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Transport Canada - Aviation Safety Letter Takeoff in Conditions of Freezing Drizzle and/or Light Freezing Rain (Fixed-Wing Airplanes)—Part I - ISSUE 4/2009

Takeoff in Conditions of Freezing Drizzle and/or Light Freezing Rain (Fixed-Wing Airplanes)—Part I - ISSUE 4/2009

by Paul Carson, Flight Technical Inspector, Certification and Operational Standards, Standards, Civil Aviation, Transport Canada

This is the first of a two-part article on this critical subject. Part II will appear in Aviation Safety Letter (ASL) 1/2010.


During the winter of 2005–2006, a Transport Canada Civil Aviation (TCCA) inspector observed a number of airplanes operated by various air operators taking off in conditions of freezing drizzle (forecast and actually reported). The inspector considered that the operations were in contradiction of Canadian Aviation Regulations (CARs), specifically CAR 605.30:

De-icing or Anti-icing Equipment

605.30 No person shall conduct a take-off or continue a flight in an aircraft where icing conditions are reported to exist or are forecast to be encountered along the route of flight unless

(a) the pilot-in-command determines that the aircraft is adequately equipped to operate in icing conditions in accordance with the standards of airworthiness under which the type certificate for that aircraft was issued; or

(b) current weather reports or pilot reports indicate that icing conditions no longer exist.

Subsequent discussion identified that air operators and flight crews have insufficient information when faced with conducting a takeoff in these conditions. These discussions also identified that nothing in the current regulations and standards authorizes takeoff during conditions of freezing drizzle and/or light freezing rain.

Certification of flight in icing conditions

Current certification practice
Ice accretion on airplanes adversely affects their performance and their operation. Where flight in icing conditions is desired, and in order to operate safely, ice protection systems (IPS) must be incorporated into the airplane design and used appropriately. The type and extent of these protection systems depends on the characteristics of each individual airplane, including its propulsion-system type, aerodynamic configuration, aerofoil geometry and overall size.

Approval of flight in icing conditions includes demonstration of satisfactory performance of the IPS and satisfactory handling qualities. A measurement of the performance degradation with the ice expected on both the unprotected surfaces and any residual ice on the protected surfaces resulting from proper operation of the IPS must also be demonstrated. Ice accretions aft of protected surfaces and due to IPS failure conditions are also considered during the certification process.

From an airplane certification aspect, it is assumed that the airplane does not have any ice accretion on critical surfaces prior to commencing the takeoff. Also, it is assumed that there is no ice accretion during takeoff until liftoff.

FAR 25, Appendix C

For certification purposes, the in-flight icing atmosphere has been characterized in U.S. Federal Aviation Regulation (FAR) 25, Appendix C. It is important to note that although these envelopes encompass most icing conditions likely to be encountered, it is possible to encounter icing conditions that exceed the certification envelope. In particular, FAR 25, Appendix C does not address supercooled large drop (SLD) icing conditions, which include both freezing drizzle and/or light freezing rain. Also, convective cloud described by Appendix C is largely stratocumulus and cumulus. Towering cumulus, and certainly thunderstorms, should be assumed to include icing conditions that exceed those established in Appendix C.

Appendix C characterizes continuous maximum and intermittent maximum icing conditions within stratiform and cumuliform clouds. Appendix C defines icing cloud characteristics in terms of mean effective drop diameters, liquid water content, water droplet size, temperature, horizontal and vertical extent, and altitude. Freezing drizzle and/or light freezing rain precipitation are not included as these environments typically contain mean effective diameters (MED) that are larger than the cloud mean effective drop diameters defined in Appendix C. Consequently, those icing conditions containing freezing drizzle and/or light freezing rain are not considered during the certification of airplane IPS, and exposure to these conditions could result in hazardous ice accumulations because the larger diameters typically impinge farther aft on airfoil surfaces. Also, mixed phase and ice crystal icing conditions are not currently considered during the certification of the engine, and exposure to these conditions could result in hazardous ice accumulations within the engine, which could then result in engine damage and power loss.

