Airspeed goes to zero


I am trying to artificially give airspeed voltage to APM AI0 pin using a Digital to Analog converter. The digital values are coming from JSBsim and are converted to voltage in DAC(0 to 2 volts) which is connected to AI 0 pin on APM 2.5.

When I use 'test airspeed' in cli mode, I see that the airspeed display correctly matches with that of JSBsim. It is also almost within +or- 5m/s variation from groundspeed data. But, when I run the simulation, Airspeed starts out fine but quickly goes to zero and stays Zero even though correct voltages are applied to the AI pin (the rest of the simulation runs fine).

I am guessing APM does some tinkering with the airspeed ratio on the fly which makes it zero ? Why else could this happen ?

PS. I am running an elaborate sensor HILS of APM with JSBsim, with servos connected, ADC, DAC etc and using a modified ArduPlanev2.70 code, Although I have not touched the airspeed part except for changing airspeed_pin from -1 to 0 in HIL mode.

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