Active Frost Conditions
 

I have read my company’s winter ops and deicing manuals but cannot find a clear answer to the following dilemma.

The other day I took over a 737-800 mid morning on a cold day. It was 2 degrees C and Active Frost conditions. The aircraft had been anti-iced with Type II twice. Once at 0400 and once at 1000. It was now 1100. Full main tanks, tank temperature around 3 degrees C. By the time we were ready, it was 1145 and a well-timed fog bank rolled in. LVPs, VIS less than a statute mile etc. Accordingly to my SOPs, that’s freezing conditions and Anti-icing not strictly required if wings are clean.

The Type II on the wings was still fluid but a contamination check would only be valid for 5 minutes before takeoff. It was a 20 minute taxi. To my mind the Active Frost HOT was no longer valid because of the change in environmental conditions, but I can find no confirmation of this in the manuals. The Type II HOT tables for this make of fluid didn’t specifically refer to fog conditions nor would the HOT for any similar type and intensity of precipitation give us long enough to get airborne (given the 1000 anti-icing start time). In other words the HOT was expired.

I elected to re-do the Type II.

Does anyone have advice on this type of situation or links to appropriate FAA/EASA sources?

I would be much obliged.

Quicker advice would come from the manufacturers recommendations than FAA/EASA sources

If there wasn't freezing fog (so fog and temperature 0C or less), or any kind of precipitation, I don't think it should be a problem, especially with wing surface (fuel tank) temperature above OAT/DP, which means you aren't even in active frost conditions anymore.

That being said, if you're not 100% sure, the easiest way to solve the problem is to de/anti-ice again. Remember, at the end of the day HOT tables are just guidance, nothing more than that.

If in doubt, there should be no doubt. Remember, nobody gets killed because you do ask for a further de/anti ice. It doesn't always work in reverse.

I've had days where I saw the only aircraft not to de-ice, but I did ask for an inspection. We were just lucky, due to positioning and the gap in the terminals compared to aircraft alongside us. Didn't need it and all was OK. They chose to, mainly I suspect because others a few stands from us had called for it. Not a problem. I had days where I was the only one asking for it too. Just have the confidence to ask when you need it. You seem to have good awareness so that's good. Like all of us, we seem to be more worried of covering our butts from the Ops manuals rather than anything else nowadays.
Happy flying.

Same with post #3, differentiating well between
- freezing conditions (OAT < 10 deg C, active precipitation, spray, visible moisture with VIS below one mile (=FG))
- freezing fog (OAT sub zero)

would probably help during the original decision making. Much easier to see from a comfy chair, sure. Wing skin temperature could overrule it all, as well covered in #3.

I would have probably asked for Type I 25:75, partly to remove any leftover non-conforming Type II, partly because I believe the mantra doubt = de-ice. Smart F/O might have tipped the scales either way, both sides really, likewise if overly smart to show where the line is drawn. Passing on freely what once was given. :)

If it had been my personal airplane with me on it single-pilot, then no additional treatment as neither the conditions for icing nor physical risk of ice formation were there. Again, based on what's reported, wearing my keyboard battledress, and only if the fuel temperature / wingskin temp. assessment would allow.

To answer the question placed:
The industry standard for winter ops in this regard is actually produced by IATA: Deicing Anti-icing Manual. Comes updated every winter with a new annual edition. Cannot find the link.

ICAO is in the making: https://www.skybrary.aero/bookshelf/books/4400.pdf

FAA: https://www.faa.gov/other_visit/avia...overTables.pdf

Makes no difference but freezing conditions are conditions in which the outside air temperature is below +3°C (37.4F) and visible moisture in any form (such as fog with visibility below 1.5 km, rain, snow, sleet or ice crystals) or standing water, slush, ice or snow is present on the runway and Freezing fog does not necessarily require subzero OAT. It is a suspension of numerous tiny supercooled water droplets which freeze upon impact with ground or other exposed objects, generally reducing the horizontal visibility at the earth’s surface to less than 1 km (5/8 mile).

Confusing “Icing conditions” (... conducive to the formation of...) with Freezing conditions (OAT below zero)?

Boeing terminology, requires EAI On in Icing conditions. Due to the local pressure drop, and the risk of nacelle icing.

Active Frost requires a temperature below 0 (either OAT or local surface temperature).

If the aircraft has flown already, some (wing) surfaces may still be below tank temperature as a result of cold soak.

To the OP: without knowing the full details, sounds like the prudent course of action. Taking the generic Type II holdover time for Freezing Fog (which is not the case as you reported), you would be also be covered for Active Frost - probably the most accurate category, based on your post.

I did make that mistake above, freezing VS. icing.

Yes, the active frost conditions had been superseded by new environmental conditions. As you say, the warm fuel uplift had increased the wing surface temperature. The new conditions were fog and LVPs.

it was not Freezing Fog which in my company is defined as 0 degrees C and <1000m visibility. It was, however, Freezing Conditions which my manuals define as temperatures of 3 degrees C or below and precipitation or standing water.

I looked at the relevant Type II HOT table yesterday and wondered whether the Freezing Fog HOT could have been used. Though technically not Freezing Fog because OAT was above 0 degrees C, it was fog in Freezing Conditions as defined by my company. The longest HOT for the Freezing Fog HOT would have covered us and would surely have been a conservative calculation?

Thanks for all the suggestions. I’ve always leaned to the conservative side when it comes to de-icing and will continue to do so, following the mantra: if in doubt, de-ice and the clean wing policy. My company puts safety first when it comes to de-icing and fuel policy so there’s no pressure to take risks.

Happy New Year all.