Typically
Typically, Type I fluid is for de-icing....cleaning off the aircraft....although
it does have to a small degree anti-icing capabilities. "Small degree"is the
key term.
Type II or IV fluids are really for anti-icing, and are to be used after the
aircraft is clean(ed).
Looking at the METAR is good, but there are other factors that must be considered. The bottom line is that there must be no snow/ice adhering
to the critical surfaces of the aircraft....for some operators, the SOPs go
further by requiring all surfaces be free of ice/snow, not just critical surfaces.
(My former company had these more srtict policies.)
Other factors are...and this list is not exhaustive:
1. Wind,
2. Is the aircraft cold-soaked,
3. Temperature of the fuel in the wing tanks,
4. Estimated time from completion of spraying to takeoff,
5. How big a problem would it be to, when getting to the number
one slot for takeoff and determining the aircraft needs more spray,
(You might just want to go with Type IV from the start of things.)
6. Dewpoint,
7. Geography (are you near water? See point #6),
8. Number of planes in line for takeoff....will ATC ask you
to snuggle up...thereby having the aircraft in front of you blow
off all your protection with his jet blast,
There are more factors....I'm sure we have numerous experienced
guys reading this who can add to the list (make a few changes and
suggestions).
Personally, if there was any precip falling, I'd almost always opt
for cleaning with Type I, and ask for Type IV. It's more expensive
for the company, but it's the safe thing to do. (maybe overkill, but
safe)