Airspeed low audible adjustment ?

[Italiancowboy]

So my "airspeed low" was working great, only coming in on final approach (set at 10 kns above stall) until I took my plane in for pitot/static and transponder certification check. Now it goes off at any altitude if below my setpoint (e.g. like taxiing).

What could my AI have done to screw up my alarm such that it now goes off on the ground ?

 

[GKC Aviation]

Are you seeing the flight director bars at the same time?
I thought this warning was only to do with the autopilot.
Might it be the AP is on (obviously not engaged), and you will get this warning.

 

[Italiancowboy]

No, this is with AP disengaged.
I think the alarm is the one associated with the AOA

Just strange that it only started to beep on taxi after the static/pitot/transponder test-calibration.

 

[GKC Aviation]

Ah, that makes sense.
If you have the Dynon pitot/AoA probe, check that the AoA inlet is not blocked off (piece of tape, etc).
When we do pitot/static leak and function tests, we have to block off the AoA port if using a standard type pitot test adapter.
Not saying it's causing your problem, but worth a check.
Does the AoA function work OK in flight?

 

[Italiancowboy]

I'll check that port but the AOA functions correctly at airspeeds above 75knts but below that "low airspeed" it continuously goes off (which it should while in the air, but not on the ground or at taxi speeds).

 

[GKC Aviation]

OK, have a look at this:
Go to your setup menu, SETUP MENU > SYSTEM SETUP > AUDIO SETUP and go to the Angle of Attack sub menu.
There is an "INHIBIT AOA BELOW AIRSPEED" in there.
Just check that your inhibit airspeed hasn't been changed to zero or something

 

[Italiancowboy]

So here's my update.
Pitot/AOA is not blocked at the probe. Blowing in the tube shows airspeed increase. However . . . . .
The AOA "below" was set at 20knts and my airspeed is reading 21knts when parked in the hanger and with the pitot cover on !!!
So I can use the above procedure to increase that to 30knts and thus eliminate the nuisance alarm while taxiing.

But now I have a new problem to figure out why the airspeed is reading 20 while not in motion. I assume it is some water pocket that has trapped air causing that reading. But the weird part about that is that I also have a G5 hooked up to the same static/pitot and it reads zero while not in motion and the Dynon and G5 read the same speed once I get to ~65knots. So if a water pocket, it must be downstream of where the branch connection goes to the G5.

The fun continues . . . . .

 

[Dynon]

So here's my update.
Pitot/AOA is not blocked at the probe. Blowing in the tube shows airspeed increase. However . . . . .
The AOA "below" was set at 20knts and my airspeed is reading 21knts when parked in the hanger and with the pitot cover on !!!
So I can use the above procedure to increase that to 30knts and thus eliminate the nuisance alarm while taxiing.

But now I have a new problem to figure out why the airspeed is reading 20 while not in motion. I assume it is some water pocket that has trapped air causing that reading. But the weird part about that is that I also have a G5 hooked up to the same static/pitot and it reads zero while not in motion and the Dynon and G5 read the same speed once I get to ~65knots. So if a water pocket, it must be downstream of where the branch connection goes to the G5.

The fun continues . . . . .

This is not uncommon to see from time to time where the system will hold a little bit of pressure while sitting statically. SkyView has a setup mode designed to zero this out. To fix this, go to SETUP > HARDWARE CALIBRATION > ADAHRS CALIBRATION > ZERO PRESSURE IAS/AOA CALIBRATION > enter this menu and follow the prompts on the screen. This will zero out the airspeed and you should be good to go from there.

It's not uncommon to have to do this maybe once a year or every couple of years. However, if you find you're needing to do this more often (every few months or more frequently) then it could point to another issue going on either with the pitot/static system or the ADAHRS unit itself.

 

[Italiancowboy]

Wow, thanks for the response. I'll hit the airport tomorrow and give this a shot and report back.

 

[swatson999]

IIRC, I was taught *never* to blow on or use other pressurization methods on the pitot tube, other than perhaps a rubber hose that you can roll up. Others may correct me if I'm wrong, but I was under the impression you can damage the electronics.

 

[Dynon]

IIRC, I was taught *never* to blow on or use other pressurization methods on the pitot tube, other than perhaps a rubber hose that you can roll up. Others may correct me if I'm wrong, but I was under the impression you can damage the electronics.

The risk of over-pressurization does exist when doing this, but the bigger concern in doing so is the possible effect that can cause the internal pressure sensor calibration to shift. The result of this are inaccurate readings and require the ADAHRS unit(s) to come back to be recalibrated by the factory. This is why we have the specific pitot/static test mode in setup to allow for pressure to be applied (reasonably) to avoid the risk of causing calibration issues.

 

[Italiancowboy]

Well, I hope the shop that did my p/s test (see initial post) correctly and put the Dynon into p/s test mode when he did the calibration. Maybe that's the root of my problem.

As for 999's comment, sure that is a concern which is why the AP just used his mouth to blow air (which raised the airspeed from 20kts to 50kts) instead of something aggressive like using compressed air or a air pump.

 

[swatson999]

Well, I hope the shop that did my p/s test (see initial post) correctly and put the Dynon into p/s test mode when he did the calibration. Maybe that's the root of my problem.

As for 999's comment, sure that is a concern which is why the AP just used his mouth to blow air (which raised the airspeed from 20kts to 50kts) instead of something aggressive like using compressed air or a air pump.

Well, he still didn't do it right...not sure I'd trust an A&P that pulled this sort of shade-tree mechanic act with *my* airplane.

Fortunately, *I'm* the repairman for mine .

 

[Italiancowboy]

thanks for all the suggestions from the Board. The zero-calibration in the set-up did the trick.