Which one do I trust
 

Today I noticed a 10 mph difference between the indicated airspeed on my GRT sport EFIS and my round airspeed indicator ,most notably in slow flight. While at altitude practicing slow flight the EFIS indicated 60 , round gauge 50 ,kind made me unsure of which one to trust once I was back in the pattern.While fueling up today I noticed white gritty stuff on the leading edge of my wings, I wiped my hands across the wings and they where covered in Sea salt, a lot of it, I live and fly at the beach but I've never seen this before . My plane is always in the hanger when not flying and I wash it regularly .Probably not related to the airspeed issue but I can't help but wonder if it's possible that the salt has partially clogged the pitot. Prior to today the speed indicators where sync.
Input?

If they used to be in sync, but aren't anymore:

1. Trust whichever one gives the most conservative reading for what you're doing at the time (in slow flight: take guidance from the one closest to the stall; in fast flight, take the one closest to Vne)

2. Accept that both instruments might have been compromised, so neither of them are strictly trustworthy anymore.

3. Start looking through your pitot system for changes: Why were they in sync before, but not now?

You're on the ground now, so perhaps (1) doesn't matter until you've sorted out (2) and (3).

Sound reasonable?

- mark

Test it out
 

Hi,

Can I suggest you go and do a GPS/Airspeed test. Info is pretty widely available and basically, set an altitude, complete a box flight plan and let each stage of the flight settle airspeed.

Check it against the GPS and round out. You will soon know which one is incorrect.

Thanks for the input.

I did a CC today and I did compare to fore flight GPS and it seems the EFIS is correct also knowing stall speeds you can fly til stall and compare to be sure.
Actually today the 2 indicators where more in sync for some reason still about 5 off though.

Hi Andy, I never put the two stories together until just now. As mentioned, something may have migrated through your pitot line. Not sure if this is indeed the case but could account for one being lower that the other and then not. If you did have salt flakes, humidity would change their effect.

All guessing here of course but it may be a factor.

Assuming both instruments are fed from the same pitot & static ports there has to be something wrong with one or other instrument. Are there any crimps in the pipes to the instruments? Carry out a leak test and rectify any leaks found, then try a calibration in the hangar using a water manometer (details on the web). Does the difference even out after the pressure is applied for 30 seconds (some kind of blockage) investigate the instrument that changes. Should be possible to get them within 1 or 2 knots.

Pete

That totally makes sense , the humidity effect on the salt.
I've got a annual coming up so I will be sure to have it checked out.

I'm curious why do the RVs have two static ports , at least mine does one on each side of the fuse. the rivet type , and do they work separately or in conjunction with each other . I'm new to this obviously.

Of course a builder could do it differently, but usually they are linked. Then a single line up to the instruments. I think it is mainly in case one gets plugged, but may help keep it more accurate in a slip?

To ensure that any sideslip is cancelled out - the ports are usually joined in the fuselage. Take the baggage bay bulkhead out and you should see a pipe attached to the next frame back.

Pete