Faulty ASI

[PaulN]

The ASI in my CT has decided to read low without any apparent cause. One flight all is OK. Tucked away for the night with cover on pitot tube then next flight it's reading about 35 to 40 kt low at cruise speed, less difference at lower speeds but still misreading. We've blown back through the pitot line and found it to be clear.

 

Can anyone offer thoughts on what I should look for to find the cause?

 

Thanks,

 

Paul

 

 

[Wilfred]

Perhaps water in the tube or an insect causing a blockage

 

Generally both of these will also cause a slow reading eg as you slow down you will find that the registered speed is slow to catch up with the real speed

 

A bit of water will let you sometimes blow through without realising it is there

 

 

[Bruce Robbins]

Paul,

 

A partial blockage in the pitot (dynamic) will do what you describe. Have you also checked for any leaks from the pitot tube right through to the ASI. I think a leak would also cause those symptoms. ie: the faster you go the more air pressure therefore more leakage therefore more percentage drop in dynamic pressure.

 

 

[Geoff]

Paul also water or blockage in the static vent will cause this prob to

 

 

[Guest J430]

Water in the static line will be also identifiable with your VSI & altimeter misbehaving also.

 

Have had water in the static line before and as you climb the VSI and altimeter jump up in steps as the water moves due pressure changes and pitch changes.

 

if you have an alternate, try that, or just remove the static line and try that, you will find that your speed and VSI will be affected by cabin pressure, try opening the vents and closing them again!

 

You will be able to tell if its static or pitot then.

 

J

 

PS could be your AI too:;)4:

 

 

[Guest Fred Bear]

Has to be either water, debris, a leak or faulty asi. I read once somewhere that someone bumped their pitot ever so slightly and this caused it to misread. Has not been bumped by anybodys head, hangar etc?

 

 

[PaulN]

Thanks for the rapid response guys, keep the ideas flowing please ;).

 

Can't tell if the pitot tube has been bumped while on the ground at Launy. It's unlikely, but not impossible, as we were parked across the apron from the deserted aeroclub building on the grass with tie downs. How could a light bump of this tube cause a mis-read?

 

The CT has no external static vent. The instruments rely on static pressure inside the a/c. Yes, did try opening a window vent but no difference.

 

The altimeter and VSI are working just fine. I cross-checked these against the alt readout on the transponder.

 

We tried a blow back through the pitot line (on the ground ) which revealed apparently good airflow. I don't know how water could have invaded the line as the weather while on the ground at Launy was dry. How would you suggest I make sure it's not moisture in there causing the issue?

 

Is it safe to disconnect the pitot line from the AI and blow back with high pressure air (compressor) to make sure it's clear of moisture and gunk?

 

I am thinking that there may be a leak in the line too. Any suggestions how this may be tested ... block one end, apply air pressure, but how would I know if it's losing pressure?

 

I guess if I could get a hold of someones working AI and swap it for mine this would soon narrow the search to either the instrument or line. I'll start asking around for volunteers. If this did show the instrument to be faulty what would be the likely cause of working fine one day and failed the next?

 

Thanks again,

 

Paul

 

 

[Guest Graham Lea]

Asked a lame?? (Current on the a/c type) :-)

 

 

[Guest brentc]

The CT has no external static vent. The instruments rely on static pressure inside the a/c. Yes, did try opening a window vent but no difference.

So that's how the CT owners are getting such high cruise speeds

 

I believe that later models have an external static fitted.

 

 

[Guest Fred Bear]

Can't tell if the pitot tube has been bumped while on the ground at Launy. It's unlikely, but not impossible, as we were parked across the apron from the deserted aeroclub building on the grass with tie downs. How could a light bump of this tube cause a mis-read?

Paul, incorrect angle of attack on the pitot.

 

 

[Guest J430]

be careful using compressed air, any joints in plastic tube may be popped apart!

 

Dodgy AI is sounding like it.

 

J

 

 

[PaulN]

Thanks to everyone for the tips. I finally got back out to the airport today to fiddle with this problem. As I had already done during our trip while stopped over at Yarram, I removed the pitot line from the AI and blew back with no result. Then applied compressed air (only gentle pressure, thanks for the heads-up J430), still no result. Next I removed the metal pitot tube from the wing and blew through with good air flow. A careful look inside the metal tube revealed a bug had built some kind of web/nest. It appears that on our arrival at Launy a bug must have slipped in before we managed to fit the cover then while parked for that couple of days it built its cosy little home.

 

All is well now and I'm happy again. Thanks ;)

 

Paul

 

 

[Guest Fred Bear]

Debris debris debris Paul!!!! ;)

 

 

[storchy neil]

quartine inspectors will be after you:;)2: yeh it had to enter while on the ground ;)cause it couldnt fly that high or fast

 

just goes to show that no matter how carefull you are tassie bugs will slip up on you neil

 

 

[Guest J430]

The incident has now been reported to the Launy Tower Manager........and he wants his bugs back!!!!!:devil:

 

Glad you have found it. Just proves airspeed indicators are not absolutely necessary!

 

Cheers

 

J;)

 

 

[Whack777]

Have had both issues with static and pitot blockages in the past. For future ref for others: Low airspeed indication (stopped or erratic) indicated pitot system. Cleared some muck out of the static line and everything was normal again.

 

If trying to clear a suspected blockage, NEVER blow into the pitot tube with the ASI connected (Chances are you will ruin your ASI)... a very gentle huff in the vicinity of the pitot will show ASI movement if it is working.

 

In my case, a STATIC blockage caused erratic behaviour of ASI and VSI (jumping erratically together as J430 described) also, ASI indicated higher airspeed than actual, red lining at cruise setting in the circuit (I don't think so) ....good time to forget the ASI and fly by feel (Subsequently noted IAS of 80kts as stall warning sounded on touchdown). Cleared the static line and all worked perfectly again.

 

 

[Guest Nelson Smith]

I fitted a $5 fuel filter to the inlet of our pitot and it didn't affect the ASI,but it stops the bugs blocking.

 

Nelson