Airspeed indicator errors

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manuals, flying instruction books and, above all, FAA regulations!


Many, many Coupes have airspeed indicators that claim airspeeds as much as 15 mph high or more.  Airspeeds listed in the manual or other sources don't apply when your airspeed indicator is in error.  Old airspeed indicators frequently have this problem but it can occur in an ASI that's just a few years old.

You should do a minimum flying speed test (a stall test in any other plane) on your first flight in a Coupe or any other plane.  This lets you find the minimum airspeed in that plane with that airspeed indicator.  It may vary considerably from reality.  (Mine did.)

Then, for your first landing, fly the approach at about 1.3 times that minimum flying speed.  This will give you some airspeed for a proper flair and soft landing.



[Before replacing or overhauling your ASI,] Make certain that your airspeed error is not caused by the pitot location.

It should be parallel to the direction of flight, [at the end of the tube, the top should be] about 3 and 7/8 inches below the wing.

I have found several Coupes that had airspeed indications too fast caused by the pitot being too close to the wing. One was way off of parallel, and was reading about 10 mph slow.



In my case, the connector to the static line on the back of the gauge looked ok but the fitting connections were leaking. (which made it indicate 10-15mph to high). I tested with a spare airpseed gauge and the old one, afterward, and both were dead-on. I would focus on pitot/static troubleshooting before replacing the gauge.



I think the airflow over the pitot tube part of the under-wing will be almost parallel to the wingís flat surface.

What I've done in the past was to eyeball it from the side, making sure the pitot/static tube is parallel to the bottom surface of the wing.  This goes along with making sure the tube is 3 7/8 inches below the wing as per the specifications.

[Lynn commented: I have noticed that when you put the tube at 3 7/8 inches below the wing, that most of them appear to be bent down just a little. They are definitely more accurate at that position.]

Then, Iíd stand in front of the wing and pitot tube, trying to judge as accurately as possible that Iím exactly in front of the pitot tube.  Iíd eyeball the pitot/static tube to make sure itís not bent left or right.

There is some installation error in the Coupeís pitot/static tube location but itís pretty minor.  As I recall, itís designed to be right on at 100 mph and the installation error at 50 or 55 mph is only about 3 mph from true.  Thatís pretty good.

The only use for the GPS when talking about airspeed is to use it to calibrate your ASI.  There are two GPS calibration procedures.

One is to fly directly into the wind at a particular airspeed and record the GPS groundspeed.  Then turn around and fly directly downwind a the same airspeed and record the GPS groundspeed again.  Average the two GPS groundspeed readings and thatís your true airspeed.

Donít forget to apply altitude and temperature readings.

The second GPS calibration method for your airspeed indicator is to fly three sides of a square, recording certain data and enter it into a website calculator.