My true airspeed as computed from my airspeed indicator is about 14 knots faster that the measured result.

Mike,

 

Uh, I would agree that your indicated TAS is 14 Kts higher than a computed TAS and if the TAS is off, so is the IAS.  I would not blame the dial on your airspeed indicator (correcting for altitude and temperature), although the airspeed device could be a problem with some sort of internal leak.  Since it is hard to get pitot pressure higher than is should be theoretically, the culprit for too high an airspeed indication is almost always a too low static (ambient) pressure sensing.  The bad part of this is that altitude would be off also.

 

Ground tests may not completely provide and answer to the source of the problem but they can certainly eliminate many problems.  There are some you can do yourself.

 

1. Pitot side.  Using a home built manometer,  you could check:

 

H2O"  Airspeed indication in Knots

2.41    70.0

4.00    78.2

6.00    95.6

8.37  113.0

 

2. Static side.  Note that a static system leak into the cockpit at cruise will give a false higher altitude and a faster airspeed.  In my airplane at 180 KIAS, such a leak would have the altitude indicate about 300  feet higher and airspeed at about 20 Kts faster than actual because of about a 4" H2O difference.  These errors are reduced at slower speed since the lower pressure in the cockpit is less.  You might run you TAS experiment at 110 KIAS and see if the correction is smaller - assuming you won't fall out of the sky.

 

You might also consider the last static system check (at least within the last two years) and any "dickering" done since then.

 

On the ground, you could seal one static port (You do have two, don't you?) and use the manometer or a slight suction device on the other static port and see how long the altitude is held as a leak indication (be gentle as the VSI and airspeed indicator will react also).

 

I would definitely want to know why the error is so great.  Luckily, if there is a static leak to the cockpit,as you slow to level flight stall speed, the error gets smaller as the ambient to cockpit pressure differential gets smaller.