"Peter R." wrote in message
...
Peter, as a person who has never flown in the Northwest US, I am
curious about your part of the country. At what altitudes are the icing
conditions during the Northwest US winters?

It depends somewhat on the weather and where you are. As Bob says, the
worst icing appears to be right along the western slopes of the Cascades,
where the moisture-laden freezing air is being lifted.

Generally speaking, the freezing level during the winter varies from as low
as 3-4000' MSL up to 7-8000' MSL, depending mostly on surface temperatures
(or at least correlating...I guess you could just as accurately say that
surface temperatures depend on the freezing level ).

In extreme cases, the freezing level is right at the surface, of course, and
I've seen it as high as 9-10,000' even in the winter (in extremely unusual
warm spells, or when a strong inversion is present).

Do you often have options to remain below those altitudes?

Depends on where you're going. If you stay within the Puget Sound basin,
MEAs are quite low, and the MVA is even lower. For most of the area, ATC
can vector you as low as 2000' or so. Furthermore, when the freezing level
is lower than the MEAs or MVA, it's almost always because of a cold high
pressure system, with very little moisture and no clouds.

However, if you want to travel more than 100 miles or so, you wind up having
to cross some terrain.

Easiest is if you're going south, with MEAs in the 6-7000' range. As long
as it's not too chilly, this gives some breathing room between the freezing
level and the MEA. Also, the freezing level usually slopes upward as you
fly south, and that's where the MEAs start getting higher. Depending on
one's destination, there is also the option of diverting out toward the
coast, where MEAs are lower, and temperatures are sometimes warmer.

Much harder is crossing the Cascades. As Bob mentioned, ATC has implemented
special "vectors for icing" procedures for use during departures and
arrivals. On departures, one may be vectored for a climb (on request) to
cruising/crossing altitude prior to continuing on route over the Cascades,
with the idea that if one discovers icing conditions beyond the capability
of the plane (and generally for piston GA planes, this just means icing
conditions beyond trace icing), they can safely abort the flight and descend
back into warmer air. On arrivals, one may be vectored at altitude,
remaining at the cruising altitude until safely over lower terrain, so that
an uninterrupted descent can be made to below the freezing level.

The worst icing I experienced was on a flight from Everett, WA (Paine Field)
to Eugene, OR. Even that icing didn't turn out to be too terribly
dangerous, but that was only because ATC was able to give me an
uninterrupted descent to below the freezing level. (The freezing level had
been forecast much higher than it turned out to be, in the Eugene area...we
had been in good conditions in the Seattle area).

I do make a definite distinction between forecast "known" icing, and
reported "known" icing. Too often, a forecast of icing is simply based on
too little information. Basically, the airmet says "icing forecast above
the freezing level in clouds". But most of the time, even around here (at
least away from the Cascades), there's not enough moisture in the clouds to
cause icing, even when the temperature is right.

Rather than cancel flights just because of a vague forecast like that, I
always make sure I have at least two "outs", preferably three, and then
proceed with the flight. One of those is almost always a 180 degree turn,
the other is almost always a descent to warmer air (requiring a freezing
level above the MEA). The third could be a cloud layer reported to be thin
enough to climb through quickly (roughly a minute of climbing), or good VFR
conditions near the route (and at the necessary altitude, of course),
something along those lines.

If there's a PIREP telling me there's actual icing, then of course I avoid
the area of the PIREP (and anywhere else nearby that might be similarly
affected). The best case, of course, is when there's a PIREP reporting no
icing at all.

Pete