The historical reasons for not including SLD conditions in FAR 25, Appendix C are unclear. However, this “unclear” history may have been the result of the limitations of the icing conditions measurement equipment, the statistical analysis methods used at the time, and the airplanes used in the atmospheric research flights. It should be noted that flights were conducted in the 1940s to develop the icing conditions environment data that eventually became known as Appendix C. It should also be mentioned that Appendix C icing conditions design standard has been used for many decades without significant question. However, the technical evolution of the airplane design has resulted in more critical aerodynamic designs—high performance, super critical wings, etc.—where recent service history has shown them to be more sensitive to critical surface contamination than older airplane designs.

For engine installation certification, additional icing conditions for ground operation are specified including falling and blowing snow, and freezing fog.

Aircraft Flight Manual limitations

In general, the Aircraft Flight Manual (AFM) will contain a statement in the limitations section such as “The airplane is approved for operation in atmospheric icing conditions.”

What is not specifically stated in the AFM is that this approval is based on the certification design standard of FAR 25, Appendix C icing conditions. In addition, the certification design standards do not require that the AFM include any wording stating what the requirements are for operating in icing conditions.

At the present time, no design standards exist for icing conditions outside of FAR 25, Appendix C; thus, no airplanes have been certified to icing conditions that exceed FAR 25, Appendix C. This includes takeoff in freezing drizzle and/or light freezing rain. No manufacturer has conducted the required analyses or tests to show that the existing IPS are effective or that the airplane performance and handling qualities are acceptable when operating in SLD conditions.

Airplane manufacturers, certification and operational authorities know that the “atmospheric icing conditions” in the AFM approval statement do not include SLD conditions. A limitation relating to severe in-flight icing conditions is contained in the AFMs of some types of airplanes and may in fact include freezing drizzle and/or light freezing rain (other than the Cessna 208 which prohibits takeoff, flight into, and landing in freezing drizzle and/or freezing rain). Other AFMs only include in-flight detection and exit strategies.

Probability of occurrence of icing conditions exceeding FAR 25, Appendix C

Because of significant geographical differences and seasonal changes, it is difficult to give a precise definition of the probability of occurrence of SLD icing conditions. However, to a first approximation, the probability of occurrence for any particular location in North America has been estimated as between one and five percent over the winter season for a large part of the continent. Some airports on the east coast of Canada have reported up to 39 “annual freezing rain days.”

Hazards associated with ground operation in SLD icing conditions

Ground contamination of ramps, taxiways and runways
Freezing drizzle and/or light freezing rain falling on ramps, taxiways and runways will likely solidify into an ice layer. This will reduce the surface friction available for airplanes manoeuvring during taxi (cornering friction) and will also reduce the friction available for stopping (braking friction).

During operations in conditions of freezing drizzle and/or light freezing rain, ground de-icing and/or anti-icing chemicals can be used to improve the friction available. In conditions of continuous freezing drizzle and/or light freezing rain, it could be expected that the braking coefficient would further deteriorate. Hence, it could be expected that even on a treated runway, the stopping performance would not be as good as it would be even on a wet runway.

Some air operators make adjustments to the takeoff reference speed V1 and reduce take-off weight, if necessary, when operating on wet or contaminated runways.

Ground contamination of airplanes
Freezing drizzle and/or light freezing rain will solidify on a parked or taxiing airplane, unless it has a recent coating of de-icing/anti-icing fluid. As per CAR 602.11(4), an airplane must either be inspected immediately prior to takeoff to determine whether there is any ice adhering to any of its critical surfaces, or, in the case of air operators, this inspection may not be required if dispatch and takeoff are conducted in accordance with an approved airplane inspection program. This inspection program includes specification of the procedures for de-icing/anti-icing and the use of Holdover Time (HOT) tables. HOT tables provide approximate times for which the de-icing/anti-icing fluid will remain effective in preventing ice adhering to the surface. These tables are provided for various ambient temperature, fluid concentrations and weather conditions for individual specified fluids. It should be emphasized that the HOT tables are based on fluid performance and not airplane performance. HOT tables are not defined as being unique to any type of airplane; they are general and unique to the fluid type.

