Replacing an airspeed indicator

On Fri, 07 Mar 2014 13:45:53 -0800, son_of_flubber wrote:

Is there any reason to suspect that an ASI (especially a non-TSO'ed one)
might be more accurate in the middle of it's range?

On this side of the pond the rules say that ASI calibration must be
checked as part of the Annual Inspection and must be within 2kts
throughout its range.

The calibration check applies a known pressure to the pitot inlet and and
records the ASI reading. This is done at 10 kt intervals from the ASI's
highest indicated speed down to 20 kts.

An ASI is a sensitive pressure gauge that happens to be calibrated in kts.
The calibration is independent of the airframe. IOW, if the IAS in flight
shown by a freshly calibrated ASI is not the same as independently
measured TAS then the error is due to the placement of the static vent
and/or the pitot. For example, its well known that pressure under the
wing is above bulk atmospheric pressure, so a static vent anywhere near
the underside of the wing will make the ASI read low. Similarly, a static
vent placed on a convex bulge in the fuselage away from the wing will see
a lower pressure and so will make the ASI read high. That's why the
static vent is commonly halfway along the boom: at that point the boom is
a straight taper and so has minimal curvature in relation to the
slipstream while the vent is placed well away from the flying surfaces.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |