- 1 Introduction
- 2 Description
- 3 Airframe Icing Effects
- 4 The Airframe Ice Accretion Process
- 5 Icing in Clouds and Precipitation
- 6 Types of In-flight Airframe Icing Accidents
- 7 Solutions
- 8 See also
- 9 Reference
- 10 Author
Atmospheric water exists in liquid form well below 0°C. The proportion of such super cooled water decreases as the static air temperature drops around -40°C (except in Cumulonimbus Cloud).
The size of water droplets and the airflow characteristics around the aircraft surface determine the extent to which these droplets will strike the surface.
The size of a water droplets determines the time required for the physical change of state from liquid (water) to solid (ice) to occur (larger droplets will take longer to freeze).
Airframe Icing Effects
In-flight icing could lead to many problems, quite different in nature, such as:
- Reduced performance
- Reduced lift - Increased drag
- Uncommanded and uncontrolled roll (altered controllability)
- Subsequent loss of control of the aircraft
- Higher stall speed at lower angles of attack
- Ultimately stall
- Structural damage due induced vibration
Hazards arising from the presence of ice on an airframe include:
Adverse Aerodynamic Effects
The latter effect may alter longitudinal stability and pitch trim requirements. Longitudinal stability may also be affected by a degradation of lift generated by the horizontal stabiliser. The modified airflow pattern may significantly alter the pressure distribution around flight control surfaces such as ailerons and elevators.
Blockage of pitot tubes and static vents
The Airframe Ice Accretion Process
Ice accretion on an aircraft structure can be distinguished as:
- Rime Icing
- Clear/Glaze Icing
The rapidity of the transition to a frozen state is because the droplets are small and the almost instant transition leads to the creation of a mixture of tiny ice particles and trapped air.
Rime ice can affect the aerodynamic characteristics of wings and horizontal stabilisers as well as restricting engine air inlets. Rime may begin to form as a rough coating of a leading edge but if accretion continues, irregular protrusions may develop forward into the airstream.
Cloudy or Mixed Ice
Some other terms which may be encountered in connection with airframe ice accretion include supercooled large droplets (SLD).
If a SLD is large enough, its mass will prevent the pressure wave traveling ahead of an airfoil from deflecting it. When this occurs, the droplet will impinge further aft than a typical cloud-sized droplet, possibly beyond the protected area and form clear ice.
Runback may occur when a thermal ice protection system has insufficient heat to evaporate the quantity of super cooled water impinging on the surface.
Any ice remaining after a de-icing system of this type has been selected off is sometimes referred to as residual ice. These de-icing systems create normal accumulation of some ice when these systems are not ‘ON’.
Icing in Clouds and Precipitation
Typical icing threats can be found in some specific weather conditions:
- Cumuliform cloud structures usually contain relatively large droplets which can lead to very rapid ice build-up.
- Stratiform cloud structures usually contain much smaller droplets which can lead sometimes to a considerable ice accumulation in even a relatively short period of level flight.
- Any drizzle or rain, when temperature is below 0°C, can generate significant ice accretion in a very short period of time, and such conditions should be excited by any appropriate change of flight path.
- Snow in itself does not present an icing threat, since the water is already frozen. However, snow can be mixed with liquid water, particularly cloud droplets, and can contribute to the accumulation of hazardous frozen deposits.
Types of In-flight Airframe Icing Accidents
- General aviation aircraft that are not equipped with ice protection systems but are flown in icing conditions. In mountainous terrain, this very often leads to a stall followed by a loss of control when the pilot attempts to maintain altitude over the high terrain
- Aircraft, predominantly propeller-driven, which rely on wing and tail ice protection by de-icing, principally by pneumatic de-icing boots, and are operated in icing conditions which exceed the capability of the protection
- VID 150259 - Creation
- VID 450012 - Wiki Integration
DATE OF SUBMISSION
- 00:36, 14 May 2021
- This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.
- The content of this documentation is intended for aviation simulation only and must not be used for real aviation operations.