Ice and aircrafts are never a good combination. Ice, which can accumulate on any part of the aircraft, is most dangerous when it accumulates on the wings or similar airfoils. These icing encounters cause many fatalities a year, fatalities that could be prevented if pilots use the strategies and tactics that are at their disposal to avoid icing dangers. In flight icing is always a concern and should be treated with an expedited response, however, proper avoidance of icing conditions begins on the ground during preflight.
This safety report will discuss the many types of icing and their effects on flight. Along with the effects of icing on an aircraft, this report will examine the procedures to follow when reacting to these icing conditions. This report will include accident data as reported by the National Transportation Safety Board (NTSB) and more importantly the research and technologies developed to help reduce icing-related aviation accidents. Aircraft Icing Aircraft Icing What are the Causes and Possible Solutions? Icing is a definite weather hazard to aircraft.
Icing refers to any deposit or coating of ice on an aircraft. Two types of icing are critical in the operation of aircraft: induction icing and structural icing. Another important form of structural icing may affect the runway or other resources used by aircraft. A runway covered with even a thin film of ice can cause loss of directional control and make braking efforts completely ineffective while the craft is on the ground (Roy, Steuernagle, Wright, 2008). In flight, including the takeoff, the threat of ice hazard is increased.
Icing Causes: Common sense tells us that winter time brings on icing conditions, however, ice is present, or potentially present, somewhere in the atmosphere at all times, no matter what the season. The secret is the freezing level of altitude, which may be around 15,000 feet during the summer and perhaps as low as 1,000 feet above ground level (AGL) on those “warm” winter days (Lester, 2004). Carburetor icing: When the temperature and dew point are close, you can be certain that water vapor is condensing within the carburetor of an aircraft reciprocating engine, and, if the engine is run at low speed, the condensation is turning into ice.
This is why some engine manufacturers recommend that carburetor heat be applied when the throttle is retarded for prolonged descent and prior to landing (Gleim, 2003). Accident summaries contain many cases of unexplained power loss. Many of these aircraft accidents can be attributed to carburetor ice. Once carburetor ice is suspected –at the first sign of engine roughness Aircraft Icing or power loss- apply full carburetor heat (Gleim, 2003). After carburetor heat is applied the engine may run rougher as the ice melts away but the rpm will return to their normal setting.
There are many cases of loss of engine power as a result of carburetor icing which forces a landing. The following accident report summary describes a similar carburetor icing situation: A 106-hour Skyhawk pilot reported that the engine began to run rough and lost power as the airplane climbed through 9,000 feet means sea level (MSL). She then switched fuel tanks and moved the mixture to full rich, but the engine continued to lose power. Carburetor heat was not used at any time. A forced landing was subsequently made in a field, where the airplane collided with a utility pole and landed in a ditch.
An examination of the engine revealed no evidence of mechanical failure or malfunction. An icing probability chart revealed that the reported weather conditions in the area were favorable for the formation of moderate carburetor icing at cruise power. The Cessna 172M owner’s manual notes that a gradual loss in rpm and eventual engine roughness may result from the formation of carburetor ice and prescribes the use of carburetor heat to clear the ice. (Civil Aviation Authority, 2006). Structural Icing:
The previous report refers to induction icing within the engine, but other forms of icing attach to the exterior of the aircraft called “structural icing”. Airframe or structural icing refers to the accumulation of ice on the exterior of the aircraft during flight through Aircraft Icing clouds or liquid precipitation when the skin temperature of the aircraft is equal to, or less than 0 deg C (Lester, 2004). Types of Structural Icing: Structural icing takes on many forms depending on the size of the moisture that comes in contact with the aircraft.
The types of structural ice are clear, rime and a combination of the two. The primary concern over even the slightest amount of structural ice is the loss of aerodynamic efficiency. The increase in drag caused by the additional ice also causes an increase in stall speed, instability and a decrease in lift (Roy, K. S, 2008). The type of ice that forms on the aircraft primarily depends on the size of the water droplets. Clear ice forms when the drops are large and the droplets impacting an airplane freeze slowly, spreading over the aircraft components gradually forming a smooth sheet of solid ice (Lester, 2004).