HOT tables include data for weather conditions of freezing drizzle and/or light freezing rain. No data are provided for moderate, heavy and severe freezing rain.

The provision of HOT tables for freezing drizzle and/or light freezing rain could be assumed to imply that operations to be undertaken in these conditions are authorized. This is quite simply not the case! The bottom of every table contains the following quote:

“Fluids used during ground de-icing/anti-icing do not provide in-flight icing protection.”

Airplane contamination due to freezing drizzle and/or light freezing rain may be more difficult to visually observe than other types of freezing contamination (e.g. frost, snow). In addition, visual inspection through cabin windows could be impaired due to anti-icing fluid on the windows.

There have been a number of accidents attributed to taking off with ice contamination of wings due to omission of, or incomplete, ground de-icing/anti-icing procedures. Airplanes with “hard” leading edges appear to be more susceptible to the adverse effects of ice accretion on the wing than airplanes with leading-edge slats.

Commercial air operators using smaller (FAR 23) airplanes frequently operate into less busy airports. Smaller airports may not have the extensive ground de-icing/anti-icing infrastructure available at larger airports. For these circumstantial reasons (in addition to technical reasons), smaller airplanes may be more at risk from ice contamination on the ground.

Windshield ice protection
Although it would be normal procedure to de-ice an airplane windshield if it was contaminated, it would not be normal practice to apply anti-icing fluid as this could result in an obscured and/or distorted view during taxi and low speed operations. Although it is believed that windshield IPS, designed to FAR 25, Appendix C icing conditions, would be effective in protecting the view in freezing drizzle and/or light freezing rain, this has not been demonstrated.

Freezing drizzle and/or light freezing rain will also be associated with conditions of reduced visibility. Specific limits are placed on the minimum visibility required for takeoff in the CARs and associated Standards.

Powerplant ice protection
Powerplant components that require protection include the engine nacelle, engine rotating and static components, and the engine sensors. It is possible that the airplane IPS may not be as effective in freezing drizzle and/or light freezing rain. However, there is evidence of engine damage and operating anomalies caused by ground operation in freezing drizzle and/or light freezing rain. Ice can accumulate in inlets and on components at low thrust levels (e.g. ground idle) without any noticeable adverse effect. This ice can subsequently be shed at high thrust levels (e.g. take-off thrust) causing engine operating anomalies and/or damage.

Other systems ice protection
Other systems that require protection include the pitot-static system, and temperature and angle of attack sensing systems. Although probably adequate, the capability of the IPS to protect in freezing drizzle and/or light freezing rain is unknown.

Takeoff into known freezing drizzle and/or light freezing rain is outside of the flight envelope for which any airplane currently operating today is certificated. Not only is it unwise to operate in such conditions, it is also unsafe, and based on the best information available at this time, also illegal.

In Part II, we will address hazards associated with in-flight operation in SLD icing conditions, and also meteorology measurement criteria forecasting/reporting freezing drizzle and/or light freezing rain vs. FAR 25, Appendix C.


  1. J. C. T. Martin, Transport Canada Aircraft Certification Flight Test, Discussion Paper No. 41, The Adverse Effects of Ice on Aeroplane Operation, Issue 2, 4 July 2006. (Paper which was the basis for Mr. Martin’s article “The Adverse Aerodynamic Effects of Inflight Icing on Airplane Operation”, published in ASL 1/2007, and available at

  2. J. C. T. Martin, Transport Canada Aircraft Certification Flight Test, Discussion Paper No. 50, Takeoff in Conditions of Freezing Drizzle or Freezing Rain (Fixed-Wing Aircraft), Issue 2, 29 September 2006.  


    This article was published by Transport Canada in TP 185E -. Reprinted with permission

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