Turbulence and airspeed

Mary and I flew to West Bend, Wisconsin (KETB) today. It was supposed to be
CAVU all day, except along the Lake Michigan shoreline (where they were
getting stomped with lake-effect snows all day), but, of course, the
forecast was just a bit off, as it often is in winter.

From Dubuque (DBQ) to Madison (MSN) we ended up flying at 3500 feet under a
broken-to-solid overcast. With an artic cold front settling into the upper
Midwest, the winds were howling pretty good, flipping around from 020 to 310
at 19 knots when we landed in West Bend.

It was a good, but busy, landing...

En route we encountered mostly light to moderate turbulence, with long
periods of little "bumps" intermixed with some pretty good jolts. I hit my
head on the ceiling once, and my seat belt was plenty tight.

Having 1:45 to study this uncomfortable mode of flight, we discerned
something about turbulence that we'd not noticed befo Upon entering an
area of more severe turbulence, air speed invariably climbs. Since our
Pathfinder (with all its speed mods) usually cruises well into the yellow
arc, this can be a real problem.

We would set the throttle and prop RPM so that we would be clearly out of
the yellow arc, and then -- just when you DIDN'T want it to climb higher
(i.e.: when hitting moderate turbulence) -- the airspeed indicator would
almost instantly jump into the yellow arc. Sometimes it would stay there
for a minute, and we'd have to reduce power to get things back in line.

We debated this phenomenon for some time, as it didn't seem to make sense.
Why would the airspeed jump when encountering turbulence? It *feels* like
an updraft, when it slams your head into the ceiling, which seems like it
should result in either a drop -- or no change -- in airspeed. And
wouldn't you think turbulence would be equal parts up- and down-drafts?

One possibility we considered: Perhaps, upon encountering an updraft, we
were instinctively pushing the nose over, to hold altitude. This would, of
course, result in an airspeed increase.

However, as soon as we postulated that theory (man, we have some
*interesting* husband/wife conversations, no?) we were able to test it, and,
no, that wasn't the case. With the yoke held rock-solid in place during
turbulence, indicated airspeed still increased.

Another likely possibility: The airflow over the pitot tube is being
disrupted, and making it LOOK like the airspeed is jumping into the yellow
arc. A good test of this would be to see if GROUND speed increased, too,
but the GPS doesn't update quickly enough to tell for sure. And the fact
that it would sometimes stay in the yellow arc for more than just a few
seconds seems to negate this possibility.

So what's happening here? Is the airspeed REALLY increasing in the bumps,
or not?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"