Use of holdover timetables and holdover time determination systems in heavy snow conditions - Civil Aviation Safety Alerts (CASA) No. 2017-09


Subpart 604, 703, 704 and 705 operators of the Canadian Aviation Regulations

Issuing Office: Commercial Flight Standards
File Classification No. : Z 5000-35
RDIMS No. : 13522490
Document No. : CASA 2017-09
Issue No. : 1
Effective Date: 2018-01-26


The purpose of this Civil Aviation Safety Alert (CASA) is to alert operators and pilots on the expected use of holdover time tables found in the Holdover Time Guidelines compared to holdover time determination reports (HOTDR) that are generated by holdover time determination systems (HOTDS), specifically when the precipitation type reported is that of heavy snow.

Transport Canada (TC) has recently been made aware of a potential gap in guidance that air operators may be providing to their crew for specific restrictions when operating in heavy snow conditions. This guidance is usually part of the air operators’ ground icing program (GIP).

This CASA serves as an awareness tool to all stakeholders with recommended remedial measures to minimize safety risks for air operations in ground icing conditions.


Holdover Time Guidelines

TC annually publishes the Holdover Time (HOT) Guidelines. This document contains tables which are typically used by air operators for operations in freezing/frozen precipitations.

The limits of testing of all de/anti-icing fluids in snow ranges from very light, to light, to moderate snow. Therefore, Transport Canada, Civil Aviation (TCCA) publishes holdover times for snow for de/anti-fluids in those values exclusively. For the purposes of determining holdover times, precipitation rate is defined as the following: grams (g) per decimeter square (dm2) per hour (hr) or g/dm2/hr.

The precipitation rate ranges for snow are the following:

  • Very light snow: 3-4 g/ dm2/hr
  • Light snow: 4-10 g/ dm2/hr
  • Moderate snow: 10-25 g/ dm2/hr
  • Heavy snow: > 25 g/ dm2/hr

Holdover Time Determination Systems (HOTDS)

As a result of technological innovations in the early 2000s, air operators are able to leverage near real-time reporting of meteorological conditions at airports compared to traditional METAR/SPECI through the use of HOTDS and apply them to the usage of anti-icing fluids. Flight crew are able to obtain a holdover time through the Aircraft Communications Addressing and Reporting System (ACARS) versus using a holdover time table.

HOTDS provide holdover times that are more precise compared to the holdover timetable ranges found in the HOT Guidelines and allow air operators to operate exclusively in heavy snow between 25-50 25 g/ dm2/hr.

In addition to requiring a functional HOTDS at the airport of departure, air operators utilizing these systems are required to meet the conditions set out in the Exemption to subsection 602.11(4) of the CARs.

The most recent exemption is found at:

In the absence of a functional HOTDS, all requirements/elements of Standard 622.11 Ground Icing Operations, must continue to apply as part of the approved GIP, including the use of holdover timetables.

Recommended Action:

Directors of safety, directors of operations, chief pilots, check pilots, pilot instructors, training providers and service providers should ensure that all staff involved in ground icing operations are aware of the differences between the use of the holdover time tables and the HOTDR generated by a HOTDS.

Guidance in approved ground icing programs should clearly indicate that no holdover time exists for operations in heavy snow when utilizing the holdover time guidelines. Furthermore, guidance should indicate that no holdover time will be issued from a HOTDS when heavy snow exceeds 50 g/dm2/hr.

Contact Office:

For more information concerning this issue, contact Mr. Roman Marushko, Program Manager, Flight Technical and Operator Certification, Commercial Flight Standards, in Ottawa, by telephone at 613-993-4692, or by e-mail at

Original signed by

Robert Sincennes
Standards Branch

The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.