Originally Posted by
Tom4
Hi guys, short and simple question.
Regarding the hold over times, usually hold over time after deicing/anticing is "always" some between, for example 0:25 - 0:40 .
Do you have any procedure in your company which time you choose? 25minutes or 40minutes?
My favorite miss-understood topic in aviation!
This is really easy, once you get started on the 'right path'.
1) The range in a table IS NOT min and max times!
-Probably the most confused topic in aviation. These times are representive of 'rate of precipitation'.
Very light snow has a liquid equivalent snowfall rate of 0.3 to 0.4 millimeters per hour (mm/h), and light snow has a rate of 0.4 to 1.0 mm/h. (For reference, moderate snow has a liquid equivalent rate of 1.0 to 2.5 mm/h, and heavy snow is greater than 2.5 mm/h.) The longer times for very light snow would correspond to the lesser rate, whereas the shorter times would correspond to higher rates.
2) For example lets have a METAR that has -SN with 1 1/2 sm visability (night, -4 &100%)...
-The 'type' which is refered to as 'precipitation intensity' is SN, but...which one? (We don't
ever use the '-' in front of the SN...Its not light snow (-SN), no,no,no!)
How to figure what 'precipitation intensity' of snow?
Snowfall Intensities as a Function of Prevailing Visibility table.
In that table, we are directed to pick a color coded box...In our example, we now have Moderate Snow (orange box).
3) Next, we go to a Type IV generic holdover table...using the conditions I randomly picked:
-Moderate snow (orange box again) is represented in the table as 0:30-0:55 minutes. From the weather scientists, moderate snow is equal to
1.0 to 2.5 mm/h.
Next, go outside on your factory installed snowfall accumulation guage. If its 1.0mm/h use 55 minutes! (2.5 millimeter per hour is only 30 mins). Done!
Don't have the 'factory installed snowfall accumulation guage' option? Hum...we know that we have Moderate Snow, however, we don't know which one.
I'll let you be the judge as to what is the best option to pick (Transport CA guidance is that the
lowest number must be used, think the FAA is getting onboard that too in the 22-23 guidelines).
4) Procedure time (from my department, written by me, plagiarized from many...
Holdover time is good for 0:30 minutes (most restrictive).
Fluid under
most optimal 'Moderate Snow' conditions good for 0:55.
We 'hold' for 30 minutes, beyond 30 minutes we 'must' do a Before Takeoff Critical Surface Inspection...(Ours is a visual check within 5 mins of Take-off to verify the fluid is still effective).
Beyond 55 minutes, a tactile check must be done (by the experts, not the pilots)...however, if good, this would require zero continuing precipitation and takeoff in 5 mins to go fly.
Hope this helps! It is a really good question. Also, thanks for asking it, that says a lot about you as a pilot. I hope I get the 'lucky draw' to ride in the back on one of your flights. As I do many positioning activites in my flying, I am very aware of the trust that others place in us. Stay safe!