D100 altitude problem and VSI strange readings

[Allegro]

On start up, the VSI fluctuates wildly showing climb and descent rates of up to 4,000 ft/min.
When the baro pressure and altimeter figures are displayed after 30 seconds, the altimeter follows the information from the VSI which continues to show climb and descent.
Also the altitude always reports about 300 ft too high and the baro setting is not remembered when rebooting the unit.
The problem happened once in flight for a few seconds but now is there all the time.
Software 3.0.0 Unit fitted with OAT and linked to GPS.
Regards
Paul

 

[khorton]

Is it possible that suddenly the static system is seeing fluctuating air pressures from prop wash, or somewhere else? Maybe some connection in the static system has  come off.   Have you checked the static system for leaks?  

If you plug all static ports but one, you can push a piece of flexible tube over the other port, and suck on the other end of the tube.  Suck hard enough to make the altimeter increase at least 1000 ft, then put your tongue over the end of the tube to block the pressure.  There will be a bit of movement in the altimeter until all pressures stabilize, then it should not decrease by more than 100 ft in 60 seconds.

 

[dynonsupport]

Does it do that only after the engine's running, or anytime the unit is on? If the former, it might be a prop / static effect as suggested. If it does it even in still air with the engine off, we may need to look at your unit.

 

[Allegro]

Thanks for the replies.
Sorry, I should have said 'when switched on'. I meant when the unit is turned on with the engine not running.
I don't think it is a static port problem because I have dual static ports and several other instruments connected to them.
Will I have to return the unit?
Regards
Paul

 

[dynonsupport]

One thing you definitely should try before we get to that point - disconnect pitot, static, and the AOA lines from the back of the instrument, and fire up the engine.

If you see the same thing, we may need to take a look at the unit, or at least get more creative in our troubleshooting. If the problem goes away, the fluctuations are really coming from the static system.

 

[Allegro]

Thanks.
OK, we disconnected the pitot and static. The readings are still wild. Sudden climb at 6,000ft/min, sudden descent to minus 400 ft. Altitude over-reads about 200ft. On reboot, baro goes from 1006 to 960. Altitude can be corrected in the menu but VSI never stabilises.
The AOA is not installed and still plugged - presumably that is correct.

The unit has 45 hours and did the same thing at about 20 hours briefly. Occasionally, it also reports unlikely wind speeds (25 kts at 1,000 ft soon after take off in a 5 kt wind). Cycling OAT "N" then "2" and manually adjusting several degrees correction seemed to cure it but that may have been coincidence.
We have a 4,000nm round trip planned from Thailand to Indonesia in July and it would be good to get the unit 100% reliable.
Regards
Paul

 

[jakej]

Hi Paul

This is a wild guess but may apply - does the fluctuation occur with changes in RPM ?  If so, temporarily disconnect the MAP line and check again.
I once saw similar issues with an Encoder connected to the Manifold Pressure line (Helo) had 3 Avionics shops baffled.

cheers,  Jake

 

[dynonsupport]

Just one more clarification - does the VSI only behave erratically within the first few minutes of power up, or does it do it continuously? The former is somewhat expected behavior as the unit takes a few minutes to warm up and settle it. But if its doing this continuously, we should probably take a look at the unit here.

 

[Allegro]

Thanks
The problem is random. The unit used to settle within a minute of switching it on with no wild VSI readings warming up.
Now the altitude always reads about 200 feet high and the VSI gives strange readings on start up, settles sometimes but can suddenly misread wildly in the air.
When the unit first arrived, the VSI would often fluctuate up/down 50 feet in level flight which I assumed was just the sensitivity of the VSI compared to a steam gauge VSI which was slower to react.

 

[PilotKris]

If you plug all static ports but one, you can push a piece of flexible tube over the other port, and suck on the other end of the tube.  Suck hard enough to make the altimeter increase at least 1000 ft, then put your tongue over the end of the tube to block the pressure.  There will be a bit of movement in the altimeter until all pressures stabilize, then it should not decrease by more than 100 ft in 60 seconds.

Be careful using Ken’s technique if you have a mechanical altimeter, ASI or VSI. If you suddenly release the vacuum you're holding with your tongue, you could damage the mechanical gages.

Also, make sure you have a long static line or flexible tube so that none of your spit (very corrosive) finds its way into the gages or Dynon unit (I don't think spit damage is covered in their warranty).

Personally, I'd pinch/fold the flexible tube to hold the vacuum.

PilotKris