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Hi, I've just had a flight where my airspeed was way out, it was as much as 15m/s or 50km/hr less than the groundspeed on a very calm day.

Can anyone help me work out if it is a leak, a sensor error, or something else?

On my second flight of the day I was doing an out and back auto mission. My target airspeed was 42km/hr (12m/s) and I noticed the airspeed reading fluctuating wildly, dropping as low as 20km/hr so I checked the ground speed which was up to 75km/hr (a new record, yay). I hit RTL to bring it back, it flew back at 75km/hr ground speed while the airspeed fluctuated wildly.

I landed, did a preflight calibration again then flew again but it wasn't any better, so I landed and physically inspected the sensor. My battery was pressing on the edge of the circuit board/cables, and the RC receiver antenna was probably touching against it so I made some space and tried again. This time groundspeed and airspeed were in near perfect agreement so I flew my mission without incident. 

When I got back I still had plenty of battery left, so with the plane in the air above me I uploaded a square circuit and tested speed vs current by doing circuits at 12, 13 and 14m/s respectively. The problem presented itself again but only at target airspeeds above the 12m/s that I'd used for the out and back mission.

I've since tested the pitot tube as described by paul riseborough and it looks fine.

I'm using APM2.5 with 2.74b firmware, 3DR pitot/sensor, APM powermodule with separate BEC for servos/telemetry. The airframe is a Bormatec Maja (pusher prop) and the pitot is mounted on a bracket that holds it 2.5cm away from the side of the fuselage, about 25cm back from the nose of the aircraft.

This graph shows the aborted mission, the big dip in the groundspeed trace at 7:47:20 is when I hit RTL. The last part where the airspeed and groundspeed are in reasonable agreement is stabilize mode.