Takeoff in ice pellets/heavy snow/with no HOT
There would appear to be two issues here. Firstly, are the designated aircraft surfaces free of frost, ice, slush and snow immediately prior to takeoff and secondly, has the aircraft been certified for takeoff/flight when snow pellets, ice pellets, heavy snow, moderate and heavy freezing rain and hail is falling (i.e. conditions not covered by the current HOT’s)?
A lot of work has been carried out to substantiate the currently published times, however, the less common 'events' detailed above have not been included in the programme to date. If you look at the holdover time tables for all current fluid types you will see that the more severe conditions are grouped under the heading 'Other' and there is a Caution stating, "No holdover time guidelines exist."
In my opinion this does not necessarily preclude takeoff under such conditions. The requirements are that i). a 'Pre-takeoff Contamination Check' is carried out to confirm that surfaces are clear and ii). the aircraft is certified for takeoff/flight under the prevailing conditions.
Obviously if the conditions are that severe, ice and/or snow will be detected during the 'Pre-takeoff Contamination Check' and either the departure delayed or further de-/anti-icing, plus additional 'Pre-takeoff Contamination Check', carried out, until the surfaces are found to be clear.
With regard to the certification issue, certain aircraft types may not be certified for flight in snow pellets, ice pellets, heavy snow, moderate and heavy freezing rain or hail. However, it is assumed that if the aircraft is certified, the anti-icing systems will be fully effective.
It is noted that the FAA have included a note in FSAT 05-02 (10/18/05) stating, "The FAA does not approve takeoff in conditions of moderate or heavy freezing rain", however, they do not appear to restrict takeoff during snow pellets, ice pellets, heavy snow or hail……