For an aircraft to be certificated for flight in "known ice" conditions, generally it must, as a minimum, have ice protection for the wings, propellers, and the windshield. Aircraft certified for flight in instrument meteorological conditions (IMC) must also have ice protection for the pitot/static system.
Pitot/Static Ice Protection
Pitot/static heat is the most common ice protection equipment on light general aviation aircraft. It consists of an electrical resistance heater that warms the pitot tube (and static port in some designs) to prevent the accumulation of ice. As a general rule, you should energize these devices only briefly during the preflight function check and during flight. If the pitot/static heat system operates continuously while the airplane is on the ground, it may overheat, possibly damaging the pitot tube or wing. Most instructors teach their students to always energize the pitot/static heat prior to entering visible moisture - the clouds.
Wing Ice Protection
General aviation aircraft use three different kinds of wing ice protection systems. The most common is the pneumatic de-ice boot. These reinforced rubber devices are cemented to wing leading edges, and sometimes to the horizontal and vertical stabilizers if certification tests deem them necessary.
Inflatable tubes or channels within the boots expand when pressurized to break apart accumulated ice. The system usually uses the same vacuum/pressure pump that operates the flight instruments (directional gyro and attitude indicator), although the pump usually has a higher capacity than those used on aircraft without de-ice boots. A typical design incorporates a shuttle valve that inflates and deflates the boots by connecting them alternately to the pump inlet and outlet. The system can be designed for manual operation, automatic operation (periodic inflation controlled by a timer), or both.
When you use de-ice boots, it's important to follow the procedures outlined in the aircraft pilot operating handbook (POH). Usually, pilots should not activate the boots until a quarter- to a half-inch of ice has accumulated on them. If you inflate the boots before sufficient ice has accumulated, the ice may reshape over the contour of the inflated boots instead of breaking apart, which renders the ice protection system useless. Coating the boots with an anti-icing fluid can improve their performance and reduce ice formation.
The "weeping wing" is another system found on some general aviation aircraft. This design uses a perforated metal strip, either attached to or built into the wing's leading edge, through which a glycol solution is pumped. The glycol flows back over the wing, coating it and preventing ice accumulation. An advantage to the weeping wing is that it prevents ice from forming on virtually the entire wing surface, instead of just on the leading edge.
Many turbine-powered aircraft use a hot wing system to prevent ice formation on the wings. This anti-ice system bleeds some air that has been compressed, and thus heated, by the engine's compressor wheels and ducts it through a system of vents in the leading edge to provide the necessary heat to prevent ice formation.
Propeller Ice Protection
On general aviation aircraft the most common method of protecting against ice accumulation on the propeller is an electrically operated system that uses resistance heaters bonded to the propellers' leading edges. Each propeller blade has inboard and outboard heating elements that are activated alternately. Alternating the heating elements reduces the maximum current draw on the aircraft's electrical system. The heating elements on the propellers are sequentially energized for a specific time. Accumulated ice begins to melt and is slung off the spinning blades. The ice may be thrown against the fuselage, which creates an enormous racket that may startle the unsuspecting pilot or passenger.
Some de-ice designs use isopropyl alcohol to prevent ice accumulation on the propellers. Small tubes pipe alcohol to the propeller and spray it onto the inboard leading edge section of each blade.
Window Heat
The most commonly seen system to prevent ice accumulation on the windshield is electric window heat. The system usually consists of a small secondary window on the outer surface of the windshield in front of the pilot. Small heating elements, similar to a car's rear window defroster, produce heat and keep ice from accumulating. Other designs incorporate electrical heating elements into the windshield. You should energize these systems only in flight, the same as pitot heat. Inadvertently operating this kind of anti-icer on the ground can melt or discolor the window.
Ice Protection System Precautions
Regardless what type of ice protection system you have on an aircraft, you should service and test it properly before using it in flight. Testing the various components is an important part of the preflight inspection for any flight during which you might encounter ice (any flight in clouds or visible moisture). Some electrically-operated systems have separate ammeters to monitor the current they draw. Monitoring these meters, or the meter for the aircraft's electrical system, helps to verify the system's operation. When appropriate, ice protection systems should be cycled prior to flight to ensure proper operation.
Even if the aircraft is certificated for known icing, pilots should avoid icing conditions. Any aircraft, regardless what level of ice protection it has, should avoid severe icing conditions. Mother Nature is quite capable of throwing enough ice at an aircraft to defeat any anti-ice or de-ice system. This caveat applies to transport category airliners as well as smaller general aviation aircraft. De-ice and anti-ice systems are intended to allow the pilot to escape icing conditions safely, not continue to fly in them.
Remember, too, that icing conditions are difficult to forecast. Icing can occur even when it hasn't been forecast. Ice protection systems aren't a cure-all for in-flight icing, but having ice protection and the knowledge to properly use it can take a big load off your mind.
Ice Avoidance Strategies
When you plan an IFR flight, be sure to get information on freezing levels and potential icing conditions. Although icing can occur in a wide variety of conditions, it's most apt to occur when visible moisture is present and the temperature is at or below 0?C.
Ice usually forms first on leading edges and small protrusions. Watch the temperature probe, fuel caps, and antennas for the first signs of ice. Once you see ice starting to form, it's time to act.
Rime ice is often associated with stratiform clouds and can extend over broad geographical areas. To avoid the icing conditions, a pilot can often climb to an altitude where the temperature is below -10?C or descend to an altitude where temperatures are above freezing. A 1,000-foot altitude change may be enough to fly clear of the icing conditions.
In freezing rain associated with a frontal system, it may be possible to climb into an area of warmer air. Remember, if performance and equipment are suitable, climbing might be the first choice of action.
Avoid cumulus clouds whenever icing potential exists, because clear ice may be present above the freezing level. The most rapid clear ice accumulation occurs when temperatures are between 0 and -15?C. Clear ice is considered to be the most dangerous because it can coat virtually the entire aircraft, is notoriously difficult to remove, and adds weight quickly.
When you climb through altitudes where icing is present, use a climb speed faster than VY. At slower airspeeds, ice might accumulate under the wing behind the leading edges aft of the de-ice boots.
If you encounter icing, make an immediate effort to get out of the conditions. Especially in severe icing, don't hesitate to declare an emergency if that is what it takes to get the assistance needed to remedy the situation.
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