In-flight or airframe Icing is the atmospheric weather condition when supercooled water freezes and accumulates on the aircraft's external surface.
In-Icing is a serious aviation weather hazard and it can lead to fatal accidents if not noticed or not taken any action on time. Aircraft will have equipment such as de-icing or anti-icing to prevent icing accumulation on the external surface.
Icing Weather condition
Icing condition is expected during the following weather conditions,
Visible water in the air
Temperature less than or equal to 0 degrees Celsius.
Weather Advisories
During en-route, In-flight weather advisories are distributed to pilots if icing is observed in any specific area. In general, these weather advisories will have intensity, type of ice and location details.
Intensity will be measured based on the rate the accumulation. The following intensity will be used in the weather advisories.
Intensity | Condition / Meaning |
Trace (TR) | Ice becomes perceptible. The Rate of accumulation of ice is slightly greater than the rate of loss due to sublimation |
Light (LGT) | The rate of accumulation may create a problem for flight in this environment for one hour. Unless encountered for one hour or more, de-icing/anti-icing equipment and/or heading or altitude change are not required. |
Moderate (MDT/MOD) | The rate of accumulation is such that even short encounters become potentially hazardous. De-icing/anti-icing required to remove/ prevent accumulation or heading or altitude change required. |
Severe | The rate of accumulation is such that de-icing/ anti-icing equipment fails to reduce or control the hazard. De-icing/anti-icing is required, immediate heading or altitude change is required. |
The type of Ice will be measured based on the droplet size. The following types will be added to the weather advisories,
Icing Type | Condition or Meaning |
Rime | Formed by small supercooled water droplets Rough or Milky white appearance Can be removed by the de-icing system or prevented by anti-icing |
Clear | Formed by larger supercooled water droplets Clear or Glaze Ice Hard to remove using the de-icing system |
Mixed | Formed by the combination of small and larger supercooled water droplets Combination of Rime and Clear icing type Hard to remove using the de-icing system |
Effects on Icing
Accumulation of Icing in aircraft structures is a serious hazard and it can impact aerodynamics by,
Destroying the smooth flow of air
Increase the weight due to accumulation of ice
Loss of Lift
Increase the drag
Increased stall speed
Blockage of pitot tubes and static vents
Radio Communication Failures
Ice Protection Systems
Ice protection systems are separated based on the following behaviour or usage,
System will be used to remove the ice after its formed (de-icing system)
System will be used to prevent the aircraft from icing formation (anti-icing system).
De-icing System
The de-icing system will be activated after observing the icing and it will remove the ice buildup. Common De-icing systems use pneumatically inflated rubber boots on the leading edge of the airfoil. These rubber tube will be inflated to crack the ice and allow air airflow over the wings to blow off the broken ice.
Anti-Icing System
The anti-icing system will be activated prior to prevent the icing formation (When icing conditions are expected during any phase of flight). The hot bleed air from the engines will be used to prevent the icing formation in the wings.
Operational Condition
Aircraft not allowed to takeoff with ice/snow/frost on the control surfaces. Also aircraft without ice protection system is not allowed to operate on icing conditions