Airspeed indicator fail (again)

[carlisle]

Finished spring inspection and cleanup on my Model II. Took it up for first flight and the ASI reads 40 mph too low. This is the second time it's failed. First one was replaced last year and had only 150 hrs on it. New one has less than 25.

Wonder if the extreme cold here in Sioux Falls has anything to do with it? Plane is hangared.

Chris

[Av8r3400]

Moisture in the pitot tube or static port?

[cap01]

just a thought but my wings were covered in 1992 and the kit sat unfinished untill last year when i completed it . after i installed the wings and the inst panel was completed i tested the pitot system and it leaked like crazy . i found the surgical tubeing that connects the pitot tube to the poly pitot hose in the wing was rotten . luckily there was a inspection plate in that area so i could get into repair it .

[Dorsal]

Another link worth looking at on the topic, I used this to check my Dynon (very accurate) and an inexpensive analog unit that was off by ~5 mph at slow speeds. As they are both in my panel it is good to know ahead of time which is right.
http://www.eaa1000.av.org/technicl/instcal/instcal.htm

[SkyPirate]

Good link Dorsal ..worth putting into the "saved" files,..thanks

Chase

[carlisle]

Thanks guys. Feel kind of silly. Started tracing back the pitot line and found the 3/8 in hole I'd drilled in it when drilling out the pop rivets for the windshield that I replaced last week. Oops. Embarassing.

Dorsal. I built a manometer last year when I was messing around with my prior broken ASI. It probably wasn't the most accurate example in the world but taught me a lot about the sensitivity and potential fragility of these instruments. Rather than re-assemble that this year, I tested this one against a known accurate one with a simpler method. Attach a 6 in or so piece of 1/8 in Tygon tubing or similar to the nipple on the back of the instrument and a 1cc insulin syringe to the other. Inject a little air, usually about 1cc, and see what it reads. If the two instruments are close, say withing 5mph of each other, it's probably ok. As a measure of how sensitive the thing is, when you plug the syringe onto the tubing, the pressure created just by that minuscule volume change gives you about 10 mph so you have to subtract that from the final reading after injecting. 1cc of air is about 80 mph using that system.

Chris

[Dorsal]

Sounds good Chris, glad you found the problem before you replaced the unit.

[SkyPirate]

it might be over kill ,.but I'm using aluminum tubing with flexible tubing between the aluminum tubing and pitot ,.. wing swivel point ..and instrument connect point ,.. with easy access to the flexible line sections ie inspection plate near the pitot. same with static line.,..it might even be lighter using the aluminum this way.

Chase