Friday, October 20, 2017
Transport Canada - Aviation Safety Letter A Holdover Time Paradigm Shift

A Holdover Time Paradigm Shift

by Doug Ingold, Civil Aviation Safety Inspector, Operational Standards, Standards, Civil Aviation, Transport Canada

This article explores a paradigm shift in the operational use of Holdover Time (HOT) information. A brief history of the origins and use of HOT will be presented. This will be followed by a historical account of the industry and authority efforts to bring about a paradigm shift to the operational use of HOT information. The potential benefits and opportunities provided by using such a system will be highlighted.

This article was made possible through the collaboration and contribution of the following individuals: Peter Graverson of D-ICE, Mike Chaput of APS Aviation, Mark Homulos of WestJet, and Bill Maynard of Transport Canada.

The operation of aircraft during ground-icing conditions poses potential safety of flight hazards that must be addressed. Contamination consisting of frost, ice, snow, and other frozen particulate create flight hazards. These contaminates must be removed prior to takeoff (Canadian Aviation Regulation [CAR] 602.11). Between 1969 and 2007, ground-icing-related accidents have contributed to over 500 deaths and significant property loss.

Dryden, Ont., March 10, 1989Dryden, Ont., March 10, 1989

The threat is very real! Some of you will remember the Dryden accident. For those who don’t, the March 10, 1989, accident of a Fokker F28-1000 claimed the lives of 24 people (see photo, above). As a result of that accident, a commission of inquiry, led by The Honourable Virgil P. Moshansky, was instituted. Public hearings lasted 20 months and 166 individuals were interviewed. Thousands of pages of transcripts and evidence were condensed into a four-volume report. Typical of many accidents, there were a number of causal factors. One of the principle causal factors was attempting to take off with contamination on the aircraft’s critical surfaces. The report concluded with 191 recommendations in 19 distinct areas. The publication of the report led to extensive regulatory changes. Furthermore, a deluge of research and development (R&D) activities were initiated. These R&D activities brought scientific support to clarify acceptable processes and procedures associated with wintertime operating conditions. Time and space preclude the discussion in this article of R&D conducted by Transport Canada in these areas over the past 20 years. Interested readers can view and download many of the R&D reports by visiting the following Web site:

There are a number of methods that can be used to remove frozen contaminates from the aircraft surfaces prior to takeoff. The most widely-used method for large aircraft is the use of de-/anti-icing fluids. The focus of this article is on the evolution of HOTs and their operational use.

Early HOT tables were produced by industry and were based on best estimates of fluid performance, see Figure 1: Early HOT table (circa 1989). At that time, there were no performance standards or criteria in place to define the fluid HOT properties. Following the Dryden accident, and a number of significant U.S. aircraft ground-icing accidents, the Society of Automotive Engineers (SAE) (, at the request of industry and regulators, put together a working group (SAE G-12) to address ground-icing issues. Its membership includes, but is not limited to, fluid manufacturers, air operators, aircraft manufacturers, aviation authorities and numerous consultants.

Figure 1: Early HOT table (circa 1989)Figure 1: Early HOT table (circa 1989)

As a result of the significant work conducted by the SAE G-12, eventually aerospace standards and recommended practices were developed and published, defining fluid properties, testing methods, acceptable application procedures, etc. By the late 1990s, some of this work led to a standardization of the HOT Guidelines, domestically and internationally, see Figure 2: Recent HOT table (2007). Transport Canada and the U.S. Federal Aviation Administration (FAA) publish HOT Guidelines for operational use on an annual basis ( The HOT Guidelines formats are by their nature limited in terms of the information they can provide to the flight crew.

The information has been simplified to ensure its ease of use, especially during the busy ground-operational phase. HOT Guidelines provide the flight crew with HOTs for a range of precipitation types, precipitation rates and temperature bands.

Establishing HOT Guidelines
Every year, the FAA and Transport Canada, on behalf of fluid manufacturers, assess the HOT performance of fluids in both laboratory and natural conditions on a cost-recovery basis.

In the laboratory, the tests are conducted under defined and controlled conditions of precipitation type, precipitation rate, and temperature.

Rectangular aluminium plates, coated with the de-/anti-icing fluid being tested are exposed to various types of precipitation.

The quantity of precipitation, also known as liquid water equivalent (LWE), is measured using pans. LWE is measured in units of grams/decimeter squared/hour.

Specific failure criteria are used to identify when the fluid on the test plates is considered failed. The amount of LWE required to reach the failure point is then documented and graphed as a data point. This allows the creation of regression curves and associated regression coefficients. These curves plot fluid failure time on the vertical axis versus precipitation rate on the horizontal axis. This information is then assessed and converted into the HOT Guidelines that many flight crew are familiar with.

Unfortunately, the existing HOT Guidelines assume that the pilot has accurate, real-world information on which to base their decision making.

What information is actually available to the pilot when using the HOT Guidelines? Remember that one needs accurate temperature, precipitation type and precipitation rate to use the HOT Guidelines effectively.

Figure 2: Recent HOT table (2007)
Figure 2: Recent HOT table (2007)
Click on image above to enlarge.

Temperature is almost always available, either through ATC or meteorological reports, or by direct cockpit reading.

Although precipitation type is available through aviation routine weather reports (METAR), there are cases where this information is not updated frequently enough for ground de-/anti-icing operations.

The precipitation rate reported in METARs (as light, moderate, or heavy) is not correlated with LWE used during fluid testing. The result is that the pilots must make a subjective assessment, integrating the various parameters and then consulting the HOT Guidelines. It is plausible that different pilots could draw different conclusions from the same reported or observed weather conditions.

