Airspeed Indicator problems

Stan: First advice: STOP BLOWING IN THE ASI! That's the best way
there is to destroy it, which you might already have done. I've seen
people walk up to a pitot tube and put their mouth to it, and I cringe
at the thought of what they're doing to the internal workings. If you
disconnect the plumbing from the instruments you can check it's
continuity by drawing a slight vacuum on it and seeing if it holds.
I've seen plastic tubing chewed through by mice, and worse yet, I've
seen instruments incorrectly plumbed by someone who was "testing the
system." If neither of these is the problem you probably need to send
the instrument to a qualified shop. Fred

On Jun 18, 10:42 am, wrote:
Stan: First advice: STOP BLOWING IN THE ASI! That's the best way
there is to destroy it, which you might already have done. I've seen
people walk up to a pitot tube and put their mouth to it, and I cringe
at the thought of what they're doing to the internal workings. If you
disconnect the plumbing from the instruments you can check it's
continuity by drawing a slight vacuum on it and seeing if it holds.
I've seen plastic tubing chewed through by mice, and worse yet, I've
seen instruments incorrectly plumbed by someone who was "testing the
system." If neither of these is the problem you probably need to send
the instrument to a qualified shop. Fred

If it is an old instrument, it could be an accumulation of dirt,
especially if flown from turf or a dusty area. Repair or replace the
instrurment and check out the plumbing as Fred suggests.

Also, plastic tubing degrades over time and can become a problem with
leaks.

Chip F.

If you are interested, I have an old Schweizer airspeed without markings
that has been on a shelf since about 1965. Make me an offer.

Bill Daniels


"chipsoars" wrote in message
ups.com...
On Jun 18, 10:42 am, wrote:
Stan: First advice: STOP BLOWING IN THE ASI! That's the best way
there is to destroy it, which you might already have done. I've seen
people walk up to a pitot tube and put their mouth to it, and I cringe
at the thought of what they're doing to the internal workings. If you
disconnect the plumbing from the instruments you can check it's
continuity by drawing a slight vacuum on it and seeing if it holds.
I've seen plastic tubing chewed through by mice, and worse yet, I've
seen instruments incorrectly plumbed by someone who was "testing the
system." If neither of these is the problem you probably need to send
the instrument to a qualified shop. Fred

If it is an old instrument, it could be an accumulation of dirt,
especially if flown from turf or a dusty area. Repair or replace the
instrurment and check out the plumbing as Fred suggests.

Also, plastic tubing degrades over time and can become a problem with
leaks.

Chip F.

wrote:
Stan: First advice: STOP BLOWING IN THE ASI! That's the best way
there is to destroy it, which you might already have done. I've seen
people walk up to a pitot tube and put their mouth to it, and I cringe
at the thought of what they're doing to the internal workings.

Apart from the possibility of moisture ingress, the resulting pressure
is likely to be too high and can easily wreck the ASI.

After I've had the panel out of my Standard Libelle, which has the pitot
and TE probe on the fin and hence a lot of plastic tubing to leak, I use
the following simple check procedures:

- start with normal or cool air temperature
- place my thumb over the end of the pitot.
- with thumb remaining firmly in place I close my hand round the pitot
tube and wait.
- after 5 to 10 seconds the ASI comes off its stop and indicates
about 10 to 15 kts
- the reading remains stable as long as I care to retain my grip on
the pitot.
- the ASI returns to zero when I let go the pitot.

This shows that the ASI is in working order and that there are no leaks
between pitot and ASI. Of course it says nothing about the ASI
calibration but that gets checked during the annual C of A.

The only blowing I do is a trick shown me by an old hand. Turn the vario
on and the volume fully up. Hold your hands parallel to each other,
fingers together and bent slightly backward, on either side of the TE
probe and with your palms about 25mm (1 inch) apart. Blow hard between
your hands with your mouth about 100-150mm (4-6 inches) away from the
heels of your hands. If the system is operating correctly you'll hear
the vario signal a climb for 2-3 seconds and then go back to zero.

If both tests work OK I can be pretty sure that the pneumatic quick
connect at the panel was sealed correctly when I put the panel back in.

HTH


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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