Airspeed Indicator

[Scott Grand]

[9089P]

[MV Case Jr]

[tomburke1]

As a rule the airspeed indicator shows lower speeds as it goes bad.
There is a website that shows how to check your airspeed. It requires that you make different runs at cardinal headings and average them out. The address is http://www.reacomp.com/index.html?true_ ... index.html
Once you establish your aircraft speed you can compare it to the true airspeed you see on the indicator.

[Zach Grant L1011jock]

Tom,
I beg to differ. As a rule the ASI will show HIGHER airspeeds as it goes bad. This is because the spring that the air acts against becomes week. Most older ASI do not show zero on the ground at rest. If your ASI does not point vertically when the plane is sitting in the hangar ( then it is showing a displacement to the right a bit), then it is showing higher than normal airspeeds. If it sits on the gound showing 10 mph, then I guarantee you will be indicating at least 10 mph high in flight. Never in the history of man has a spring become stronger after use, rather than weaker...
-Zach

[tomburke1]

I stand corrected. I should have said mine showed lower airspeeds. At stall it was reading abou8t 45 mph when I finally had it overhauled.

[Zach Grant L1011jock]

Tom,
Just curious, what, if anything did the overhaul shop say was wrong with the ASI. Was the bellows bad? Most of these get caught on static checks as it will cause mixing of air pressures between the static and pitot systems.
-Zach

[tomburke1]

It has been so long ago that I have forgotten. I don't remember how much it was but I remember I thought I could buy 2 new ones for the price,,

[9089P]

Hi Zach,

My faulty asi was caught on a pitot static check. The tester sited a system leak so we took the left sidepanel out which is a big pain only to find no problem. We then engaged our brains and the IA asked if the asi was inaccurate. I said yes it seemed high to me and so we tested it by putting a tube on the back and carefully blowing in to put a SMALL amount of pressure to the gauge. Raised the needle to 200 mph then put thumb over the end of the tube. The needle slowly returned to 0 indicating a leak.

Given that the gauge was original (ie '66) and incorrectly marked (the mods to restore the vne were installed) I ordered a new one from the gyro house. Only problem was that I had to convince them to mark it correctly as they use the poh and mine had the lower speed.

The new asi is very accurate and two added benefits are that getting to gear speed is much easier and landings improved as final speed was now accurate.

Good luck, Don

[Scott Grand]

Tom,

Thanks for link - I'll give it a try.

Zack,

The ASI does indicate zero on the ground, and I had it bench tested as well. I'll post the results of the TAS test.

[Zach Grant L1011jock]

Scott,
I would take the plane to a shop that can do full Pitot/Static tests. That will show any leaks in the static system that could have low pressure static air from cabin venting, causing a greater delta P and a higher airspeed indication even with a perfect ASI. Also, what type of static ports do you have, the flush holes, or the button type that are slightly raised. If you have the flush ones, you may need to put a "fence" in front of the hole to get a true static pressure, not a low pressure. Good luck.
-Zach

[Scott Grand]

[Zach Grant L1011jock]