Safety question for all you EFIS users

[terrykohler]

What does your EFIS NOT display if you lose pitot /static input (for example, blocked due to ice). If the input source is from VHF rather than GPS (you're on an ILS), do you lose attitude? How does the screen display lack of pitot?
Curious as to how Dynon, Advanced Flight, Grand Rapids, TruTrak, and MGL all handle this.
Terry, CFI
RV9A N323TP

[dlloyd3]

Page 2-4 of the downloadable user guide says if the pitot becomes blocked, it will use GPS info for speed and display a message GPS Assist. If the GPS then fails, it will use the last speed for reference. The GPS info can come from either their GPS or an external source.
Others may or may not do the same, each needs to be studied to find individual actions and pitfalls.

[Mike D]

GRT sport,
I have lost pitot once. From my memory, the GRT just did not display an airspeed, but everything else displayed fine. It was quite obvious because the airspeed was red.

[N941WR]

I've actually tested this feature on the Dynon D100 and in talking with the gang at Dynon, it works the same on the SkyView.

With the pitot line disconnected the background changes color and you get a warning that it is using the GPS speed.

Remember, when flying with the GPS speed, you are working with ground speed, not airspeed and when approaching to land in a strong headwind, your airspeed will be higher than your ground speed, if you have a headwind on final.

(My airplane has a steam AS gauge and the D100. To conduct the test described above, I disconnected the pitot line from the D100 but left it plumbed to the steam gauge.)

[Brantel]

For Dynon, even if you have a VHF Nav radio as the source driving your HSI, the GPS ground speed is still available for the EFIS to use for aiding in the event you lose pitot info.

[Jamie]

One caveat for Dynon's GPS aiding: If your pitot line becomes completely clogged (i.e. it ices over and the drain is iced over/clogged), the GPS aiding will not activate and the heading becomes unreliable after a while. This is because the IAS will 'stick' (not drop to zero) and your pitot system now behaves like a static system. Dynon's GPS aiding appears to only work when the IAS is at or near zero.

This is easily testable by temporarily plumbing a valve into your pitot system. This is precisely what I did.

IMHO, the important thing here is understanding and being able to detect any failure mode. I plumbed in a valve that allows me to vent the pitot system to allow GPS aiding to kick in if necessary.

[Av8torTom]

Thanks Jamie

[Noah]

How does your EFIS "Know" that the pitot or static system is blocked? The pressure will remain constant (or will greatly lag the true pitot or static pressure). I can see that your EFIS could have an algorithm which compares indicated (static pressure derived) altitude and GPS altitude and revert to GPS altitude if there is a deviation greater than some value (say 150 ft). But when and how does the EFIS decide that the problem has been corrected (ice has melted)?

As far as a pitot system blockage goes, I just don't see how the EFIS could detect a blocked pitot. Differences between IAS and groundspeed are completely normal, resulting from winds aloft, so you can't use that to detect pitot system blockage.

Yes, the EFIS might be able to detect an electrical failure of the IAS PRESSURE SENSOR, and put a big red X on the airspeed or revert to ground speed, but a blocked pitot? I just don't see how an algorithm could be designed to sense this without other inputs.

What say you, EFIS manufacturers?

[dynonsupport]

Noah,
You are 100% correct. A frozen pitot is no different than any other airspeed. Depending on how it freezes, it can drop to zero or stick at some value (which will change as you change altitude). But there is no way to detect this, since it's still just pressure at a sensor.

Sure, you can try and compare against GPS speed, but now you are 100% dependent on GPS, and what do you do in an LSA or Cub where winds can actually be higher than airspeed and zero ground speed is actually reasonable in flight, while also dealing with a L-39 that can be up at flight levels and seeing 120 knot winds. You also can't use the altimeter since a frozen pitot won't mess with your static, and GPS and pressure altitude can and will vary by hundreds of feet by design.

If you read a G1000 user manual, it talks about what happens when you lose "air data" but in this case they mean you lost the "air data" sensors or data connection to them, not just that a tube popped off or your pitot froze.

So, in Dynon's case, we did testing and found that while we have errors if the pitot freezes and sticks to an airspeed, these errors are not catastrophic, and a normal IFR pilot that is paying attention to heading, altitude and turn rate won't have any issues. We display level when we're level, and will have a small roll error when you are in a turn if your airspeed is way off. We've flown this as our only reference for many hours and it's perfectly usable.

What was an issue was if your pitot froze and went to zero airspeed. This could show you level when you were not, which is dangerous. GPS assist solves this problem by using GPS data when we have no airspeed indication, and this GPS assist mode is clearly indicated on all Dynon products when it's active. It also serves as a really nice indicator that your pitot is having a problem.

Also, as you point out, heading does use the attitude sensor platform in the short term. This lets the displayed heading move immediately as turns start, and also allows heading to be correct in all attitudes. Over the long term, though, the magnetic sensors "win". So when not maneuvering, heading will gradually lose any errors induced by the effects of a partially plugged pitot, and you should see the same correct headings your used to after it settles out. It just may over or under react during turns, similar to a normal "wet" compass.


--Ian Jordan
Dynon Systems Engineer

[terrykohler]

Ian:
So if one is planning to use your EFIS for instrument flight, an internal GPS is a must-have, correct? A feed from a garmin 430 wouldn't suffice.
Just to complete the circle, what would happen to the display on a unit without internal GPS if the pitot blocked and AS went to zero? Same if the drain hole on the pitot also blocked?
Do competitive units behave similarly?
Great info for serious instrument pilots. Thanks.
Terry, CFI
RV9A N323TP