blocked pitot/static pipes
Last week I went to my aircraft with the intention of giving it a good look over as it has been standing under its full set of covers over the winter. I went prepared to drain the remains of the now stale fuel out, wash and clean all over and re-lubricate the usual points in the control system. I also wanted to run up the engine and warm it up to heat the oil and refresh the battery. When I removed the covers I was pleased to find the aircraft in a much cleaner state than expected, certainly cleaner than previous years when it has been in a ventilated hangar through the winter months. So I did the lubrication of hinges etc then fired up the engine (took a bit of cranking to prime the carbs) and sat in the cockpit while it warmed up. I usually concentrate on the oil pressure and temperature at this point but my eye was drawn to the ASI which was reading some 50KTS, quite a lot for an aircraft parked on a calm day.
On the way home I contemplated the situation and figured out that there must be some sort of blockage in the pitot tube. Had it been in the static the altimeter and ASI would both be reading odd. So, today I went back to the aircraft armed with a bucket full of tools, a vacuum pump (for sucking the debris out of the pitot tube), a small compressor for blowing the system through and a selection of sizes of plastic tube to allow connection of stuff to the pitot system. On arrival I saw that the ASI was still reading a bit but less than before so I lifted the pilot seat and pulled open a connection in each of the two pipes going to the ASI. Immediately the ASI fell to 0. So I went to the pitot assembly, which sticks out from the front of the wing strut on my aircraft, and using my mouth tried gently blowing into the two pipes, one at a time. No problem, no detectable back pressure found. Odd. Then I noticed a few drops of water had escaped from the opened ends of the connection under the seat and a little water was visible inside the plastic pipes running along the cockpit floor in the direction of the instrument panel. It seems that a little water pooled in the pitot pipes can give sufficient pressure due to gravity to cause a significant reading on the ASI. Must make a better pitot/static tube cover, one that doesn't allow water to head up the tubes by capillary action.
If I had known how quickly the problem could be sorted I would have taken all the other clobber needed to commit aviation today. Maybe I'll find a slot tomorrow?
Rans6........................................