IFR just 5.4% of the time

"Jay Honeck" wrote in message
ps.com...
As I sit on the ground, on a day off, not flying due to (yet more)
ice, I thought I'd share these interesting results with the group...

Pilots are always surprised when I tell them that Mary and I have
traveled the country extensively by light plane for 12 years, all
VFR. While it's true that we have to be flexible, my experience has
been that it is rare, indeed, that we must cancel a flight due to IFR
conditions that we would have flown in our Pathfinder (a Piper
Cherokee 235), even with the rating.

Many people have questioned the validity of our experience, wondering
if we scud-run everywhere, or are simply not telling the truth. Well,
in a strange twist of fate, a friend of mine recently completed a
study of ASOS observations from 2002 - 2004 here in Iowa City. His
primary goal was to determine prevailing wind direction while IFR
conditions existed, but he inadvertently turned up some interesting
data that supports my informal observations.

During that two year period, he looked at ~33,000 recorded hourly
observations at KIOW. Just 1765 of those observations were IFR, or
5.4%.

Now, of course, there were an unknown number of marginal VFR
conditions in the data set, but these results pretty well confirm my
(non-scientific) observation that showed us canceling just a handful
of flights each year due to weather, and a truly tiny set that were
canceled due to "soft IFR" conditions that we would feel safe flying
Atlas in. Most of our IFR weather in Iowa City is due to icing, fog,
or thunderstorms, meaning that we're not about to challenge Mother
Nature in a Piper Spam Can anyway.

What does this mean? A few conclusions:

1. VFR conditions prevail roughly 95% of the time, even here in the
rough-and-tumble Midwest.
big snip
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

If you're a fun flyer, maybe work for yourself, and don't have a hard and
fast schedule you need to follow, being VFR only ain't a bad thing. On the
other hand, if you need to get somewhere on a schedule, and get back on
schedule, VFR only doesn't get it done.

Your stats showed only 5% or thereabouts IFR weather. Fair enough. So, 5% of
the time, you won't be able to get in or out due to IFR conditions. I'd
venture that there is another 5% where marginal VFR exists and you wouldn't
venture too far from home in those conditions. So, now we're at a 10% no-go
rate.

Extrapolate that to a destination location, which would be no-go 10% of the
time, and you're down to an 80% "go" probability, assuming there isn't
something nasty between here and there. If the X/C is of any distance, there
is probably at least a 5% chance that there is weather bad enough that you
wouldn't cross it VFR. So, now we're down to a 75% chance of launching on a
cross country trip.

The return trip a day or three later has the same weather odds, so if you
multiply the 75% chance of a good trip out by a 75% chance of a good trip
back, the odds of meeting a schedule on a round trip X/C are about 56%...

Time of year and where you are based play a large role here. Presumably if
you live in Arizona you don't see much IFR. On the other hand, on the East
side of the Mississippi, the winter can bring days and even weeks of
marginal weather, and the spring and summer bring fronts and convective
activity that isn't to be trifled with.

KB
(A VFR only pilot with a VFR only airplane...)