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This article provides guidance for controllers on what to expect from an aircraft experiencing the effects of in-flight icing and some considerations which will enable the controller, not only to provide as much support as possible to the aircraft concerned, but also maintain the safety of other aircraft in the vicinity and of the service provision in general.
In-flight icing poses serious threat to the safety of a flight. Water droplets accumulate on the airframe as ice under specific conditions. The ice deposits alter the wing profile and disrupt the flow of air. This drastically changes flight parameters such as lift, drag, controllability, etc. As drag increases, lift rapidly decreases - a natural pilot action to compensate for this would be to apply power and increase angle of attack to maintain level, however this leads to even faster ice accumulation as larger airframe surface is exposed. Ice formations on wing and control surfaces can lead to increased stall speed, sudden uncontrolled pitch or roll with difficult recovery and, potentially, eventual loss of control.
Moderate or severe icing conditions could overwhelm the anti-ice system commonly fitted on modern aircraft. Safe continuation of the flight could become impossible.
Some hazards associated with in-flight icing include:
For further information, see the separate article on the Aerodynamic Effects of In-Flight Icing.
Type and speed of ice formation is, in part, a function of the variables of the cloud liquid water content, the mean effective diameter of the cloud droplets and the ambient air temperature. The shape of the aerofoil and the speed of the aircraft also directly affect the rate of ice accretion. The following table shows the risk classification, depending on the cloud type and temperature:
|In-flight icing risk||Cumulus clouds||Stratiform clouds||Rain and drizzle|
|High||0° to -20°C||0° to -15°C||0°C and below|
|Medium||-20° to -40°C||-15° to -30°C|
|Low||< than -40°C||< than -30°C|
Figure 1: In-flight icing risk in various atmospheric conditions. Source: Aircraft Icing Handbook, CAA, NZ (www.caa.govt.nz)
According to studies conducted on icing-related incidents and accidents, “the worst continuous icing conditions are found near the freezing level in heavy stratified clouds, or in rain, with icing possible up to 8,000 ft higher. Icing is rare above this higher altitude as the droplets in the clouds are already frozen. In cumuliform clouds with strong updrafts, however large water droplets may be carried to high altitudes and structural icing is possible up to very high altitudes. Further, in cumuliform cloud the freezing level may be distorted upwards in updrafts and downwards in downdrafts, often by many thousands of feet. This leads to the potential for severe icing to occur at almost any level.” (Aircraft Icing Handbook pp2, see Further Reading)
The following icing classification is derived from the same source and is based solely on the speed of ice formation:
In-flight icing could lead to many problems, quite different in nature, such as:
The stress level for the crew on a flight with icing conditions is significantly higher than during a routine one. The pilots should maintain high situational awareness and closely monitor ice formation as immediate diversion could be necessary.
The procedures for avoiding icing conditions include autopilot disengagement, change in altitude/heading or both and search for areas clear of clouds or with warmer temperature. All these pose significant amount of workload to the crew.
Once the first signs of flow disruptions are shown, the crew will put all efforts and attention to regain control. Therefore, during this phase an ATCO could expect broken, incoherent communication with the aircraft.
Best practice embedded in the ASSIST principle could be followed (A - Acknowledge; S - Separate, S - Silence; I - Inform, S - Support, T - Time):
A - acknowledge the problem, ask for the crews’ intentions
S - separate the aircraft from other traffic, provide accurate and optimal vectors
S - silence the non-urgent calls (as required)
I - inform other aircraft and all concerned parties according to local procedures of the reported icing
S - support the flight experiencing icing with any information requested and deemed necessary
T - provide, in a timely fashion, a revised clearance to move the affected aircraft out of the icing conditions.
In addition, the controller might be required to:
In icing conditions:
In icing conditions it may be appropriate to remind trainees or less experienced pilots of turboprop aircraft to:
Practical advice for pilots on stall and upset recovery:
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