Airspeed and GPS speed disagree: the airspeed sensor is suspect

What ArduPilot says:

• Airspeed sensor N failure. Disabling
• Airspeed sensor N now OK. Re-enabled
• Airspeed sensor N, Re-enabled as GPS fall-back
• Airspeed N warning N.Nm/s air to gnd speed diff
• Airspeed primary changed: N

Five times a second the plane compares the airspeed sensor with the GPS speed. A difference above ARSPD_WIND_MAX, more than any wind you fly in, lowers a health score; after about 4 s of it, and if ARSPD_OPTIONS bit 0 is set, the sensor is taken out of use (“failure. Disabling”) and the plane flies on its own airspeed estimate. The “warning … air to gnd speed diff” line comes earlier, at ARSPD_WIND_WARN, at most every 15 s. “now OK. Re-enabled” follows about 30 s of agreement if bit 1 is set; “Re-enabled as GPS fall-back” when the GPS loses its fix.

What to do, most likely first

1. The pitot tube is blocked: water, ice, an insect, or the cover left on.

   CheckIn the log the airspeed sits near 0 or falls away while the GPS speed stays at flying speed.

   FixKeep the speed up and land. Clear the tube, blowing away from the sensor, and check for the cover before every flight.

2. A hose came off or leaks, so the reading is low or erratic.

   CheckThe airspeed in the log is noisy and jumps when the throttle changes.

   FixRefit and secure both hoses; replace any that have gone hard or cracked.

3. The wind really is stronger than ARSPD_WIND_MAX.

   CheckThe warning comes and goes with the heading: flying into the wind and with it, and the sensor reads sensibly.

   FixSet ARSPD_WIND_MAX above the strongest wind you fly in, with a margin; it is meant to catch a broken sensor, not a windy day.

The disabling is not saved: after a reboot the sensor is in use again, so find the cause before the next flight.

Parameters to look at