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Aviation Learning Center Document Winter Flying Tips - P-8740-24
Author: FAA Date: 1996
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Weather conditions vary considerably in cold climates. In the more remote sections of the world weather reporting stations are generally 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. Winter weather is often very changeable; one pilot may give a good report and five or ten minutes later VFR may not be possible.

Remember, mountain flying and bad weather don't mix. Set personal limits and stick to them.

Snow showers are, of course, quite prevalent in colder climates. When penetration is made of a snow shower, the pilot may suddenly find himself without visibility and in IFR conditions. Snow showers will often start with light snow and build. Another hazard which has claimed as its victims some very competent pilots is the "whiteout." This condition is one where within the pilot's visibility range there are no contrasting ground features. Obviously the smaller the visibility range the more chance there is of a whiteout; however, whiteout can occur in good visibility conditions. A whiteout condition calls for an immediate shift to instrument flight. The pilot should be prepared for this both from the standpoint of training and aircraft equipment.

Carburetor Ice

Three categories of carburetor ice are:

  • Impact ice - Formed by impact of moist air at temperatures between 15 and 32 degrees F on airscoops, throttle plates, heat valves, etc. Usually forms when visible moisture such as rain, snow, sleet, or clouds are present. Most rapid accumulation can be anticipated at 25 degrees F.
  • Fuel ice - Forms at and downstream of the point where fuel is introduced, and occurs when the moisture content of the air freezes as a result of the cooling caused by vaporization. It generally occurs between 40 and 80 degrees F, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50 percent.
  • Throttle ice - Forms at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve. Since the temperature drop is usually around 5 degrees F, the best temperatures for forming throttle ice would be 32 to 37 degrees F although a combination of fuel and throttle ice could occur at higher ambient temperatures.

In general, carburetor ice will form in temperatures between 32 and 50 degrees F when the relative humidity is 50 percent or more. If visible moisture is present, it will form at temperatures between 15 and 32 degrees F. A carburetor air temperature (CAT) gauge is extremely helpful to keep the temperatures within the carburetor in the proper range. Partial carburetor heat is not recommended if a CAT gauge is not installed. Partial throttle (cruise or letdown) is the most critical time for carburetor ice. The recommended practice is to apply carburetor heat before reducing power and to use partial power during letdown to prevent icing and overcooling the engine.

To prevent carb ice:

  • Use carb heat ground check
  • Use heat in the icing range
  • Use heat on approach and descent

Warning signs of carb ice include:

  • Loss of rpm (fixed pitch)
  • Drop in manifold pressure (constant speed); rough running

Pilot response to warning signs should be:

  • Apply full carb heat immediately (may run rough initially for short time while ice melts)

In the chart below, the curves encompass conditions known to be favorable for carburetor icing. The severity of this problem varies with different types, but these curves are a guide for the typical light aircraft. Light icing over a prolonged period may become serious. When you receive a weather briefing, note the temperature and dewpoint and consult this chart.


Carbon Monoxide Poisoning

Don't count on symptoms of carbon monoxide to warn you: It's colorless, odorless, and tasteless, although it is usually found with exhaust gases and fumes. If you smell fumes or feel any of the following symptoms, you should assume that carbon monoxide is present.

Initial symptoms include feelings of sluggishness, warmth, and tightness across forehead, followed by headache, throbbing, pressure at the temples and ringing in the ears. Severe headache, nausea, dizziness, and dimming of vision may follow. If any of the above conditions exist, take the following precautions:

  • Shut off the cabin heater or any other opening to the engine compartment.
  • Open a fresh air source immediately.
  • Don't smoke.
  • Use 100 percent oxygen if available.
  • Land as soon as possible.
  • Be sure the source of the contamination is corrected before further flight.

Spatial disorientation can also be expected any time the pilot continues VFR flight into adverse weather conditions. Flying low over an open body of water during low visibility and a ragged ceiling is another ideal situation for disorientation.

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