Winter Flying - From our friendly NTSB people
Most pilots are familiar with winter conditions in their particular area. However, a distance of only a few miles can change the environment enough to present unforeseen problems to an inexperienced pilot. Flight planning during winter months requires special knowledge in order to protect the aircraft as well as the pilot. Extra precautions should be taken.
For example, consider routing your flight along a well-traveled road. With today's extensive highway system, most flights would not be extended more than a few minutes this way. You will have an out in case of emergency, and vehicles on the road will provide valuable information about the weather ahead -- if you see cars and trucks coming toward you with fresh snow on them, you can expect reduced visibility ahead, and may as well start making a 180-degree turn. (Remember, however, that certain roads that are well-traveled during the summer months may be abandoned in the winter.)
Always file a flight plan. A flight plan, in conjunction an ELT and a little knowledge of winter survival, may save your life.
When making business appointments, always give yourself an out by informing your contact that you intend to fly and will arrive at a certain time -- unless the weather conditions are unfavorable.
Experience has proven the advice of operators who are located in the area where an operation is contemplated to be invaluable. However, as the pilot, you have complete responsibility for the go/no go decision. Do not let compulsion take the place of good judgment.
* PREFLIGHT PREPARATION *
When the aircraft is out in the cold it is tempting to hurry through the preflight, but this is the time when you should do your best preflight inspection. Here are a few items which you should be certain to consider:
Cabin Heater -- Many aircraft are equipped with cabin heater shrouds which enclose the muffler portions of the exhaust system. Each year accident investigations reveal carbon monoxide as a probable cause of accidents occurring in cold weather operations. It is imperative that a thorough inspection of the heater system be made too eliminate the possibility of carbon monoxide entering the cockpit.
Wheel Wells and Wheel Pants -- During thawing conditions mud and slush can be thrown into wheel wells and wheel pants during taxi and takeoff. If frozen during flight, this mud and slush can create landing gear problems. The practice of recycling retractable gear after a takeoff in this condition should be used as an emergency procedure only.
Oil Pressure Controlled Propellers -- Propeller control difficulties can be encountered due to congealed oil. Caution should be taken when intentionally feathering propellers for training purposes to assure that the propeller is unfeathered before the oil becomes congealed.
Fuel System -- Extra care should be taken during changes in temperature, particularly when it nears the freezing level. Ice may be in the tanks, change into water when the temperature rises, filter down into the carburetor and cause an engine failure. If fuel does not drain freely from the sumps, a line or sump may be obstructed by sediment or ice. Fuel tank vents plugged by ice or snow can cause engine stoppage and/or collapse of the tank.
Ice, Snow and Frost -- A common winter accident is trying to take off with frost on the wing surface, which reduces lift and may prevent the airplane from becoming airborne at normal takeoff speed. Also, don't count on snow blowing off during the takeoff roll. There is often frost adhering to the wing surface below the snow. Caution should be used if an aircraft is taken from a heated hangar and allowed to sit outside for an extended length of time when it is snowing. The falling snow may melt on contact with aircraft surfaces and then refreeze. It may look like freshly fallen snow, but it usually will not blow away when the aircraft takes off. If an aircraft is parked in an area of blowing snow, special attention should be given to openings in the aircraft where snow can enter, freeze solid and obstruct operation. These areas include pitot tubes, heater intakes, carburetor intakes, antitorque and elevator controls, and main wheel and tail wheel wells where snow can freeze around elevator and rudder controls.
Engine Starts -- Be sure to follow the manufacturer's recommendations regarding preheat and cold starts. In moderately cold weather, engines are sometimes started without preheat. Particular care is recommended during this type of start. Oil is partially congealed and turning the engine is difficult. Also, there is a tendency to overprime, which can result in washed-down cylinder walls and scouring. This also results in poor compression and, consequently, harder starting. Sometimes aircraft fires have been started by overprime, when the engine fires and the exhaust system contains raw fuel. Other fires are caused by backfires through the carburetor. It is a good practice to have a fire extinguisher handy during these starts.
Another cold start problem that plagues an un-preheated engine is icing over the spark plug electrodes. This happens when an engine only fires a few revolutions and then quits. There has been sufficient combustion to create some water in the cylinders, but insufficient combustion to heat them up. This little bit of water condenses on the electrodes, freezes to ice and shorts them out. The only remedy is heat. Engines can quit during prolonged idling because insufficient heat is produced to keep the plugs from fouling out. Engines which quit under these circumstances are frequently found to have iced-over plugs.
