Joint TAA-OAA Definition and Guidance for Icing Conditions

Effective Date: 26 June 2014

Version in effect: 2 (revised 26 January 2026)

Reference: N/A

OPI / Telephone: DTAES 7-5/343-551-5476

SSO OA/+1-204-833-2500, ext. 6649

1. Purpose

1.1. This joint TAA-OAA Advisory provides clear definitions and guidance with respect to Icing Conditions, snow conditions and other forms of frozen precipitation.

2. Applicability

2.1. This advisory is applicable to all aircraft fleets operated by the Canadian Armed Forces (CAF).

2.2 This advisory establishes the baseline against which the TAA and OAA interpret and provide guidance on a prohibition or permission granted in a Flight Manual (FM) and/or Aircraft Operating Instructions (AOI) for flight in icing and/or snow conditions. It provides supplementary guidance and procedural means to operate in such conditions. The information contained herein shall be read in conjunction with the FM and/or AOI for each fleet. Where a conflict exists between this advisory and a FM/AOI, the FM/AOI shall take precedence.

3. Related Material

3.1 Definitions

1. Aerodrome Meteorological Report (METAR). An aerodrome report describing the actual weather conditions, at a specified location and at a specified time, as observed from the ground.
2. Aircraft. Any machine capable of deriving support in the atmosphere from reactions of the air (reference 3.2.a). The term 'aircraft' applies equally to fixed-wing aircraft, rotorcraft, and Uncrewed Aircraft (UA) regardless of weight.
3. Air Temperature (AT). The air temperature that would be measured by a temperature sensor that is not in motion relative to that air. For the purposes of this advisory, for in-flight use, Static Air Temperature (SAT), Free Air Temperature (FAT), Outside Air Temperature (OAT), true outside temperature and ambient temperature are interchangeable.
4. Area Forecast Amendment (AIRMET). Information message to advise pilots of the occurrence or expected occurrence of weather phenomena, which may affect the safety of aircraft operations, and which were not already included in the Graphical Area Forecast (GFA).
5. Flight Manual (FM). A technical document, normally provided by the Original Equipment Manufacturer (OEM), which contains the Technical Airworthiness Data (TAWD). A typical Flight Manual would contain operating limitations, Normal/Abnormal/Emergency operating procedures, performance data and loading information. It is submitted to the TAA and the OAA for airworthiness approval as part of the Type Certification process (reference 3.2.a). The term FM applies equally to fixed-wing aircraft and rotorcraft. For those fleets that do not have a FM, those sections of the AOI that contain TAA-approved TAWD constitute the FM for that fleet.
6. Graphical Area Forecast (GFA). A series of temporally adjusted weather charts, each depicting the most probable meteorological conditions expected to occur below 24 000 ft over a large area within a specified timeframe.
7. Pilot Report (PIREP). An in-flight pilot report of actual weather conditions encountered during flight.
8. Significant Meteorological Information (SIGMET). Information message issued by a meteorological watch office to advise pilots of the occurrence or expected occurrence of specified weather phenomena that may affect the safety of aircraft operations.
9. Significant Weather Charts (SIGWX). Charts showing for the mid and high levels occurring or forecast weather conditions considered to be of concern to aircraft operations.
10. Terminal Aerodrome Forecast (TAF). A description of the most probable weather conditions expected to occur at an aerodrome, together with their most probable time of occurrence.
11. Weather Check Flight. A flight conducted to validate forecasted weather conditions over a specific geographic area. Such flights shall be defined and conducted in accordance with local flying orders.

3.2 Regulatory References

1. C-05-005-001/AG-001 – Technical Airworthiness Manual (TAM)
2. Federal Aviation Administration (FAA) Advisory Circular (AC) 91-74B – Pilot Guide: Flight in Icing Conditions, dated 8 October 2015
3. B-GA-007-001/PT-D01 – Air Command Weather Manual, dated 1 November 2012
4. Environment Canada – AWARE: The Atmosphere, The Weather and Flying, January 2011
5. National Transportation Safety Board (NTSB) Order EA-585, NTSB 940 – Administrator vs. Bowen (1974)
6. NTSB Order EA-603, NTSB 950 – Administrator vs. Bowen, Petition for Reconsideration (1974)
7. NTSB Order EA-3770 – Administrator vs. Groszer (1993)
8. FAA Letter of Interpretation – Office of the Chief Counsel to Ms Leisha Bell (Aircraft Owners and Pilots Association), dated 16 January 2009
9. Title 14 Code of Federal Regulations (14 CFR) 121.321 – Operations in Icing, dated 13 March 2013 (effective 21 October 2013)
10. Regulation (EC) No 216/2008 of the European Parliament and of the Council of 20 February 2008

4. Discussion

4.1 Background

1. As stipulated in the TAM (reference 3.2.a), aircraft may include allowances or prohibitions for Flight in Icing Conditions. The TAM does not, however, provide a definition for such conditions. These allowances or limitations with respect to icing are generally documented in the FMs.
2. In some cases, the FM does not provide a clear definition of what constitutes Icing Conditions. Furthermore, the wording used in FMs is inconsistent: references are made to 'Icing', 'Icing Conditions', 'Known Icing Conditions' or 'Forecast Icing Conditions', suggesting they are different.
3. The training material available to the operators (references 3.2.b through 3.2.d) provides most of the necessary knowledge on the physics of the atmosphere, ice formation and its effects on aircraft. However, it fails to establish a clear and unambiguous definition that could be used by all operators.
4. The lack of a clear and unambiguous definition is not a problem unique to the Department of National Defence (DND), and many safety and regulatory agencies, such as the NTSB and the FAA, have long faced this issue. The guidance in this advisory is developed based on DND experience, as well as guidance developed over the years by other agencies (see Annex A).

