Help Diagnosing EFIS Airspeed Issue

[Prof D]

While on the taxi out for a post condition inspection test flight I noticed the airspeed of both the pilot EFIS (AFS 3500) and co-pilot EFIS (AFS 3400) flipping from 0 KIAS to about 25 KIAS every 5 to 10 seconds. It seemed to be an abrupt transition, not a continuous roll up/down. Not sure if they were doing it in synch or if they were just both doing it. It was also only occurring during taxi. It settled out at the run up area while the plane was not moving.

I taxied down the runway at about 35kias. The airspeed seemed stable and was pretty close to the GPS speed.

Both Pilot and co-pilot were doing this. The G5, which is plumbed into the same pitot/static system was reading "---" on the taxi.

The pilot side EFIS and the G5 were both removed during the inspection, so the electrical connections as well as the pitot/static systems were disturbed. The co-pilot side EFIS was not disturbed. We also did a pitot static leak check during the CI.

I've never seen this before today. I did not take the plane up.

Anyone see anything like this previously or have an idea? I'm leaning towards a bad electrical connection, but am open to thoughts.

 

[AZSteve053]

While on the taxi out for a post condition inspection test flight I noticed the airspeed of both the pilot EFIS (AFS 3500) and co-pilot EFIS (AFS 3400) flipping from 0 KIAS to about 25 KIAS every 5 to 10 seconds. It seemed to be an abrupt transition, not a continuous roll up/down. Not sure if they were doing it in synch or if they were just both doing it. It was also only occurring during taxi. It settled out at the run up area while the plane was not moving.

I taxied down the runway at about 35kias. The airspeed seemed stable and was pretty close to the GPS speed.

Both Pilot and co-pilot were doing this. The G5, which is plumbed into the same pitot/static system was reading "---" on the taxi.

The pilot side EFIS and the G5 were both removed during the inspection, so the electrical connections as well as the pitot/static systems were disturbed. The co-pilot side EFIS was not disturbed. We also did a pitot static leak check during the CI.

I've never seen this before today. I did not take the plane up.

Anyone see anything like this previously or have an idea? I'm leaning towards a bad electrical connection, but am open to thoughts.

Some time ago, AFS (AdvFlt) changed their software to have a minimum airspeed - this changed the EFIS read out such that at rest (zero airspeed) I think it reads "---" but don't hold me to that. As airspeed picks up to the "Min" speed, the airspeed does start reading - it could be as high as 25kts... therefore if you were taxiing near this speed (including headwind) then you may have been coincidentally right at the speed where it starts reading - therefore no issue. As you returned to the hangar from your run-up I presume the opposite direction would cause a change in headwind so issue should have disappeared. Of course airspeed is Pitot minus Static so any air pressure to static could also change situation.
If your EFIS normally gradually ramp up from zero to 2-5-8-10kts then my supposition above is likely not the cause.
Hope this helps.

 

[Jpm757]

Check for water in the static system.