November 2002Features

Frigid-Flight Fundamentals

How To Get Winter Flying Down Cold


Unless coconuts are growing in your backyard or all the birds in your neighborhood wear tuxedos, preparations for winter flying will likely present you with challenges. The first and greatest is actually encountered even before leaving the ground: that bracing adventure of coaxing your airplane from hibernation and preparing it for flight.

It's easier to assimilate the collective wisdom of cold-weather flying if we consider each part separately, from preflight to postflight. In winter, there can actually be one additional step, and that's preheat, so let's start there.

When it's below freezing, a thorough preheat is just as essential as the studious scrutiny of preflight. Without one, your engine's moving parts will be straining to prevail over viscous, syrupy oil, and your cold-soaked battery will struggle against the increased resistance and a greatly increased load. Bone-dry cylinders will be scored as pistons grind against them; crankcase valves can stick; and then there is the risk of iced plugs, a flooded engine, or even fire when one misinterprets the cause of sluggish starting and overprimes the engine - or even worse, pumps the throttle. For any frozen engine, coercion by force only brings higher operating costs and an early overhaul.

Opinions vary as to what the cutoff is, but below 32 degrees Fahrenheit (0 degrees Celsius), a gentler means of persuasion should be de rigueur. A lucky few of us have the best preheat option of all: a warm hangar. In all my flying, I've enjoyed such a refuge exactly once. If you do have a hangar, even it's an icebox but it has electricity, an oil sump heater or preheat system is a good idea. For the rest of us, preheats epitomize life: If you won't plan on investing the time now, you can plan on investing the money later. If you rent the aircraft you fly, ask your flight school or fixed-base operator what its policies and capabilities are regarding preheating.

Perhaps born of Yankee ingenuity is a preheating option available in the flying club to which I belong. Years ago, some inventive soul removed the blades from a secondhand lawnmower, installed a sheet-metal cover under it, and attached a flexible hose to its discharge chute so that its main product became hot air. Even though it is the old pull-cord type, since there is no blade there is little inertia to overcome, and it's very easy to start. We simply direct the flexible hose upwards toward the engine by the nosewheel well, using any number of convenient points for attaching it via a hook installed at its business end. We are careful not to direct heated air directly onto fuel, oil, or hydraulic lines! Twenty minutes later: a warm engine. Lacking such inspiration, however, the accumulated expense of paying your FBO to send a lineman out with a preheater is still likely less than that of an engine overhaul.

Even before the preflight, what you do after a previous flight - in this case replenishing fuel - can be important. Condensation of water in partially filled fuel tanks can be bad news in the liquid phase, and potentially worse if it freezes. Water vapor can condense inside half-empty fuel tanks, and pieces of ice can do more than momentarily break your engine's stride, which is what entrained slugs of water can do in warmer weather. Ice can block fuel lines. Consider filling up after landing (winter or summer), so long as full fuel won't be a liability for the airplane's next pilot or your next flight-remember that many two-seat trainers can end up significantly over their maximum gross weight with full fuel and two large occupants. Another thing you can consider is removing wheel pants, which will help to prevent slush and compromised braking ability.

When I walk up to an airplane, regardless of how cold it is, the first thing I do is waggle the wings. By the time I get around to draining the sumps, maybe 10 or 15 minutes later, I've allowed some time for any water to settle to the lowest points. Then I get that preheat going while I complete the rest of my preflight.

It's natural to want to expedite a preflight when the outside temperature is low, but just when you want to hurry is precisely when you should take your time. Also, interruptions can lead to skipped checklist items, so if you get distracted by a friendly conversation (or a mug of hot chocolate), recheck the item you left off with and resume from there. Thoroughly check the fuel drains for entrained water. Examine the heater shrouds for cracks or gaps. And make sure you have a carbon monoxide testing patch in the cockpit (and that it hasn't expired).

Check all vents and even the crankcase breather line (underneath, usually by the nosewheel) for obstructions or condensed vapor. Frozen water vapor can plug up the breather line and give your engine a terminal case of iceclerosis, where pressure builds up and causes the oil filler cap to blow off, or ruptures a seal. Don't let water or melted ice get near hinges or other critical moving parts. Of course, check for ice or water in any vent, static port, and the pitot tube. All openings are targets for obstruction by ice.

Do not use an automotive scraper to remove snow and frost from the airplane! You can fashion a push-broom-style remover. You should carry several towels or cotton rags in your car to rub off all frost from the wings, or perhaps your flight school or FBO can provide deicing fluid for the task - ask for instruction in its proper use. Conscientious removal of snow and ice from airfoil surfaces is important, because even a light frost can sap the wing's lifting capacity.

Be sure to use the correct amount of oil (obviously), as well as the right grade, as indicated in your pilot's operating handbook - it might call for a different viscosity during cold-temperature operations. When it's cold, things get less flexible, and different materials react differently: check hoses, hydraulic fittings, and seals. In blowing snow, make sure all openings are clear, such as carburetor and heater intakes, and fuel vents. Also, this isn't exactly a preflight item, but if you have an anemic heater, blocking drafts from the baggage area can make a difference in cabin comfort.

