Air Speed Indicator won't

[Old Koreelah]

My ASI used to work well. Now it doesn't. All the plumbing seems OK. I made a "manometer" to see if the instrument was faulty, but it worked well, with an error of less than 3%. I even tested it when sitting in the pilot's seat- a light puff into a tube connected to the pitot swings the needle nicely, then it comes to rest on 20kt. A puff into the static brings it back to zero. There is no movement in the needle when taxying or flying (except once, when for a few seconds it climbed to 40kt on take-off.)

 

What have I missed? Any help greatly appreciated.

 

Lyle

 

 

[robinsm]

Silly question, but is the pitot opening facing into wind (thr pitot hasn't moved has it). Also check and see if the air hole in the front of the pitot hasn't been knocked and squashed a little.

 

 

[Old Koreelah]

Thanks for the reply, Robinsm. The pitot and static probes still point in the correct direction, are undamaged and their tubing is unblocked. The instrument responds to puffs of air via a tube attached over the probes, but not to air movement in flight. What have I missed?

 

Lyle

 

 

[FlyBye]

If you have an alternative static source maybe try that. If not and the plumbing is ok maybe the instrument needs to be checked.

 

 

[facthunter]

pressure/ airspeed indicator..

 

Most of the aircraft we fly you could disconnect the static line and just use the cockpit pressure. FOR A CHECK OF SERVICEABILITY ONLY. as there will be a pressure drop, and associated error, but it will still work. Sorry about the blowing bit. It will wreck the instrument, most times. Nev

 

 

[Old Koreelah]

Thanks for the replies, people. I suspected the instrument because of its age, but when tested with a home made manometer (I FLY EZ) and it proved accurate to within 3 knots at 100 kt. This test required a fairly hefty puff into the tube, and I have replicated that puff when in pilot's seat ready for takeoff, and the needle swings nicely. I will try itwith the remote static removed, but doubt it will make a difference.

 

If all else fails, where can I get it tested professionally?

 

Lyle

 

 

[Owi]

Mate,

 

Is it a mechanical ASI you're talking about? If it is, the below may not help you but I've pasted this from the Dynon EFIS-D10A manual for reference anyways...

 

"The EFIS-D10A is factory-calibrated to be accurate for airspeeds between 15 and 325 knots (17 to 374 mph). As airspeed increases from 0 knots, the indicator becomes active at 20 knots. The indicator remains active until airspeed drops below 15 knots. The EFIS-D10A may display airspeeds above 325 knots but it is not guaranteed to be accurate."

 

Perhaps that is what you're missing?

 

Cheers,

 

 

[Spriteah]

OLD,

 

PUffing will not necessary damage the device. If you want send it to me and I will test is for you at not cost other than return mail.

 

Jim Tatlock

 

Austec Avionics.

 

 

[Old Koreelah]

Thanks for the offer Jim. I'll give it one more try this weekend.

 

If no better I'll remove the gauge and send it to you. I'll need an address.

 

Thanks,

 

Lyle

 

 

[Old Koreelah]

Thanks, Spriteah the offer to test my ASI, but it's working again. I disconnected the static line, (as suggested by Facthunter) and it worked, showing 10% over ground speed (GPS) on three triangular runs at different speeds. I had blown thru the line and although constricted, it allowed air thru. Is it supposed to allow the same airflow as the pitot line, or is some restriction normal?

 

Lyle

 

 

[Spriteah]

By the sound of it you have an external static source. It sounds like there is a problem there somewhere. Disconnect from instruments check all tubing and source for obstructions (bugs, dirt etc.).

 

Jim

 

 

[Old Koreelah]

ASI is now working

 

My ASI is now working. After I followed the suggestion of Facthunter to remove the static line, the ASI worked, but read c.20Kt too fast. I blew the static line clear and reconnected it. The ASI still read ziltch in flight.

 

It finally dawned on me that the static probe shouldn't have air rammed in the open front end, just like the pitot. (None of the internet sources had mentioned the actual design of the probe.)

 

I just plugged the front of the static probe, to prevent any pitot-like entry pressure. The ASI now agrees with the GPS on averages of 4-way runs at 4,500ft, but is still c.12Kt fast near the ground.

 

How do you correct instrument error without pulling it apart?

 

If I open out the two tiny holes each side of the probes will I get a more accurate reading?

 

Lyle

 

 

[Spriteah]

Your messing with a very important instrument for your survival. I do not recommend you try and fix it! If it was reading slow that is acceptable. If reading fast then you are likely to find yourself in a stall spin.

 

Buy a new one!

 

Jim

 

Austec.

 

 

[Spriteah]

Try and not connect any static and see what it does. As your plane does not pressurise you only need the air to be able to get to it. No real requirement to have a probe. You can use a little tube and then put a small restriction in the end to stop rapid changes.

 

jim

 

 

[Old Koreelah]

Thanks for your input, Spriteah, but I have tried it with static line disconnected;

 

it reads about 20Kt fast. That was an improvement. (I flew it for weeks with the static line connected, but no ASI reading throughout the flight envelope.) After I plugged the front of the static probe, it's spot-on at altitude, but near the ground (where it's really needed) it reads only about 12 kt fast. Perhaps I will fly lots of test squares to compare readings with GPS and establish where on the ASI the reliable stall speed is.

 

 

[old man emu]

Old Koreelah,

 

It is going to be cheaper, and quicker for you to pull the instrument out an send it to Spriteah to check out (if he has the correct equipment). If he has the credentials, he'll do the equivalent of an Instrument 8/9 insection on it and send you back a certification.

 

I don't mean to be rude, but from reading your posts, I wonder if you have studied the theory of airspeed measurement and have considered the differences between IAS, CAS and TAS?

 

Also NEVER blow air into a pressure instrument. I have an ASI on display that has been blown to smithereens by someone who tried to clear a blocked pitot line with workshop air. The damage was so bad that even the face glass was blown out.

 

Old Man Emu

 

 

[Old Koreelah]

Thank you for your suggestion, Old Man Emu. Perhaps you should read all that I have posted before making any assumptions. Yes, I have blown high pressure thru my pitot and static lines, but no, I was not dumb enough to have had them connected to the instrument at the time.

 

Yes, I have gently blown air from my mouth via a manometer into the pitot port, and it verified instrument reading as 3% fast.

 

Yes, I have a pretty fair appreciation of the difference between Indicated, Calibrated and True Airspeed. I have been flying quite safely without any ASI, relying on feel, attitude and my GPS, and always staying well above stall speed.

 

All I really need is to know how far above actual stall speed I am when in the landing phase.

 

A properly-calibrated ASI would be best, but first I have to ensure the external plumbing is OK. Understand?