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.

FMI: Aero-Tips