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Aircraft and In Flight Icing Risks
From SKYbrary Wiki
| Article Information | ||
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| Category: | Weather | 
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| Content source: | SKYbrary | 
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| Content control: | EUROCONTROL | 
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| WX | |
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| Tag(s) | Icing |
Description
Flight in ‘icing conditions’ brings two risks which are independent of each other. These are, the possibility of ice accreting on the airframe and the possibility of ice affecting the normal operation of the powerplant(s) fitted to the airframe (because of ice formation in or around the air inlet path).
Of fundamental importance is whether or not the aircraft is certificated for flight in icing conditions. This information is contained in the AFM or, for a small aircraft, the Pilots Operating Handbook (POH) and is part of the Operating Limitations. Aircraft Limitations are often transcribed into an approved Company Operations Manual.
If an aircraft is not approved for flight in icing conditions, then flight should be planned to avoid them. However, if icing conditions are inadvertently entered, the only wholly safe option is to exit them as soon as possible. It is important to note that aircraft which are approved for instrument flight rules (IFR) operation are not necessarily also certificated for operation in icing conditions.
If an aircraft is certified for flight into known icing conditions then, for smaller aircraft, there may be specific AFM restrictions on such flight. For most larger aircraft however, there will not be any specific restrictions, but it can be safely assumed that icing certification never implies that operation in severe icing conditions is approved - or feasible. Freezing Rain and Freezing Drizzle constitute severe icing conditions.
For any aircraft type which is certificated for flight in icing conditions, the AFM or POH will contain a manufacturer’s definition of ‘Icing Conditions’ for the purposes of the selection or activation of ice protection equipment. This is usually given as the presence of visible moisture and an OAT, SAT or TAT reading of less than a figure between +3°C276.15 K
37.4 °F
497.07 °R and +10°C283.15 K
50 °F
509.67 °R Operation of ice protection equipment is never based upon the appearance of visible ice or on the flight deck annunciation of external ice detector activation.
Although the limiting severity of icing conditions in which an aircraft can be operated vary, low performance aircraft usually experience a higher exposure to whatever icing conditions prevail. This factor alone tends to distinguish the significance of ‘routine’ icing exposure for jets versus turboprops. Whilst both may be similarly certificated for “flight in icing conditions”, the effects of light or moderate icing is greatly reduced if less time is spent in it. Additionally, the use of jet engine bleed air for anti-icing is much more effective than the cycling of pneumatic boots installed on the leading edges of most turboprops.
A crucial requirement which applies to all aircraft is that at rotation on take off, the wings and empennage must be completely free of accreted ice. Use of appropriate ground de/anti-icing fluids may be required to achieve this. It should be noted that these fluids provide no protection against freezing rain or freezing drizzle, so that if such precipitation is occurring, no aircraft is able to get safely airborne.
Finally, whilst certification for flight in icing conditions requires evidence that an aircraft can operate safely within a specified flight envelope, it is often extremely difficult to objectively establish the actual icing conditions which exist. This is because crucial parts of the airframe are not readily visible to the flight crew, and even when they are, the reliability of visual inspection is poor. This is particularly so at night.
Related Articles
- In-Flight Icing
- Aircraft Ground De/Anti Icing
- High Level Ice Crystal Icing
- Piston Engine Induction Icing
- Ice Formation on Aircraft