After the engine starts, use of carburetor heat may assist in fuel vaporization until the engine generates sufficient heat.
Taxiing -- A pilot should keep in mind that braking action on ice or snow is generally poor. Do not taxi through small snowdrifts or snowbanks along the edge of the runway -- often there is solid ice under the snow -- and you want to avoid kicking snow into the wheel wells or pants. On a hardpacked or icy surface, the aircraft will slide sideways in a crosswind and directional control is minimal particularly during taxiing and landing roll when the control surfaces are less effective. If it is necessary to taxi downwind and the wind is strong, get help or don't go.
* TAKEOFF *
Cold weather offers a distinct performance advantage -- power output increases by about 1% for each ten degrees of temperature below standard. However, cold weather also offers some special problems.
Here are a few points to remember:
+ Take care not to overboost supercharged engines. This is easy to do given lower density altitude resulting from lower than standard air temperatures. Care should be exercised with normally aspirated engines as well -- at minus 40 F an engine will develop 10% more than rated power even though RPM and/or manifold pressure limits are not exceeded.
+ Use carburetor heat as required. In some cases, carb heat is necessary to vaporize the fuel since gasoline does not vaporize readily at very cold temperatures. Do not use carburetor heat in such a manner that it raises the mixture temperature barely to freezing or just a little below. In such circumstances, the use of carburetor heat may actually induce carburetor icing. Partial carburetor heat is not recommended if a carburetor temperature gauge is not installed.
+ If equipped, turn on the pitot tube heat prior to takeoff. However, it is prudent to anticipate the loss of an airspeed indicator or most any other instrument during a cold weather takeoff, especially if the cabin has not been preheated.
+ During climbout, keep a close watch on head temperature gauges to avoid overheating the engine. Cooling air can be restricted if baffles are installed for cold weather operation. Another factor can be extreme temperature inversions. If the head temperature nears critical, increase airspeed or open cowl flaps or both.
* EN ROUTE *
Weather -- Weather conditions vary considerably in cold climates. Call Flight Watch on 122.0 to establish contact with an En Route Flight Advisory Service (EFAS) station for actual weather along your route. In more remote areas, reporting stations are few and far between and reliance must be placed on pilot reports. However, don't be lured into adverse weather by a good pilot report. A pilot may give a good report and five or ten minutes later VFR flight may not be possible. A particular hazard which has claimed some very competent pilots is the "whiteout." This condition exists when there are no contrasting ground features within the pilot's visibility range, potentially leading to spatial disorientation, and calling for an immediate shift to instrument flight. Flying low over an open body of water during low visibility and a ragged ceiling is another ideal situation for spatial disorientation.
Carburetor Ice -- In general, carburetor ice will form in temperatures between 32 F and 80 F when the relative humidity is 50% or more. If visible moisture is present, it will form at temperatures between 15 F and 32 F. Warning signs include loss of RPM (fixed pitch), drop in manifold pressure (constant speed) and rough running. If you suspect carburetor ice, apply full carb heat. In aircraft equipped with a fixed-pitch propeller, you should note a decrease in RPM due to enriching the mixture and then a gradual increase in RPM as the ice melts away.
There are three categories of carburetor ice:
+ Impact ice formed by impact of moist air at temperatures between 15 F and 32 F on airscoops, throttle plates, heat valves, etc., and usually forms when visible moisture is present. Most rapid accumulation can be expected at 25 F.
+ Fuel ice forms at and downstream from the point that fuel is introduced when the moisture content of the air freezes as a result of the cooling caused by vaporization. It can occur whenever the relative humidity is more than 50% and generally occurs between 40 F and 80 F.
+ Throttle ice is formed at or near a partially closed throttle valve. The water vapor in the induction air condenses and freezes due to venturi effect cooling as air passes the throttle valve. Since the temperature drop due to venturi effect cooling is usually around 5 F, throttle ice will most often occur between 32 F and 37 F, although a combination of fuel and throttle ice could occur at higher ambient temperatures.
