View Full Version : airspeed indicator

09-13-2004, 08:18 PM
Upon recently flying wing to wing with another supercub I noticed that my airspeed indicator reads a constant 20-25 knots too slow. Unless I am flying with a constant tailwind, my GPS would seem to confirm this. Just wondering what you guys would suggest doing about this and some prices for different options. Thanks in advance.


09-13-2004, 08:26 PM
First thing to do is take a 4 foot length of tubing..plug one end and slip the other end over the pitot tube. Slowly roll the tubing while you or someone watches the A/S. It should climb as your roll and if you hold the roll steady the A/S should stay steady. IF it returns to zero or drops you have a line leak somewhere. Start there and then see if you really need a new airspeed indicator.


09-13-2004, 08:37 PM
It could be worse. I looked at my airspeed indicator last week and noticed on climb out it was indicating 140 MPH. Not bad ......

Then I looked out and noticed that the flapper was stuck down on the pitot tube. That's bad..... Keep the wax away from that flapper.

Tom From Iowa

09-13-2004, 08:44 PM
Yep, that proves it. Plugged lines read high and leaking lines read low! Another one when the A/S reads high is a spider crawling part way down the line to lay their eggs. Pop the line off at the A/S and use a compressor to blow back out the tube end.


09-13-2004, 08:53 PM
Good question:

First thing, visually look at pitot/static probe: Rule-of-thumb, they should be ~parallel with bottom of wing, and the pitot (hole-in-the-front) probe should be parallel with the static (holes-in-the-sides) probe. Also check for cracked metal or plastic tubes. Some aircraft have statics on the bootcowl or fuselage sides, but this isn't typical of Supercubs. I'm amazed at how many ragwing Pipers I've preflighted have problems in this area.

More likely, check airspeed (and altimeter) static hose connections, largely behind the panel. I've seem some dumping to the cockpit, not the official static connections. Frequently the tygon (or whatever) connections have come loose somewhere in th the line. You should trace it all the way back. from the instrument to the pressure/static source. Sometimes some flakey "Y" connections behind the panel, or cracked connectors.

Another related check, see what happens when you open the window/door in flight. If ASI changes, something's wrong with the plumbing, probably ASI static to cockpit or somewhere.

Third thing, I'd drag a GPS along, and compare with the other 'cub.

This effect sometimes also shows up as flakey rate-of-climb indications, if so equipped.

If you've got a Mode C Transponder Enconder the altimeter will get checked on a static basis, but depending on the day, might test OK at 29.92" but be flaky elsewhere; the Airspeed never gets checked.

If this doesn't show up anything, borrow a Magnahelic or Shortridge differential pressure meter from your Friendly Local Heating Ventilating and Air Conditioning technician, and compare readings between your pressure and static side of ASI and the other 'Cub. This can also be real handy in analysing engine cowling anomalies. (Used to have one of each in my engineering desk drawer until the layoff....).

Last chance, send A.S.I. out for check/repair to certified station. Rarely needed, in my experience.

Actually, a basic Magnahelic isn't that expensive from, e.g., Grainer's. Some of the glider guys have one one their panel hooked to a trailing-edge flap "wake rake" to measure Delta P between upper and lower tailing edge (lowest delta=best flap setting for a given condition....) But that's maybe more than you wanted know.

Hope this helps. Cubscout

Marc Olson
09-13-2004, 11:28 PM
My ASI went south a couple of months ago. The problem turned out to be the instrument housing--it was old enough that the 'back' was part of the pressure system and had failed ('47 PA-12).

It was original, and old enough to have cracked and started leaking (showing O ASI at all speeds). They re-sealed the inside of the housing and it's back to normal.

Jerry Gaston
09-14-2004, 09:23 AM
Marc I think I flew by your place in the San Juans. Is your landing strip on the east side of the island running north and south and the buildings along the strip have tall tower like structures? One being blue as I recall.

09-14-2004, 12:07 PM
We needed a new ASI for a cub we rebuilt that had the 2000 lbs gross weight increase. Got one from CC for $320.00 or so that had all the new markings for the GW increase. It was TSOd, non TSOd with markings were around $200.00 or so.

09-15-2004, 08:02 PM
thanks for all the help guys. As soon as I get home I will use some of your tips and tricks to find out what the hell the problems is.