter flying we would leave our cozy home after checking the weather and filing a flight plan; drive to our heated hangar; pull the car inside through a remotely controlled door; load the baggage and flying paraphernalia; preflight and board the airplane; push a button to open the hangar door; yell "Clear!"; start the engine; and taxi to the active runway as the hangar door closes behind us. The airplane engine, instruments, and cabin would always be warm and ready for flight, the pilot and passengers would always be protected from the elements, and there would never be any snow or ice on the airplane's wings or windshield. Unfortunately, this scene is more likely to be a dream than reality. General aviation pilots have to dress up so they look like Nanook of the North before hauling out the engine preheater to prepare their airplanes for another hour or two of winter flying. If it's such a hassle, why do people go out of their way to fly during the cold winter months?

Winter flying can be a lot of fun and is often the source of many pleasant flying memories. The cold, dense winter air boosts engine power, aids wing lift, and is often very stable and smooth. After a winter front has passed, entire sections of the country will lay beneath crystal-clear air that provides 50-mile-plus visibilities.

Those who are lucky enough to fly ski-equipped airplanes can land on almost any well-frozen lake, descending down onto a white landing area that's surrounded by the dark greens, grays, and blacks of a northern winter. Often the only signs of man's presence are the newly carved ski trails in recently fallen snow and the ticking and creaking of the cooling engine.

Winter flying can be an anticipated adventure, or it can be a huge struggle for an airplane owner. Taking action to prepare your airplane for winter flying will increase winter flying safety and comfort.

Winter and the airplane

If winter weather brings snow and ice to your region, remove your airplane's wheelpants in the fall. Wheelpants can pack with snow and then lock up one or all wheels. Non-rotating wheels make for exciting and expensive landings. Wheelpants prevent a thorough preflight inspection, and when the brakes freeze after taxiing in snow and ice, the pants make it very difficult to break the wheels loose. Take the pants off in the fall and store them until spring.

Since the carburetors and fuel-injection units on general aviation engines are not capable of measuring and adjusting for changes in air density, the mixture and idle speed adjustments on the carburetors and fuel-injection units should be reset when the seasons and temperatures change. These engine adjustments should be done when the engine is at normal operating temperature. Pilots can self-test the idling mixture of their engine by setting the engine rpm in accordance with idle mixture adjustment procedures in the aircraft service manual or appropriate service bulletin, and slowly pulling the mixture control knob out to stop the engine. Watch the tach for any rpm change just before the engine dies. For instance, the rpm is set at 1,000 when checking the idle mixture on a Continental fuel-injected IO-520. Whatever the specified rpm for your engine turns out to be, a correctly set idle mixture will yield a slight rise of approximately 25 rpm just before the engine dies. If the rpm rises excessively, the mixture is too rich; if it doesn't rise at all, the mixture is too lean. Correct adjustment makes the engine easier to start, and helps prevent spark-plug fouling and operational roughness.

Engine breather tube and whistle slot

One of the hazards of winter operation is that the engine breather tube may freeze shut. This tube provides a vent for normal crankcase pressures that occur during engine operation. If the tube freezes shut, the engine's internal pressures will increase until the crankshaft oil seal is pushed out of position. When the seal is displaced, the result is the speedy exit of all the engine oil as it flows aft over the fuselage and windshield.

As the engine warms up to operating temperature, moisture that has accumulated in the engine and oil vaporizes, and is then vented overboard through this tube. If this tube is not insulated, or lagged, prior to the onset of cold temperatures, the water vapors may freeze and block the tube. The so-called "big bore" Continental engines, such as the 470, 520, and 550 series, are particularly susceptible to having these tubes freeze, since the tubes exit the engine case a few inches aft of the propeller flange and then are routed rearward along the top of the engine. During the run from the front of the engine to the aft baffle the aluminum tube is ex-posed to cold air coming in through the cowling openings.

The "whistle slot" should be checked before winter operations. This slot, which is actually a hole located six to nine inches up from the exit end of the breather tube, is there to provide engine venting if the end of the breather tube ices over and is blocked off because of impact ice. Sometimes these slots are covered by a clamp or insulation—so make sure there is a hole and that it is open during every pre-winter airplane check, and after engine maintenance.

Fuel system checks

When air temperatures get down to the vicinity of freezing, any water in the fuel system will cause big problems. Fuel selectors can freeze in position, and engines can stop when fuel lines are blocked by frozen water. Pilots can avoid these unwanted adventures by "de-watering" their airplane's fuel system in the fall. This is easily done by adding isopropyl alcohol to the fuel in the proper proportion of one percent by volume, which translates to one quart of alcohol for every 25 gallons of avgas. The proper amount of alcohol can be diluted in a couple of gallons of avgas and then added to the tanks, or it can be poured in slowly during fueling. Ethylene glycol monomethyl ether (EGME) can also be added to fuel when water is suspected. This additive is sold under the name of Prist PF 205 Lo-Flo and can be ordered from most aviation mail-order houses in an aerosol can that treats up to 90 gallons of avgas. Prist is sold with a dispensing tube and is added during fueling. If there's any reason to believe that the fuel being used isn't filtered properly, continue to treat the fuel with either additive. Both of these additives prevent any water in the fuel system from freezing.

Fuel tank vents that can be affected by impact ice should be checked for correct positioning in accordance with the aircraft service manual and adjusted if necessary.