Carbon Monoxide Poisoning -- Although CO is usually found with exhaust gases, don't count on symptoms to warn you. CO is colorless, odorless and tasteless. However, if you smell fumes you should assume that CO is present. Since CO poisoning is a form of hypoxia, you should take precautions if you feel sluggishness, warmth, tightness across the forehead followed by headache, throbbing, pressure at the temples or ringing in the ears. Severe headache, nausea, dizziness and dimming of vision may follow. If any of the above exists, take the following precautions:
+ Shut off the cabin heater or any other opening to the engine compartment
+ Open a fresh air source immediately
+ Decrease altitude and/or use 100% oxygen if available
+ Land as soon as possible
+ Ascertain the source of the problem and confirm it has been corrected before further flight
* LETDOWN *
Engine Operation -- During letdown there may be a problem keeping the engine warm enough for high power operation if needed. It may be desirable to use more power than normal, which may in turn require extension of landing gear and/or flaps to keep airspeed within limits. Carburetor heat may also be necessary to help vaporize fuel, enrich the mixture and prevent icing and overcooling the engine.
Blowing Snow and Ice Fog -- If the weather pattern indicates rising winds, blowing snow may be expected, possible requiring an alternate course of action. Ice fog is a condition opposite to blowing snow and can be found in calm conditions at about minus 30 F and below. It is found near populated areas, since a necessary element in its formation is hydrocarbon nuclei, such as those found in automobile or smoke stack exhaust. Both of these conditions can form very rapidly, are only a few feet thick (usually no more than 50 feet) and may be associated with clear en route weather.
* LANDING *
A landing surface can be very treacherous in cold weather. Caution is advised regarding hazards such as snow banks on the sides of runways and poorly marked runways. If advance information is not available, take the time to circle the field and look for drifts or other obstacles. Be aware that tracks in the snow on a runway do not ensure safe landing conditions. Often snowmobiles will use runway areas, giving the illusion that aircraft have used the airport and snow is not deep.
* POST FLIGHT *
Here are a few items to consider before leaving the aircraft after flight:
+ Fill the tanks as soon as possible to minimize condensation, even if the aircraft is going into a hangar.
+ If the aircraft is to be left outside, put on engine and pitot covers.
+ If the weather forecast is for snow or "clear and colder," put on wing covers and save yourself a snow or frost removal job in the morning.
+ Lock controls and tie down.
+ During engine shutdown, a good practice is to turn off the fuel and run the carburetor dry. This lessens the fire hazard during preheat the next morning.
* SURVIVAL *
After a crash landing, vacate the aircraft as soon as possible. Take care of any injuries first. Stay away from the aircraft until all gasoline fumes are gone. Take time to analyze the situation. Sit down and think. Keep in mind that survival is 80% mental, 10% equipment and 10% skills. Establish a goal to conquer regardless of the consequences. Don't run off without taking time to think out each problem. Don't imagine things that are not there. There are basic fears in each of us.
Fear of the unknown
Fear of darkness
Fear of discomfort
Fear of being alone
Fear of animals
Fear of death
Fear of punishment
Fear of personal guilt
Your mind is the best tool for survival. Use it.
The number one enemy is yourself
The number two enemy is injury
The number three enemy is temperature
The number four enemy is disease
Whether to stay with the aircraft or start out on foot is a major decision. Did you file a flight plan? If you did, it may be best to let them find you. Is your ELT operating? Do you have a survival kit? Here are some items you can find in the aircraft that can be used to aid in survival:
+ The compass will keep you going in one direction
+ Gasoline will help make a fire
+ Oil can be used for smoke signals
+ Seat upholstery can be wrapped around feet and hands
+ Wiring can be used as tie strings
+ The battery can be used to ignite fuel
Don't fight a storm. Most storms are of short duration. Stay put and find shelter. Use whatever is available to protect the body from loss of heat. Don't waste body heat by eating snow. Make a fire and heat water before drinking. You can conserve energy to last three weeks if you have water and stay dry. Body heat can escape 240 times faster from wet clothing than from dry clothing. It is best to eat small amounts of sugary food to replace the energy lost through body heat.
A good survival kit is well worth its weight.
The following would be a useful kit:
+ Metal container with a water-tight lid. This container will store the items in your kit and can be used to heat water, as a signal mirror and as a digging tool.
+ Boy Scout knife
+ Small candle
+ Box of matches (wrapped in plastic)
+ 2 Large leaf/trash bags (pull one over head, cut hole for face; step into the other and tuck into pants or tie around waist -- you now have full body protection from heat loss.)
+ Sugar cubes (wrapped in plastic)
+ Plastic tape