4.2 TAA-OAA Position

Below are the definitions and interpretations of terms and conditions to be used when referring to aircraft limitations/allowances or to supplement current FM and operational limitations, guidance and procedures, where they exist:

1. The following combinations of terms shall all be interpreted as 'Icing Conditions':

(1) Icing (Conditions);

(2) Known Icing (Conditions);

(3) Flight in Icing (Conditions); and

(4) Actual (Observed) Icing (Conditions).

2. The following definitions shall be used in the interpretation of all Icing Condition-related material and in conjunction with para 4.2.c of this advisory.

(1) Fog: Suspension of condensation that causes visibility to be reduced to less than 1000 m (5/8 SM, 3280 ft).

(2) Liquid Precipitation: Rain, freezing rain, drizzle, freezing drizzle, recirculating spray and mixed precipitation (a mixture of snow and any other form of liquid precipitation).

(3) Visible Moisture: All forms of cloud, fog and liquid precipitation.

3. The following interpretations shall be used in conjunction with those above for the purposes of predicting whether a flight is likely or not to encounter icing:

(1) Icing Conditions exist when:

(a) There are weather reports in which Icing Conditions are forecast, unless provisions of paras 4.2.f or 4.2.g apply;

(b) Icing Conditions are observed or reported;

(c) Any form of frozen precipitation (to include all forms of snow, hail, ice pellets, snow grains, etc.) adheres to any aircraft surface.

(2) Icing Conditions exist in visible moisture at a SAT of +5°C and below.

(3) Icing Conditions may also exist if induced by hovering in recirculating snow at a temperature greater than -5°C.

4. In determining whether Icing Conditions exist, operators shall use all weather forecasts and reports reasonably available to them, including, but not limited to, SIGMET, METAR, GFA (see exceptions at para 4.2.f below), AIRMET, SIGWX, TAF and PIREPs.

(1) While a pilot may consider multiple forecast sources in their flight planning, it is expected that a reasonable level of caution is applied when forecasting reports are conflicting. This is further reinforced by the legal position in section 1.3 (reference 3.2.g).

(2) Pilots shall consider more than just PIREPs, Actuals, Forecasts, etc. when determining if a permissive environment exists to conduct flight operations in icing conditions. Specific aircraft capabilities, sortie profiles, sortie duration, etc. must also be considered.

5. Since PIREPs are samples of the atmosphere limited in time and space, they shall not be used on their own to negate a forecast of Icing Conditions.
6. When permitted by local Flying Orders, Weather Check Flights that give due consideration to all available environmental factors may be used to determine if Icing Conditions exist in a specific operating area. Such a weather check must consider the rate of change of weather conditions and sample the atmosphere where flight is intended.
7. The following shall be considered with respect to Graphical Area Forecasts (GFAs) only:

(1) Areas of moderate or severe icing are graphically depicted as defined in reference 3.2.d. The spatial extent is clearly indicated horizontally and vertically.

(2) When icing is expected to be light, it is indicated in the comments box rather than on the depiction itself. This format does not provide sufficient granularity to assess the likelihood of encountering these conditions at the local level.

(3) It is therefore probable that significant areas of clear air (i.e., non-icing conditions) will exist within a region where light icing is forecast. For this reason, a GFA of light icing shall not be included in the definition of Icing Conditions in para 4.2.c(1), above. For example, with an ambient temperature of -5°C at ground level, during daytime, and a GFA showing nil-to-light icing above the freezing level, Icing Conditions will not exist in those areas clear of visible moisture.

8. An aircraft prohibited from Flight in Icing Conditions may be flown in areas of forecast icing provided the pilot can remain clear of visible moisture. Planning for such a flight must consider that avoidance of visible moisture is exacerbated by poor ambient lighting conditions (e.g., night, low sun with hazy conditions, heavy overcast cloud, etc.).
9. An aircraft certified for Flight in Icing Conditions shall be operated with consideration that icing certification verifies only certain types of Icing Conditions. Despite being certified, Icing Conditions may be encountered that could exceed the aircraft’s capability (e.g., supercooled large droplets).
10. Operators are encouraged to file relevant PIREPs, particularly when encountering actual conditions that are significantly different from those expected. Detailed PIREPs that have broad extent in time and space are most useful when considering the risk of encountering Icing Conditions.
11. If the application of this TAA-OAA Advisory adversely affects operational capability, then a risk assessment (airworthiness or operational) is required to quantify the risk associated with operating the aircraft in Icing Conditions.

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