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


Subpart 604, 702, 703, 704 and 705 operators of the Canadian Aviation Regulations

Issuing Office: Commercial Flight Standards
File Classification No.: Z 5000-35
RDIMS No.: 19241602
Document No.: CASA 2017-09
Issue No.: 2
Effective Date: 2023-02-17


The purpose of this Civil Aviation Safety Alert (CASA) is to alert operators and pilots on the expected use of holdover timetables 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 that are typically used by air operators for operations in freezing/frozen precipitations.

The limits of testing all de/anti-icing fluids in snow range 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, the 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)

Air operators can leverage near real-time reporting of meteorological conditions at airports compared to traditional METAR/SPECI with HOTDS and apply them to the usage of anti-icing fluids. Flight crews can obtain a holdover time through the Aircraft Communications Addressing and Reporting System (ACARS) versus using a holdover timetable.

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 g/dm2/hr.

General Operating Flight Rules Standard (GOFRS) Standard 622.11 - Ground Icing Operations set out the requirements for air operators on the use of HOTDS, including the minimum assurance requirements and performance specifications.

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

Recommended action:

Director of Flight 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 timetables and the HOTDR generated by a HOTDS.

Guidance in approved GIP 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 a Transport Canada Centre; or contact Commercial Flight Standards in Ottawa by e-mail at

Original signed by

Stacey Mason
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.