Icing Avoidance
Ice does not accumulate in clouds until you fly into the super-cooled droplets that exist from freezing level down to -4-degrees F. At +5-degrees F droplets freeze on the leading edges of the flying surfaces. From 28-degrees to 32-degrees the water runs back on the wings and forms clear ice. At 14-degrees to 24^ ice may freeze on the trailing edges. From the FSS you want to know where is the water and how large are the droplets. The standard briefing will always include icing information. Any forecast with 50% humidity means that there is icing up there.
A pilot should watch TV weather for several days before going on an extended flight. Only by understanding the formation, growth, movement and changes in the weather can the pilot learn to anticipate what is coming. Interestingly over half of all weather related accidents show no evidence of the pilot having either sought or received pertinent weather information. TV weather is just the beginning. A FSS/Flight Watch specialist can pick up the loose ends by providing currency to information that applies to a specific route of flight.
1. Depart the area
2. Climb to above freezing temperature
3. Descend to above freezing temperatures
4. Frost should always be removed form an aircraft before flight.
5. Freezing or super-cooled rain is most hazardous of all conditions.
6. A 180 out of icing is a crap shoot.
7. Climb at reduced angle of attack to improve stall margin.
8. You can only avoid icing by avoidance of icing conditions.
Where's the Ice:
--Where's the water
--How much water
--Droplet size
--Backflow around a low?
--Upslope lifting related to cold front?
--Get tops, bases, and temperatures.
--Relative humidity trends.
--Have TAFs been amended
--Where are the escape routes?
If encountering icing, you must get clear as soon as possible. This means climbing or descending at least 2500'. Given the choice I would recommend climbing if within aircraft capability. My reasoning for this is that if you can't climb clear at least you will have acquired some additional obstacle clearance altitude. If you can get into -20 C you are not likely to get more ice.
Ice is possible in the six to ten thousand foot level year-around when lifting occurs. Winds carry moisture so determine if you are flying the warm side or the cold side of the fronts. Amendments to forecasts mean that the forecasts were wrong in the first place. Freezing level charts are renewed every twelve hours.
Misinformation as well as misinterpretation is the major causes of icing accidents. Freezing drizzle is more dangerous than freezing rain because it leaves a rough surface.
Ice and Performance
Carburetor ice
Induction ice
Fuel system ice
Rime ice -2 to -10C
Clear ice -10 to +2
Worst ice at cloud tops
Loss of thrust
Increased drag
Less lift
Higher stall speed
Trim effects
Stall differences
Instrument problems
Induction Ice
The specific kind of induction ice of the several available is not always determined by the kind of carburetion used by the aircraft. Impact icing that accumulates on the exterior air intakes can and will accumulate on any aircraft. Impact icing only occurs in actual IFR conditions with temperatures in the 25 F area. The first signs of impact ice appears at the edges of the windows and on the sharper protrusions of the aircraft. Freezing temperatures in the cockpit can cause the grease used to lubricate the throttle cable to congeal and make any throttle movement impossible. The pilot reaction when suspecting induction ice is to utilize alternate air. With alternate air comes a reduction in power.
A richer operating mixture comes as a result of both carburetor heat and alternate air. Leaning will improve both engine operation and increase engine heat. While Lycoming says detonation is not possible with less than 75% power with both carburetor heat and alternate air, a full power go-around presents a detonation probability. Leaning will increase engine heat and improve anti-icing capability.
Ice cannot exist unless there is water vapor present that can be frozen by cooling due to venturi effect in the carburetor or by temperature drop due to the vaporization of fuel. In either case, ice will accumulate on the interior and protrusions inside the carburetor.
The presence of ice initially causes a drop in rpm, followed by engine roughness and finally stoppage. This sequence can occur through a rage of outside temperatures from 20 F to 90 F when humidity is above 50%. My experience has been that taxiing in 50-F is a high probability zone at CCR. When outside temperature is below 14F it will be too cold to form carburetor ice UNLESS heat is applied. Don't!
Throttle ice occurs when a prolonged descent cools the engine so that insufficient warm air is available to melt the ice. Since such icing can occur on relatively short notice, this is one reason not to make power-off landings and is one of the reasons for the FAA gave for making the change to partial-power landings as being the standard.
Defenses against Ice
Get priority handling from ATC
Have an escape plan
Use carburetor heat and leave it on
Stay clear of clouds even under IFR
Check POH for minimum speed with ice
Climb immediately if you can
Descend quickly
Control effects at altitude
Increase in approach speed
No flaps
Smooth maneuvers
Fly down to the ground
Long runways
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