Does AIRMET Zulu
always imply known icing conditions?
In a word, no.
Although AOPA's Air Safety Institute (ASI) would make you
believe that AIRMET Zulu is considered known icing conditions based on
their Precipitation and Icing
interactive online course. As shown on the right, the ASI
clearly states in this course that known icing conditions
exist if icing AIRMETs/SIGMETs exist for your route. If you
are familiar with AIRMETs you understand that AIRMET Zulu all by itself
does not imply known icing conditions as suggested by the ASI course.
Zulu is an advisory for widespread moderate structural icing.
would seem plausible that an AIRMET for moderate icing would be a
forecast for known icing conditions. However, AIRMETs
are what forecasters at the Aviation Weather Center (AWC) refer to as
time-smeared forecasts. That is, they
are forecasts valid over a six (6) hour period.
In fact, the AWC provides a good explanation on their
website. They say,
AIRMET items are considered to be widespread because they must be
affecting or be forecast to affect an area of at least 3,000 square
miles at any one time. However, if the total area to be
during the forecast period is very large, it could be that only a small
portion of this total area would be affected at any one time."
This is very clearly stated. Icing
conditions that are expected to develop, move or dissipate within the
six hour forecast period must be included within the bounds of the
AIRMET. Consequently, AIRMETs can cover large regions of
airspace because of their time-smeared nature.
Let's take a look at a specific example.
Assume that you are proposing a short 52 nautical
mile IFR flight at 7,000 feet leaving shortly after 1500 UTC from
Fitchburg Municipal Airport (KFIT) in north-central Massachusetts to
Taunton Municipal Airport (KTAN) located in northern Rhode Island
(shown below by the red arrow). You receive a standard
briefing from Lockheed Martin Flight Service and briefer advises you of
an AIRMET (shown to the left) that was just issued at 1445 UTC for moderate ice below 12,000 feet
that covers your entire proposed route. (Click here to view a larger
image.) According to the online ASI course mentioned above, an AIRMET
is one of
the parameters that triggers known icing conditions. Assuming
you are not flying an aircraft with a certified ice protection system
(IPS), are you effectively grounded because of this AIRMET?
Not in this
case. As of 1500 UTC the air was completely free of clouds with
excellent visibility between your
departure and destination airports as shown
by the surface observations (METARs) on the right (and below) that are
valid shortly before 1500 UTC (a green square implies clear below
12,000 feet). (Click here to view a complete
KFIT 071452Z AUTO 27015G24KT 10SM CLR
M01/M11 A2937 RMK AO2
KORH 071454Z 26021G31KT 10SM CLR
KSFZ 071455Z 29009G15KT 240V300 10SM CLR M01/M09 A2936
Also notice that the METARs for
Fitchburg and Taunton above report a fairly large dewpoint depression
(temperature-dewpoint spread) of 10 and 8 degrees Celsius,
respectively, which implies very dry conditions near the surface.
in mind that surface observations only show the conditions near
the airport. So it plausible that clouds and potential icing
conditions could exist
between these reporting stations.
Or perhaps this is just a case of a bad forecast by the
Aviation Weather Center?
Possibly. But weather was anticipated
to move in from the west with time. Looking at the latest
visible satellite image (shown to the left), the air between Fitchburg
and Taunton is actually clear of any significant clouds (click here to view a complete
image). Moreover, there were no pilot reports (PIREPs) for
icing anywhere near this route as shown below (click here to view a larger
So based on the ASI's definition
contained in the online course, known icing conditions would exist even
if the sky were clear over your departure
airport and along your route of flight as long as an AIRMET for icing
existed. Of course, this does not make common sense.
This is a case where the ASI clearly does not understand the
limitations of AIRMETs.
Perhaps the ASI is just following the
guidance provided by the FAA? In this letter dated January 16,
2009, the FAA's Chief Legal Counsel clarified the definition of known
icing conditions once and for all. In that letter sent to AOPA, it states,
"Known icing conditions
involve instead circumstances where a reasonable pilot would expect a
substantial likelihood of ice formation on the aircraft based upon all
information available to that pilot."
In this proposed flight, would a
reasonable pilot would expect a substantial
likelihood of ice based on all available
information, not just a single AIRMET. Given the METARs,
visible satellite image, the likelihood of structural icing on
a flight from Fitchburg to Taunton at 1500 UTC was certainly not substantial.
Perhaps if the departure was several hours later when
moderate icing had moved in, then you can be sure the NTSB/FAA will use
the AIRMET as evidence that the pilot should have known better even if
there are no pilot reports for icing.