Mauledriver(at)nc.rr.com
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 Posted: Tue Aug 18, 2015 8:04 pm Post subject: Bug in the (Gretz) Pitot |
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Well, it finally happened to me - an insect built a home in my uncovered
pitot tube.
At about 600 hours I had my first declared emergency. I lost my
airspeed indication in IMC conditions. Fortunately I was hand flying
the plane at the time and didn't notice the drop until I engaged the
Trutrak AP. When I did that, the airspeed was still 'okay' but
dropping. When it reached a certain threshold, the AP responded by
flagging "AS" and diving to prevent a stall.
I disengaged and repeated the same operation while not quite allowing
the "AS" flag to sink in.
It seemed like a good idea to ask for vectors to VMC and the nearest
airport which I did. When asked if declaring I said 'yes' and landed
without incident in the unfortunately named town of Lynchburg. The '10
flies quite easily without reference to airspeed so no problem.
Problem determination pointed to some kind of clog in the Gretz unit
which in fact seemed clogged. I was hesitant to stick anything in it so
I blew it out with shop air and soaked in alcohol overnight hoping to
dissolve any organic material.
It seemed to still be partially clogged but I decided to test fly it and
all seemed well.
The next several flight involved flying in rain but each time I
enountered even the lightest precip, the airspeed would begin to drop.
Heavy rain would effectively disable use of the AP. But after some dry
air flying things would return to normal.
A call to Andrew at Angus Aviation, current owner of the Gretz heated
pitot, indicated that the pitot is simply a copper tube with external
heating elements and that poking it with a pipe cleaner or whatever
would be no problem.
I poked some things through there and determined that yes there was
organic material in there. Apparently just enough to restrict airflow a
bit but not enough to prevent accurate airspeed indications. However,
with application of a bit of moisture it swelled up and blocked the
pressure.
All good now. Use your pitot cover!
Bill "exhausting every possibility before doing the obvious" Watson
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