Aero-Tips!
A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.") It's part of what makes aviation
so exciting for all of us... just when you think you've seen it
all, along comes a scenario you've never imagined.
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators, and as
representatives of the flying community. Some of them, you may have
heard before... but for each of us, there will also be something we
might never have considered before, or something that didn't
"stick" the way it should have the first time we memorized it for
the practical test.
It is our unabashed goal that "Aero-Tips" will help our readers
become better, safer pilots -- as well as introducing our
ground-bound readers to the concepts and principles that keep those
strange aluminum-and-composite contraptions in the air... and allow
them to soar magnificently through it.
Look for our daily Aero-Tips segments, coming each day to you
through the Aero-News Network. Suggestions for future Aero-Tips are
always welcome, as are additions or discussion of each day's tips.
Remember... when it comes to being better pilots, we're all in this
together.
Aero-Tips 05.25.06
I was flying into a tower-controlled airport on a clear, cool
morning. Light winds from the southeast were blowing wet air from a
nearby river toward the airport, and I could see the fog bank was
encroaching on the runway.
ATIS (Automated Terminal Information System) weather was being
updated rapidly with visibility reports and, as the tower itself
was nearly engulfed in the fog, it began reporting prevailing visibility.
Sayeth the Feds: "Prevailing visibility is the greatest
visibility equalled [sic] or exceeded throughout at least one half
of the horizon circle, not necessarily contiguous. Segments of the
horizon circle which may have a significantly different visibility
may be reported in the remarks section of the weather report; i.e.,
the southeastern quadrant of the horizon circle may be determined
to be 2 miles in mist while the remaining quadrants are determined
to be 3 miles in mist."
KEY CONCEPT: If a prevailing visibility is
reported, by definition at least part of the sky around the
reporting point has significantly lower visibility.
Let's translate this to my arrival in that Skyhawk. About half
the area around the tower (the visibility reporting point) had
unlimited visibility—the VFR weather I was enjoying to the
west. Slightly less than half the area around the tower, however,
was foggy and visibility was limited to about a mile. The ATIS
might have reported (it's been a while, and I forget the precise
report) "Prevailing visibility greater than 10 miles; southeastern
quadrant visibility one mile in fog."
Measurement
More from the Aeronautical Information Manual (AIM): "When the
prevailing visibility at the usual point of observation, or at the
tower level, is less than four miles, certificated tower personnel
will take visibility observations in addition to those taken at the
usual point of observation. The lower of these two values will be
used as the prevailing visibility for aircraft operations." In
other words, when visibility begins to drop at a tower-controlled
airport, trained human observers will double-check automated
visibility reporting systems, and whichever visibility report is
lowest is the one that's reported.
VFR... or not?
Since the "prevailing" visibility was still in excess of 10
miles in my case, the airport was still VFR. Beware, though -- the
fog rolled in quickly, and by the time I arrived I needed a Special VFR clearance to
land.
Aero-tip of the day: Understand what's meant by
prevailing visibility, and be prepared for overall visibility to
drop if conditions are variable enough that a prevailing visibility
is reported.