"Paul Tomblin" wrote in message
...
How good
are the boundaries of airmets? Are you ever going to encounter the
conditions outside the boundaries, or are they pretty conservative about
them?

Icing is hardly ever as predictable as we'd like but my personal experience
is that I've found that the boundaries are usually broader than actual icing
conditions, but the accumulation rates can be both more and or less severe
than indicated in the airmet because it is also unpredictable and is highly
dependent on your speed and time spent in the conditions. I also fly around
the great lakes, and downwind from the lake is a great place to find ice
building faster than predicted and faster than you can get out of it.
Usually when the windward side is CAVU, the lake effect layer on the leeward
side isn't very thick, but you sure as hell don't want to be stuck in it
very long. If the windward side is IFR and tops into the teens or higher,
the lake effect moisture is just added to those conditions on the leeward
side, and you've probably lost all your "outs". Stay away... stay far away.

I've found that the ADDS forecast has been pretty accurate. If I can get
above a potential icing layer that isn't too thick, fly towards warmer
conditions and then do a slam dunk down through it into warmer and clear
conditions near my destination, I'll give it some serious consideration. If
I don't have at least 3 "outs", (altitude, distance, improving weather,
higher temps, between layers, ect.) I won't consider it.

This weekend in WI was a perfect case of airmets galore, but good planning
wouldn't have prevented flying due to ice. Overcast at 2700, clear above
3000.(4000 MSL) light to moderate rime ICIP airmet blanketing the entire
state. VFR both south and north of us, surface temps between 40 and 45F.
Freezing level 4000 MSL north sloping to 6000 MSL south. ADDS was
predicting a 80-90% chance of icing at 6000, nothing at 3000, and nothing at
9000. The "go" decision could be made because the clouds at 2700-3000 AGL
were below the freezing level and it was clear above, so the airmet didn't
have any weather that it applied to. Be careful when using ADDS that you
remember ceilings are AGL, icing predictions are MSL. So, we could have
taken off, climbed through the ceiling into clear air.

What if we come upon a layer at 6000 where ADDS said 80-90% chance of ice?

What are our outs?
1) Climb above it, ADDS predicted 0% at 9000, climb is always my first
choice. you can climb before you accumulate ice, you can always go down.

2) descend below it and continue, MEA is 3000 MSL

3) descend through it and land

4) turn around and fly back into clear weather

5) turn 90 degrees to the system and fly around it, this requires knowledge
of the area of coverage and conditions surrounding it, plus additional fuel.

I hate airmet zulu as well, it always throws the unknown at you. All we can
do is explore all our options, set minimal standards, do extra planning, and
be willing to stay on the ground. Fly safe.

Jim