Once the preheat is done, it's time to start the engine and resume the checklists. Wait too long and things may get cold quickly; you can do any cockpit resource management after the engine is really warmed up. First, always use your manufacturer's recommendations for cold-weather operations, and with cold-weather starts, always have a fire extinguisher handy. Once things are warmer under the cowling, here are some points to consider between the warm-up and the start-up. In cold weather, you will obviously need more primer than normal. If your engine isn't as preheated as you thought, it is possible to get frosty sparkplugs. This happens when it runs a few seconds, then quits; the cylinders are still cold, and water vapor freezes on the spark plugs' electrodes. If that occurs, you're not going anywhere quickly. If it's really cold, a slow idle may not keep the plugs warm enough, either. Experience (yours or someone else's) can tell you what rpm to use.

Your engine may be warming up, but be aware that your control cables (such as for the throttle and carburetor heat) may still be working under the load of congealed lubricants. Be wary of sluggish starting; if you don't preheat long enough, you may mistake sloth for starvation and overprime the engine. Overpriming could also result in scored cylinder walls from having washed them down with avgas (not to mention poorer compression, which will make starting even harder). Don't try to start even a warm engine by pumping the throttle. Engine fires in flight are rare; engine fires on the ground are not. (Think about that.) For constant-speed props, cycle the prop several times to fill the propeller hub with warm oil. Just be careful not to "deep-cycle" it.

Oil pressure should be in the green within 60 seconds at about 1,000 rpm. (That's a rule of thumb. Follow your POH.) Now you're sitting in a warmed-up airplane, ready to taxi. In a word, do it s-l-o-w-l-y! This goes double when there's snow and ice on the ground. Braking may be poor to nil, and even your tricycle-gear airplane may weathervane into a breeze. Avoid short turns and quick stops. Snow may be covering an ice slick. And give snow banks a wide berth! During the first few seconds of your runup, make sure you're not moving. Also, before you take off into any weather (and especially instrument meteorological conditions, if the weather allows and you're appropriately rated), be sure that your cockpit and instruments have warmed up. A vacuum-driven gyro, for example, can be unreliable below about 40 degrees Fahrenheit.

Finally, we're ready to fly. The best insurance item for wintertime (or any time) is to file and open a flight plan. Carry extra fuel, keep track of what winter winds may do to your fuel planning, and continue to keep track of the weather at your destination. Know where the better weather is. Over remote areas, plan your route of flight within range of roads. Dress warmly enough to walk home. Carry extra clothes, warm waterproof headgear, food, and especially water on long-distance winter flights, in case of an unplanned landing (see "Survival Scenarios," ). Making a long-distance phone call to confirm the conditions of taxiways and runways wherever you're headed is often a wise investment. And if you have a cell phone, bring it. Keep abreast of conditions en route via Flight Watch, HIWAS, and local ATIS stations.

Winter can bring some wonderful things: severe clear, below-sea-level density altitudes in the East, great tailwinds if you're headed East, better performance, and clouds low enough to be overflown. But aside from shorter days, what can be howling winds, annoying turbulence, and the weather-prone zone on the northeast side of a low-pressure system, ice is your biggest concern in flight. Using carburetor heat is obviously more critical when it's already cold, and particularly in partial-throttle operations such as slow flight, approach, and descent. If you do encounter structural ice, you can usually escape it by changing altitude, but waiting isn't an option. Climb up through it at a high speed and shallow angle, and down through it at a lower speed, but high rate of descent. Remember that ice can cause an aircraft to have sudden and violent stall characteristics, and a much higher stall speed. Ice usually forms first on small-radius fixtures. And if you see ice on the wings, chances are good that you already have ice on the tail, so don't slow down or use flaps.

Winter weather systems are smaller, and fronts often move faster, so things change (and also clear up) more quickly. Even your Piper Cherokee 140 can take you into an entirely different weather system. The world below can become a homogeneous sheet of white, and some of your previously well-contrasted favorite landmarks (even lakes) may be covered over with snow. Always be alert for the odor of exhaust fumes - carbon monoxide has no odor, but the other gasses do - or feelings of sluggishness, unexplained headache, dizziness, or other hypoxia symptoms. Be ready to shut off that heater and open a window! You may have missed something or developed a carbon monoxide leak. Winter brings more gusty winds, so go up with your favorite instructor and get crosswind current.

After you land, remember that cold may make flight more efficient, but it has the opposite effect on ground operations. What you see is not necessarily what you'll get. Depth perception suffers when snow covers everything. Snowdrifts may be larger than they appear, and ice may be invisible. Plan to use all of the available runway length and be on guard for wind shifts. Be prepared for ineffective braking, and lower your crosswind limits by half for snow (and to near nil for ice). If your destination is a nontowered field, contact an aircraft that has just landed and ask about runway conditions. Remember that snow plowing begins with the longest runway, and the GA ramp is usually last.

A few other thoughts: During engine shutdown, consider turning off the fuel and letting the engine run the carburetor dry. (This reduces the fire hazard during the next preheat.) Fill the tanks right away if doing so is appropriate, put on those control locks and tiedowns...and close that flight plan! Be sure to install the engine cover, if you have one, as well as the other familiar ones (pitot covers, etc.). For folks up in Frostbite Falls, remove wet-cell batteries when below freezing if they're not fully charged.

Oh, and one last thing: In mid-winter, never book the first rental slot of the day.

Jeff Pardo is an aviation writer in Maryland with a commercial private pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,100 hours in 12 years of flying. An AirLifeLine mission pilot, Pardo has also flown for the Civil Air Patrol.


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