To recap, the HOT Guidelines are generated using a scientific approach to ensure their accuracy and consistency. The operational use of the guidelines is based largely on a subjective assessment of the weather conditions. This is due in large part to the fact that the current weather reporting and observing infrastructure was not designed for use with de-/anti-icing activities.

Is there a better way of making use of all this scientific HOT data?

Holdover Time Determination Systems (HOTDS)
In 2003, Transport Canada was approached by DAN-ICE, now called D-ICE. D-ICE is a Danish company developing meteorological and support equipment for use with HOTs. At that time, they were requesting approval or certification of a system that would simplify the way flight crews obtained their HOTs.

Largely as a result of that original request, Transport Canada supported industry initiatives that would improve the way HOT information is provided and used by pilots and operators in making safety-critical decisions, thus directly promoting safety and aeronautics in Canada.

Transport Canada, as the regulatory authority involved with wintertime aircraft operations and associated R&D, was poised to promote new systems that assist pilots and operators in better coping with wintertime operations. The system can also be thought of as an automated electronic version of the HOT Guidelines. The term Holdover Time Determination System (HOTDS) was coined to describe this system, and will be used throughout the remainder of this article.

D-ICE HOTDS equipmentD-ICE HOTDS equipment

The HOTDS uses the regression curves and coefficients generated during the fluid endurance testing previously described in this article. The regression coefficients are published in a Transport Canada report. The current report only contains coefficients for the two fluids used at sites where the HOTDS is currently installed.

This was the first time such a system would be implemented anywhere in the world, and therefore, a shift in cultural and operational thinking was required. To begin with, there were no standards or design requirements associated with this type of system. Furthermore, the availability of LWE information for ground de-/anti-icing operations through regular meteorological channels was, and currently still is, unavailable.

Transport Canada contracted APS Aviation to conduct R&D into the development of a performance-based standard that would initially be included as part of a regulatory exemption (similar to the automated weather observation system [AWOS] exemption). Eventually, the performance-based standard could be incorporated within the CARs as a regulatory standard. The development of the performance-based standard took two years. Simultaneously with the development of a performance-based standard, Transport Canada was formulating the necessary exemption criteria to support operational implementation of the HOTDS.

WestJet had shown a keen interest in the potential for using the HOTDS. WestJet and D-ICE paired up to conduct initial operational suitability trials during winter 2005–2006 and 2006–2007.

Operational use of the HOTDS was kept as simple as possible. The flight crew would initiate a request for a HOT via the Aircraft Communications Addressing and Reporting System (ACARS). The latest information from the HOTDS, which provides updates every 10 min, would be sent back to the flight crew through the ACARS as a single HOT value for the current weather conditions. The HOT information would then be displayed on the flight deck flight management system (FMS).

Potential benefits associated with the HOTDS included:

  • providing pilots with better HOT information on which they could base their decisions;
  • providing pilots with the most appropriate HOTs, thus minimizing confusion and errors during the extremely busy ground-operational phase;
  • the ability to select the most appropriate fluid type for the given conditions, thereby minimizing environmental impact;
  • a potential cost savings associated with optimum fluid selection.

In December 2007, Transport Canada issued a one-off regulatory exemption to WestJet, allowing them to use a HOTDS in place of paper HOT Guidelines at a limited number of airports. It is expected that full operational use of this system will be in place for the 2008–2009 winter season.

Use of the system is contingent on the operator:

  • revising their company operations manual;
  • conducting the appropriate training;
  • having contingency plans;
  • ensuring the HOTDS equipment is declared as meeting the performance-based criteria; and
  • having the appropriate HOTDS equipment in place at selected airports.

The use of the exemption approach allows limited operational usage of HOTDS, since the onus is on the air operator to ensure that the HOTDS is installed and declared compliant. To truly reap the full benefits of HOTDS, it is necessary to ensure that the requisite meteorological information for de-/anti-icing purposes is disseminated through regular meteorological channels.

The Future
In order to obtain maximum benefits to aviation for improved decision-making capabilities regarding anti-icing and de-icing decision making, it is important that a common global approach be taken, to the extent practicable. Transport Canada is working within the International Civil Aviation Organization (ICAO) and the World Meteorological Organization (WMO) to develop common methods for assessing and communicating information in support of operations under icing conditions. At present, efforts are focusing on whether this data should be added to METARs or aviation selected special weather reports (SPECI) at select stations, and the FAA already has plans to do so for some aerodromes in the not-too-distant future.

2008–2009 Ground Icing Operations Update

In July 2008, the Winter 2008–2009 Holdover Time (HOT) Guidelines were published by Transport Canada. As per previous years, TP 14052, Guidelines for Aircraft Ground Icing Operations, should be used in conjunction with the HOT Guidelines. Both documents are available for download at the following Transport Canada Web site: If you have any questions or comments regarding the above, please contact Doug Ingold at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Flight Crew Survey on Takeoffs in Freezing Drizzle or Freezing Rain

Transport Canada (TC) has initiated a Working Group to better understand the current operational practice of taking off during freezing rain or freezing drizzle conditions. To this effect, an independent third party will be administering a survey on TC’s behalf.

As a pilot, your participation in this survey will assist TC in determining whether additional guidance material, further interpretation of regulations and standards, or additional regulations and standards are required in the area of takeoff in these conditions. TC is also taking this opportunity to collect information related to takeoff during conditions of ice pellets.

The survey is targeted predominately to IFR-rated pilots who operate in winter conditions. We encourage these pilots to complete the survey, which can be found at the following Web address:


This article was published by Transport Canada in TP 185 Issue 4/2008 -. Reprinted with permission

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