Airspeed Indicator

[johneeb]

[quote="Randolph"] I ordered a Service Manual from a place we're not going to talk about here, but it hasn't come yet. I understand there is a pic in there of angle and location. Thanks Randolph[/quote]

Jim you are a quick study!

[N2865C]

One other thing I can think your problem could be is possibly a partial restriction in the pitot line like a dead bug or something. If you check the lines and they are clear the only other thing you can do is have a complete pitot/static check using a test box to check the calibration of your ASI.

Well duhhh... I completely forgot why I learned to do the rubber hose trick in the first place, and that reminded me. When I flew my plane home after buying it my airspeed was also reading low (around 90 in cruise). What cured it was removing the line from the back of the ASI and blowing compressed air through the pitot line (air going away from the ASI). I hope your problem is that simple.

There were wasps nests all over that plane. After takeoff I opened the air vent and saw a wasp nest in the tube. Scared the crap out of me. Fortunately, there was no one home.

jc

[GAHorn]

Many years ago I put a valve to the static line in my '53' as a alternate static source for those times when the outside static port iced up. Whenever I opened it to cabin pressure the airspeed indicater would indicate about 8 to 10 mph slower than the outside port. My first '52' 170B had rotten static hoses when I got it and when they were replaced the indicated airspeed increased about 10 mph. So if you are indicating low I'd check the static side of your system.

That is an EXCELLENT point! Sometimes we forget that simple, unpressurized airplanes can have complicated difficulties.
The airspeed, VSI, Altimeter ALL use the static vent sytem, and high cabin air pressure (due to wing-root air vents being open and ramming air into the cabin) can induce large errors in the indicators. Having our pitot-static systems checked periodically (every 2 years is required for IFR) is a good idea even on VFR aircraft.

[Randolph]

Gentlemen, a year ago I had a problem with my ASI. I posted here and got a lot of ideas and support. Unfortunately I never solved the problem.
For reasons I won't bore you all with I sort of just quit worrying about it. Here I am a year later and I have been working on the same problem again with a mechanic. I was up flying four different times today and we still haven't solved it. If I disconnect the static from the back of the instrument I get some pretty close readings (I'm comparing airspeed in all four directions with GPS speed in all four directions on fairly calm morning). I read another thread on here about a fellow with a 48 170 (which is what I have) that was having a similar problem. Eventually I believe he just disconnected the static line from the back of his ASI and called it good. I haven't quite become that exasperated yet. I would truely like to solve this. Oddly enough I recall vividly that when I first got this plane it showed 120mph cruise but it doesn't anymore, at least not on the ASI.
We've checked and rechecked the pitot line and blown air through it in both directions.
I've compared the pitot to the picture in the book and flown with string tied to the front end of the pitot tubing.
I've replaced the instrument with another that is "supposed" to be working correctly.
I've removed the disc on the inside of the static port and reattached it to the fuselage skin with something called "gorilla snot"?
I've removed the four machine screws holding the static assembly together and replaced them with rivets.
The mechanic has checked over the tubing and the fittings between the port and the instrument and declares them to be "OK"
I've bypassed all the other instruments and gone straight from the port to the instrument to rule out any problems associated with the other instruments.
Again, it flies around 15mph faster than it registers, it worked when I got it, and it looks pretty good when it is disconected trom the static at the back of the instrument. Everybody in the shop is scratching thier heads over this fellows.
Thanks Randolph

[GAHorn]

Hello, Randolph!
Well, I re-read the posts leading up to the present, and I see you sent the indicator out to a "shop".... Was that a genuine aircraft INSRUMENT SHOP that had your problem described to them?
Did they have any suggestions as to how to quit all these "farmer" troubleshooting methods we've described, and let them or another genuine repair station troubleshoot this for you?

[Randolph]

George, I dropped off the instrument at the local Cessna shop. They were going to do a pitot/static test on it but their machine wasn't functioning correctly so I asked them to "send it out" When I picked it up, they told me it was fine. I called just now to see where they sent it. I couldn't get a hold of the fellow that I talked to last year. I spoke with the parts guy and he said probably the "Gyro House" which I believe is a place in Auburn that specializes in that sort of thing. I'm sure others on the board, in this locality, can/will comment on them. I have the receipt but it is from the local shop. It indicates that the instrument was shipped somewhere but says nothing about the condition of the instrument or where it was sent. My recollection of the discussion that ensued when I picked it up was something like: They said it was fine, they couldn't find anything wrong with it. That's the best I can answer your question.
I printed off an article from the C 140 website about a similar problem in 140's but I have this very distinct recollection from when I first bought the plane of looking down at the ASI and seeing it read 120mph and thinking,to myself, that's right where it should be on this airplane.
Thanks, Randolph

[GAHorn]

Randolph, there's a distinct difference between a "receipt" for service and a "yellow tag" or "serviceable tag" (also known as a Form 8130-3). The yellow tag/8130 certifies that the instrument was tested in accordance with it's mfr's specs and is approved for return to service.
The "receipt" is a business matter with no validity with regard to airworthiness. This situation with your instrument may be nothing more than instrument error, position error, installation error, etc., ...or... it could be a defective instrument. If all that's been done so far is to use shadetree methods of troubleshooting (which I've contributed to here myself) then very little has been accomplished over the last year to resolve your problem.
I suggest you have a qualified, certified avionics shop give your aircraft a complete pitot/static system recertification and also have your airspeed indicator, your altimeter, and your altitude encoder re-certified for flight. (You're required to do this every two years anyway if you fly IFR, and you're required to do at least a txdr/encoder check every two years even if you only fly VFR. See FARs 91.411 and 91.413) This should difinitively help you resolve the problem.
Let us know how this proceeds.

(I presume you are familiar with the True Airspeed, and how that relates to groundspeed and how, depending upon atmospherics, that will result in a lower indicated airspeed indication?)

[N8249A]

I have the same problem with airspeed. 10 to 15 MPH slow. Had the local avionics shop do the IFR pitot static check and it passed. I will try the template next. The ASI was overhauled prior to my purchase of the A/C two years ago and the Airspeed has always indicated slow.

[FredM]

If you have the original tube type pitot tube I would also inspect the tip very closely. If the tube was cut with a tubing cutter and not properly de-burred the inside diameter at the tip will be reduced and the airflow slightly restricted. Checking it with the template previously mentioned was a good idea also. If the tube has been bent too sharply the tube can flatten reducing it's crossection area and causing a restriction. If it has passed a pitot static check and you can't find a defect with the pitot tube I would still recommend changing the tube. If you make your own I would use a cutoff wheel to cut the tubing instead of a tube cutter.

[GAHorn]

That would seem logical, but impact pressure is still impact pressure. In only milliseconds the tube would fill with it and the indicator (within the bounds of it's accuracy) would indicate the total pressure. Unless the tube is radically reduced/restricted, the gauge should still indicate the total pitot-pressure.
A common failure of old air-pressure instruments is a leaking aneroid and/or hysteresis...the inability of the aneroid capsule to remain elastic and flexible, which results in inaccurate readings. (This is a problem with alitmeters also, and a major reason they must be re-tested every two years to remain ifr-airworthy.)
Another problem with old gauges is increased mechanical resistance due to worn bearings and gears. The needle simply doesn't want to move as easily (sorta like me.) Time for overhaul/replacement if bench-test warrants.
Remember that density alittude will affect airspeed indications....the less dense the air...the lower the reading even though the aircraft is maintaining the same speed. As summer approaches the air is becoming less dense and indications from more standard, winter weather will occur less and less.