Clear ice is the most dangerous form of structural icing because it is heavy and hard; it adheres strongly to the aircraft surface greatly disrupting airflow. Clear ice will normally form while flying through cumuliform clouds and through freezing rain (Lester, 2004). Rime ice is the most common icing type and forms while flying through stratified clouds and freezing drizzle (Lester, 2004). It forms when water droplets freeze on impact, trapping air between the small frozen drops, giving the ice a milky white appearance.
Mixed ice has characteristics of both types making it a combination of rime and clear ice. Aircraft Icing The following NTSB summary describes the dangers of inadvertently encountering ice and the effect it will have on the ability of the aircraft to maintain lift and stability: CE 182. One serious and one minor injury. Pilot received a weather briefing approximately one hour prior to flight during which “A chance of light icing” was forecast. Approximately 30 minutes after takeoff, while at 6,000 feet, a small amount of ice began to form on the strut in the light rain.
Although the aircraft was then cleared to climb above the cloud layer, heavy icing began to accumulate. The aircraft could not climb above 7,300 MSL and a 300 – 400 feet per minutes (f. p. m). descent developed. The aircraft was cleared to an alternate airport via radar vectors. Over the runway at about 50 feet above ground level (AGL), the aircraft uncontrollably veered to the left and struck the ground hard, collapsing the nose gear. A witness stated that there was ? inch of ice on the fuselage and an inch on the belly. The aircraft was also loaded approximately 200 pounds over gross weight.
NTSB cited the probable causes as icing, improper weather evaluation, and deteriorated aircraft performance. (Watson, 2007) Pilots need to avoid ice especially if their aircraft are not approved for flight into icing. The aircraft in the NTSB report summary above was not approved for flight into icing conditions. Although ice forecast retrieved via weather briefings are in some cases inaccurate the pilot needs to have an escape route should icing be encountered. Accident data as reported by the NTSB is most useful when it brings about the development of technologies that help reduce icing-related aviation accidents.
Aircraft fall into two categories, those approved for flight into icing and those that are not. Aircraft equipped with ice protection system allow them to keep ice from accumulating Aircraft Icing on the wing structures while in flight. The evolution of aircraft has provided advanced and useful technologies that have made our aircraft safer in less favorable atmospheric conditions. Icing protection systems: The types of icing protection systems are pneumatic deicing boots, thermal devices, and electro-mechanical systems (Burrows, 2002).
The pneumatic deicing boot is a rubber tube attached to the leading edge of an aircraft wing. When ice is encountered during flight, portions of the rubber device inflate breaking off the ice (Burrows, 2002). Pneumatic deicing boots are used on propeller driven aircraft and jets. Thermal systems use electricity to heat protected surfaces of equipped aircraft. Thermal deicing systems have a more advanced function than deicing boots in that it prevents ice from forming on the heated protected surfaces. The electric heaters can be used as de-icers or anti-icers (Burrows, 2002).
The newest technological advance in de-icing is called electromechanical de-icing, the system use a type of mechanical actuator that physically knocks the accumulated ice off the flight surfaces. The technology works in conjunction with previously developed ice detection systems and is triggered automatically once sensors detect ice. First, an electro-thermal strip heats the wing’s leading edge to just above freezing, melting the ice. Then other electro-thermal systems heat the leading edge enough to evaporate moisture on contact, preventing it from escaping and refreezing elsewhere as “runback” ice.
The water flows downstream and eventually freezes where Aircraft Icing the aircraft is less sensitive to airflow disruptions. That’s where [the deicers] hit it. ” An electrical current is sent through one set of coils at a time, and as the current loops through the coil, it flows in one direction and then the opposite, inducing a magnetic field. Jolted with electrical energy pulses that last . 0005 second, the coils deliver impact accelerations of over 10,000 Gs to the airfoil skin once a minute, shedding ice as thin as . 06 inch. Despite the high G-load, the impact amplitude—the amount of movement of the aircraft skin—is only about . 025 inch. The skin accelerates so rapidly, though, that ice de-bonds as if hit with a hammer (Smithsonian Air and Space).
Conclusion Ice and aircrafts are a dangerous combination when pilots don’t utilize weather services to determine freezing levels. When a pilot doesn’t understand when to deploy his ice protection system or doesn’t do a proper preflight including weather briefings, icing encounters become a reality. Many fatalities a year could be prevented if pilots use available resources to avoid icing dangers.