Asi fault

mike hurn · Post by **mike hurn** » Sun Jan 18, 2009 7:20 pm

Hi All

I went out flying on take of my brauniger ASI was only reading 20kts and flashing red light at me after climb out it was only reading 50kts my gps was showing 160kts ground speed (windy day) thought it was probably ice in pitto tube on landing at desternation removed pipe from indicator and tried to blow down tube air was going through but felt restricted after sitting plane in the sun for a few hours ruled out ice refitted pipework and flew home with same result if not worse has anyone had any experience of this ? or possible cure ... going to try sticking some mig welding wire down pitto tube at weekend to if anything is jammed in pipe
I have 2 Asi's fitted one efis one standard both undereading

Today

Well i ran a wire up the pitto tube from Asi's to the pitto tube found nothing in the pipe or anywhere else best i can tell there is no kinks in the hard plastic tube BUT the Asi's still do not work

so am now a bit stuck as to what to do all i can think of is replace the whole tube system and see what that does .. although im not holding out much hope with that ... i don't see how it worked one day then plane was parked up for 8 weeks while i flew another and now it doesn't ... anyone have any ideas that i have missed ???

Mike

Steve Brown · Post by **Steve Brown** » Mon Jan 19, 2009 12:35 pm

At the risk of stating the obvious, since both mechanical and EFIS are not fully working the problem (almost) must be in the lines - a partial leak ie split tube, sheared fitting or tube disconnection allowing the pitot pressure to dissipate somewhat or a partial blockage (water, muck, bug nests or combination of these).

(It could also be related to the static line but unlikely based on your symptoms)

I'd suggest it is an air leak since a partial blockage would cause lagging ASI readings but at a steady airspeed the ASI would eventually read correctly (it is a closed system) , whereas a partial leak is likely to cause a permanent underread.

To check this, I would connect up a pipe to the pitot tube and gently blow down it to pressurise it a bit, to see how / if it registers on the ASI. Dont blow too hard or you will damage the ASI and possibly the EFIS pressure sensor (less likely).

Since you are getting a partial ASI reading when flying, if it is a blockage you will initially get a slow reaction / partial reading on the ASI, but when you stop blowing and release the pressure, the ASI reading will stay where it is for a while then decrease ie the blockage will cause the pressure to increase AND erode more slowly than normal.

If it is a air leak, you will have to keep supplying a small feed of air (equivalent to the leak rate) to maintain an ASI reading - not normal in a closed pressure system that a pitot system is.

If that doesn't work I'd connect a tube directly to the pitot connection of the ASI and /or EFIS and see what happens then.

Best of luck Sherlock!

mike hurn · Post by **mike hurn** » Mon Jan 19, 2009 1:23 pm

Hi Steve

Thanks for the post and idea's i will check them through at the weekend

another idea that someone on another forum came up with was that mabe water froze in the hard plastic pipe and split it somewhere i cant see (as its been standing for a couple of months in a cold hangar ) i have some new pipe orderd so will be replacing it

Thanks Mike

Bill McCarthy · Post by **Bill McCarthy** » Mon Jan 19, 2009 1:27 pm

You could cause a fair bit of damage by blowing into the pitot tube. You can check the accuracy (or indeed if there is a leak in the system) by connecting a manometer tube to the pitot. Establish a level in the manometer first, then connect. By careful addition of water using a syringe to the free end, the difference in levels equates to an indicated airspeed. This method is detailed in the EAA website somewhere. I'll have a look for the hard copy that I ran off a couple of years ago.

Apologies, I don't know how to paste the web details but it is on :

http://home.hiwaay.net/~langford/airspeed_calibration/
hope that works for you.

ian herdis · Post by **ian herdis** » Mon Jan 19, 2009 2:43 pm

Thanks for posting that link Bill great info I have that filled away, when the time comes to install my pitot etc I will use it to test for leaks and be able to prove that the system works.

Thanks, Ian

mike hurn · Post by **mike hurn** » Mon Jan 19, 2009 3:10 pm

Thanks for that Bill a very handy simply made little tool for checking asi and tubing system i will make one and test it at the weekend

great stuff

Thanks Mike

mike hurn · Post by **mike hurn** » Tue Jan 20, 2009 12:05 pm

Hi Bill

Made up one of the gauges you suggested tried it out on a asi i had on the bench and found it very accurate against the scale you provided so will be very helpful in finding fault with my asi in saving me not having to keep test flying it

Many thanks Mike