Read the Transport Canada guide page 11.1.7
TP 14052 - Guidelines for Aircraft Ground - Icing Operations - Transport Canada
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Inspection Immediately Prior to Takeoff
General An inspection is not required until the lower (more limiting) HOT has been exceeded, provided conditions do not exceed those represented by the table. When making this assessment, the impact of other factors (e.g. jet blast) shall be taken into account. If there is doubt surrounding the conditions associated with using the lowest time as the decision making criteria, an inspection prior to takeoff would be prudent.
Pre Takeoff Contamination Inspection A Pre-takeoff Contamination Inspection shall be completed if the elapsed time is within the range of time given in the HOT table for the conditions present. The check will consist of a visual inspection
performed from the aircraft cabin of the upper wing surfaces within 5 minutes prior to the commencement of takeoff. If evidence of ice, snow or frost accretion is observed, or any doubt exists over the condition of the critical surfaces, the aircraft should return for additional de/anti-icing.
Pre Takeoff Contamination Check Should the higher (less limiting) time in the HOT table be exceeded, the only option available to permit takeoff is a Pre Takeoff Contamination Check, which shall be
completed externally by authorized ground staff. If it is not possible to conduct this check, or it is not possible to takeoff within 5 minutes of conducting the check, the aircraft must return for de/anti-icing