We show airspeed starting at 20 knots (23 MPH) when accelerating and turn off at 15 knots (17 MPH) when slowing down.
The TSO allows +/- 5 knots error at 20 knots, so it's actually perfectly allowed for one instrument to read 25 knots and the other 15 knots and both meet the requirements for an airspeed indicator. The error is only 3 knots above 100 knots.
The reason for the error allowance is that 20 knots is only 0.009 PSI, so it's pretty hard to detect. The difference between 20 knots and 15 knots is only 0.004 PSI! Most certified gauges start at 40 knots (0.040 PSI) for this reason. At 100 knots the pressure is 0.240 PSI and it takes 0.015 PSI to get 3 knots of error.
There is a "zero airspeed cal" you can run in the AHRS Hardware cal to make low speeds more accurate. We still don't show speed until 20 knots though.
Just for personal interest, what backup instrument do you have that is showing airspeed at less than 20 knots?