Heater system checks

Both the heat muff-type heaters that are common on most single-engine airplanes and the avgas-fired combustion heaters that are found on many twins should be given the best possible maintenance, since even a small crack in either system can admit carbon monoxide to the passenger cabin. This fatal gas, a by-product of incomplete combustion, can't be smelled or seen, so a portable carbon monoxide detector should be in every winter pilot's flight bag. Prices for high-quality portable CO detectors that provide parts-per-million readouts and aural warnings run about $50. One Web site that supplies units suitable for aviation use is www.aeromedix.com. The inexpensive plastic stick-on detectors, like the Deadstop and the Quantum Eye, which have a "dot" that changes color in the presence of CO, are also effective as long as the expiration dates are current and they are scanned regularly during every flight.

Oil system realities

Pilots' operating manuals and engine operators' manuals all have a section that specifies when to change from summer-weight oil, which is usually 50 weight, to winter-weight oil, which can be either 30 weight, 40 weight, or a multigrade oil such as 15W-50 or 20W-50. The Continental Operators' Manual for the O-470 series engines recommends that 50 weight oil be used when the average ambient air temperature is above 40 degrees F and 30 weight or multigrade oil be used below 40 degrees F. The change to less viscous oil is done so that the oil circulates quickly throughout the engine immediately after engine start.

Failure to switch to a less viscous oil, or failure to adequately preheat an engine when temperatures drop, can result in metal-to-metal contact within the engine because of lack of lubrication. This type of wear is called scuffing. Camshaft lobes and lifter bodies, pistons, and cylinder walls are especially susceptible to scuffing wear caused by inadequate lubrication during cold starts. Some engines are more susceptible to cold start damage because the lubrication of these high-stress areas is done by splash lubrication. Splash systems depend upon the reciprocating motion of the internal engine parts to splash or distribute oil within the engine. When the oil is thick, or the engine isn't preheated well, splash systems can't adequately distribute the oil during engine start. Rapid and sufficient lubrication are the primary reasons we switch to more viscous oils and preheat our engines when air temperatures drop.

Worst-case starting

With persistence, auxiliary electrical power, lots of priming, and the application of carburetor heat during the initial start and warmup period, aircraft engines can be started and will run when cold-soaked at temperatures below zero degrees F. But the engine will be damaged because of the lack of lubrication already mentioned, and because the excessive priming will dilute and wash off the existing oil film on the cylinder walls. Excessive priming becomes necessary because avgas doesn't vaporize very well below approximately 20 degrees F.

Mike Busch, who has written extensively about turbocharged engine operations, estimates that the wear incurred during one cold start like the one described is the equivalent of 500 hours of normal engine operation. Fred Potts, author of F.E. Potts' Guide to Bush Flying, a book that distills the experience gained during 20 years of Alaska flying, preheats all his engines when the ambient temperatures drop to 30 degrees F. because, as Potts says, "I like to get my full TBO." Once again, preheating is done to ensure adequate lubrication during the start and initial engine warmup phase of operation, and to aid in better fuel vaporization during cold temperatures.

Winterization kits

Winterization kits, also called winter fronts, are installed to maintain cylinder head temperatures and oil temperatures. On some airplanes these kits also add a restrictor plate to the carburetor air intake to compensate for the dense, cold winter air. A winterization kit for a Cessna 182 includes a left and right cowl inlet air plate, a carburetor inlet air restrictor plate, lagging for the engine breather tube, and a decal for the instrument panel. The decal directs the owner to install the kit when the average ambient temperatures drop below 20 degrees F. In addition to restricting the air over the cylinders, the right cowl inlet plate restricts the airflow to the oil cooler.

The most critical part of this kit is the one that restricts airflow to the oil cooler—it's vital to maintaining working oil temperatures. Since winterization kits are no longer available for some general aviation airplanes, and there don't appear to be any STCed replacement kits, many northern pilots restrict the airflow through the oil coolers by the application of a strip of aviation-grade duct tape. If used, the tape should be applied across the middle of the cooler's width, leaving the upper and lower areas clear.

Operational tips

Warm up the engine at 1,000 to 1,200 rpm unless it's necessary to reduce rpm to keep from exceeding the oil pressure redline. As the oil warms up, the rpm can be increased. Allow plenty of time for the engine to warm up. The hydraulic lifters, which adjust the valve lash to compensate for engine expansion during warmup and operation, are dependent on warm oil to work correctly.

Some carbureted engines, like the O-470 in the Cessna 180 and 182, run markedly better if partial carburetor heat is applied during cruise flight in cold weather. A carburetor temperature gauge is necessary to practice this art, which consists of adding carb heat to maintain a 10 degree C. /50 degree F. carb throat temperature.

Watch oil temperatures carefully. Really cold temperatures can cause oil to congeal in the cooler. When this happens the oil temperature climbs rapidly. Short of landing and waiting for the oil to warm up, slowing the airplane may reduce the amount of cooling air and permit the oil to flow again.

Planning should include minimum brake usage when landing and when taxiing through snow since warm brakes melt any snow upon stopping. Then the snow refreezes, locking the plane in position. A wise pilot always gives each main tire a kick during preflight to check for frozen brakes.

Cold temperatures also may require higher-than-normal engine power settings to keep the temperatures up in the green arc.

Winter flying requires the correct mental attitude, a commitment to pay extra attention to the care and maintenance of the airplane, and a willing-ness to wait out some weather. But these are minor inconveniences compared to the payoffs that can only be experienced by those willing to take the winter flying plunge.


Links to additional information about winter flying may be found on AOPA Online ( www.aopa.org/pilot/links/links0012.shtml). E-mail the author at steve.ells@aopa.org.