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In true medical
fashion, I would like to request of you all 2nd and 3rd opinions about the cause
of an Airspeed Indicator (AS) problem I developed last week.
The problem: I am
getting erroneously low (by 20-40kts) AS readings. This came about suddenly, and
has never been a problem before.
The history:
First, you must know
that the plane (L-IVP) has 970hrs TT and has always had a very accurate AS,
matching very closely GPS readings (adjusting for wind and altitude) over the
last 10 years.
It is my habit to
always verbally call out "airspeed alive" as I accelerate down the runway for
takeoff--the AS usually becomes active at about 40kts or so. Recently,
however, the AS did NOT become active at the usual spot down the runway during a
takeoff run, despite a normal runup and full power settings. Just as I pulled
the power to abort the takeoff, the AI seemed to bump to life. I
exited the runway and regrouped. I shut down and inspected everything
externally--all looked good. Oh well, perhaps I had inadvertently left the pitot
cover on, and it later blew off as I ran down the runway, I thought (no
dice, as it turns out, as I later found the cover safely stowed in the baggage
compartment). Sooo, I tried another takeoff with the same result--this time the
AS became active, but far down the runway, and I aborted
again.
The local A&P
did a thorough inspection of the pitot system (all A-OK), and did a full test of
the AS with a meter attached to the pitot tube showing that indicated AS was the
same as the metered setting on the calibration device through a full normal
range of speeds up to VNE. He also "set" an airspeed with the pressure on the
pitot and clamped it off, making sure there was no decay in the indicated
airspeed, which might have indicated a leak in the pitot system. He also
disconnected the pitot system within the cockpit and reverse blew some air back
out toward the pitot tube, on the outside chance a bug or something had
temporarily blocked the system. All appeared normal, so I went out today
to do another test flight.
Results: I ran down
the runway today once again after normal runup at full power. Once again, NO
active AS at the usual point. I decided to proceed with the takeoff referring to
the GPS speed. After takeoff and climbing into the pattern at 120kts (GPS)
the AS started swinging back and forth between 70 and 88kts--it never stabilized
and I could not even retract the gear, indicating that the squat switch did not
think I had enough airspeed to trip the solenoid. I brought the plane around the
pattern and landed without incident, using the GPS speed indication +
a good margin for safety. The AS throughout the flight was always significantly
below normal reading by anywhere from 15-40kts.
So, we have here a
situation in which the AS tested out fine on the ground, and bad in the air. Any
idea why?
The mechanic is
stumped. I am stumped. One opinion has suggested that the gear speed switch
diaphragm may have ruptured, but wouldn't that result in a decay
in indicated speed with a test device putting clamped air pressure on
the pitot tube?
Help and ideas
appreciated.
Doug Johnson
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