Airworthiness Notice - D008, Edition 1 - 14 November 2006

Safety Information Regarding Ground and Airbourne Icing

Purpose

The purpose of this Airworthiness Notice (AN) on ground and airborne icing is to highlight the fact that continued aircraft operations in icing conditions introduces additional risks.

An airplane flight manual (AFM) may indicate that the aircraft is “approved for flight in icing conditions”, or, “approved for operations in atmospheric icing conditions” however, this does not automatically imply that the aircraft can safely dispatch, take off and operate in all foreseeable icing conditions.

Discussion

Flight in icing conditions is an inescapable fact of life for Canadian air operators conducting all-weather operations. As is discussed below and in greater detail in Commercial & Business Aviation Advisory Circular (CBAAC) 130R, there are many factors involved in determining an aircraft’s capability to operate safely in icing conditions, and not all aircraft are equal in this regard. Nevertheless, there is questionable benefit in continuing operation in icing conditions, regardless of the aircraft’s de-/anti-icing capability. Pilot workload is increased, performance and controllability are degraded, and fuel consumption increases through operation of engine and aircraft anti-ice systems.

Ground and airborne icing are very complex issues. There are environmental aspects, aircraft design features and flight phase factors that determine the type and severity of the ice accumulation.

For example, Transport Category aeroplanes in Canadian commercial air service certificated for flight into known icing conditions are certificated to the standard contained in Appendix C to Federal Aviation Regulation (FAR) 25. The Appendix C icing envelopes are the design standards for the ice protection equipment. However, potential icing conditions inside or outside of cloud, such as freezing rain/drizzle exceed the Appendix C icing condition envelopes. Currently, the design and certification of aeroplanes, including the anti-icing and de-icing equipment, is conducted only with respect to the requirements of Appendix C.

The parameters that are used to define the Appendix C icing conditions do not relate directly to the more pilot-familiar meteorological terms for freezing precipitation, such as freezing rain (FZRA), and freezing drizzle (FZDZ). In practical terms this means that the ice protection equipment on some aeroplanes certificated to Appendix C may not be adequate to cope with all icing conditions encountered.

Ground icing operations require the coordinated effort of numerous highly specialized people so that the aircraft arrives at the take-off point in a “safe for flight” condition.

Recommendations

Transport Canada is reviewing the interpretation and application of current regulatory requirements related to    take off and flight in icing conditions. In the interim, operators and flight crews are strongly encouraged to:

  1. Ensure that the aircraft is certified for flight into known icing conditions (if necessary contact the manufacturer for clarification);

  2. Review the limitations section of the AFM to determine whether there are specific prohibitions with respect to flight into freezing drizzle, freezing rain or other atmospheric conditions and comply with any such limitations;

  3. Consider that the operation of certain aircraft types in icing conditions poses a greater risk (e.g.: operation of reciprocating or turbo-propeller aeroplanes with pneumatic de-icing boots and un-powered controls pose a greater risk than larger turbojet aeroplanes with powered flight controls, leading edge high lift devices, and thermal anti-icing systems);

  4. If possible, avoid dispatch or takeoff during freezing precipitation (freezing drizzle, freezing rain, etc.) conditions. This cautionary action is more applicable to those aircraft whose AFM’s recommend exiting those types of icing conditions as soon as possible after they are encountered or for reciprocating/turbo-propeller aeroplanes with pneumatic de-icing boots and un-powered controls;

  5. As a follow on to d., consider the severity, and horizontal/vertical extent of icing conditions and assess safe exit strategies (the best alternative may be to wait it out on the ground);

  6. Ensure that the aircraft is properly de/anti-iced prior to departure and that the flight crew has determined immediately prior to takeoff, or in accordance with an approved ground-ice program, that contamination is not adhering to the critical surfaces;

  7. Ensure that the ramps, taxiways and runways are suitable for use and, if appropriate information is available, adjust takeoff performance for reduced runway friction;

  8. Consider that hold over times (HOTs) guidelines have not been defined for certain weather conditions (e.g. moderate and heavy freezing rain) because the protection times are expected to be of such short duration that they are operationally unusable;

  9. Consider the appropriate course of action relating to possible failure conditions, such as a critical engine failure during the take-off phase.

For the Minister of Transport, Infrastructure and Communities

D.B. Sherritt
Director
Standards
M. J. Eley
Director
Aircraft  Certification

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