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 Aircraft Flight Manual (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, to a lesser extent, Freezing Drizzle represent severe icing conditions, because clear ice is formed by Freezing Rain.
For any aircraft type which is certificated for flight in icing conditions, the AFM or POH will contain a manufacturer’s definition of the threshold for ‘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 Outside Air Temperature (OAT), Static Air Temperature (SAT) or Total Air Temperature (TAT) reading of less than a figure between +3°C and +10°C. Operation of anti icing systems is never based upon the appearance of visible ice or on the flight deck annunciation of external ice detector activation, although de icing systems may be. Visible moisture can be defined in flight as clouds, fog with visibility of 1500m or less, and precipitation. On the ground this can include standing water, slush or snow present on the taxiways or runways.
Although the limiting severity of icing conditions in which an aircraft can be operated vary, low performance aircraft are likely to be exposed to relatively more risk in flight through 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. However if a low performance aircraft and a high performance aircraft fly the same trajectory through a portion of airspace containing light or moderate icing conditions, very similar amounts of ice will potentially accrete on both aircraft by the time they leave the icing conditions. The ice potentially accreting depends on the integral along the path of the aircraft of the liquid water content. For a given aircraft flying a given trajectory, increasing the airspeed will cause a higher proportion of the liquid water to accrete to the aircraft. Note though that if a high powered aircraft can climb more steeply than a low powered aircraft, and hence exit the icing layer in less time, the two trajectories are different and the high powered aircraft may well accrete less liquid water than the low powered aircraft. 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. However, many aircraft certified for 'flight in icing conditions' do not have that term defined at its extreme and even in jet transports, severe icing should almost always be avoided - or exited promptly if unexpectedly encountered.
Consideration should also be given to the aircraft angle of attack during climb-out in icing conditions. A high angle of attack may result in ice forming aft of its leading edge, possibly to areas where there is no de/anti-ice system present.
The lower temperature limit for icing to occur is generally accepted at -40C, below which temperature no significant engine and/or wing icing occurs. However, icing can occur at temperatures below -40C in conjunction with cumulonimbus cloud, in particular in the anvil regions.
A crucial requirement which applies to all aircraft is that, at rotation on take off, the wings and empennage must be completely free of frozen deposits. Prior use of appropriate ground de/anti-icing fluids may be required to achieve this. A typical anti icing product will give a specified time period in which it is effective (holdover period). After the holdover time elapses the aircraft must receive another treatment before it can safely take off. The holdover period is effected by the prevailing conditions like temperature and type of precipitation. Holdover period validity does not guarantee no ice accretion or reformation within that time. Visual inspections may be needed to ascertain that no ice has reformed or accreted, but such inspections may be difficult, particularly for larger aircraft - where critical lifting surfaces are not visible from the cockpit and even the cabin, and at night. It should be noted that de-/anti-icing fluids provide either no, or at best very limited, protection against freezing rain or freezing drizzle, so that if such precipitation is occurring, departures are generally prohibited by air operators for safety reasons.
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.