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terrykohler

Well Known Member
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
 
Dynon Skyview

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.
 
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.
 
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.)
 
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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.
 
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.
 
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?
 
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
 
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That's What I Was Looking For

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
 
terrykohler said:
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.
...
Click to expand...
Terry,

Ian will be along shortly but the quick answer is, the Dynon EFIS can be fed by an external GPS. In my case, I used a 496 to feed the GPS ground speed to my D100 in the test I described above.
 
For WIW, I suspect frozen pitot tube will not happen instantly and to cause the airspeed to get stock at pervious cruise speed. It will probably be a rapid decrease in speed till the pitot is fully frozen and then speed remain constant at that pressure altitude.

The one time that I discovered frozen pitot tube my IAS was rapidly going down while my attitude/power was unchanged. This prompted me to turn on the pitot heat which rather instantly got back to the cruise speed.

In that episode, my GRT functioned normally with the exception of seeing IAS rapidly decreasing. It was for too short of the time for me to notice any heading/attitude discrepancies.
 
dynonsupport said:
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.
Click to expand...

My own testing agreed with what IAN said here. I followed the Dynon attitude indication like a naive pilot in the soup (did this several times) and it banked no more than 20 degrees and pitched no more than +- 10 and the constant heading turn indicated that something was definitely amiss.

If you blindly follow the attitude indication of any instrument, including in a six-pack configuration, you are doing it wrong. This is the reason for an IFR scan -- to cross-check your instruments to ensure that everything is in agreement.
 
terrykohler said:
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?
Click to expand...

The D100/D10A/D180 don't have an internal GPS, and any external GPS works, including a 430 or 430W. The SkyView systems can use the internal or any external GPS for assist (or both if you have more than one like we suggest).

A Dynon EFIS with no GPS connected and a pitot reading zero will slowly revert to level in long turns. If the pitot freezes at a fixed airspeed, then there will be a roll and maybe pitch error, but it will be correct at level.

You will have to ask other vendors very direct questions about their performance without pitot. Ask them if they would personally fly their system in IFR with a frozen or blocked pitot with and without GPS connected. I can't answer that for other vendors, but with a Dynon I would if I had a GPS. We try and be very open with the performance of our systems so that people can know the limits and fly them safely. Also ask if anything else in the plane must be working for attitude to be safe to use.

By the way, one huge advantage of the way we do things is that you can boot the EFIS in flight and have usable attitude within seconds. Some other EFIS systems can take minutes to recover in flight (if ever). There's no perfect solution to attitude sensing, everything is a tradeoff.

--Ian
 
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Funny how this is about safety for efis users. At least there is some form of backup whereas certified steam gauges are much more of a safety issue when pitot or static fail. I wonder how much safer certified aircraft would be if tried and true experimental efis displays, autopilots, etc were allowed.
 
Different? Yes, I wouldn't go so far as to say safer

Flybuddy2 said:
Funny how this is about safety for efis users. At least there is some form of backup whereas certified steam gauges are much more of a safety issue when pitot or static fail. I wonder how much safer certified aircraft would be if tried and true experimental efis displays, autopilots, etc were allowed.
Click to expand...

I'm considering an EFIS as part of a panel upgrade. Do they give you more information? HITS, Flight Path Indicator, Synthetic Vision- oh yeah. BUT, I've learned I should have some constant form of GPS input, that I should turn on pitot heat on ANY flight in IMC, and that I either need a backup battery for the unit, or more likely a backup attitude indicator. Also, I now know how some of the failure modes will present themselves. This is great info.
With an "old school" six-pack,if the vacuum pukes, I've got a turn coordinator and mag compass. Pitot input doesn't effect my attitude. Safer? Not necessarily.
Terry, CFI
RV9A N323TP
 
This is an excellent thread. I hope some of the other EFIS manufacturers chime in.
 
dynonsupport said:
The D100/D10A/D180 don't have an internal GPS, and any external GPS works, including a 430 or 430W. The SkyView systems can use the internal or any external GPS for assist (or both if you have more than one like we suggest). <SNIP>
Click to expand...
Something I've wondered about for the SkyView system is which GPS takes "priority" if there are wildly different positions being supplied?

By the same token, if I've got a Skyview system with two ARHS modules connected, and they disagree, how does the "system" know which one to follow, or which one is the "correct" attitude solution?
 
Today, SkyView does not do any cross-checking of GPS or AHRS data.

For GPS, there is a priority order. It uses POS1, then GPS1, GPS2, GPS3, GPS4, POS2, POS3, POS4.

GPS devices report their own lock status. If the GPS is telling us it has a lock, we believe it and use it. So if POS1 (99% of the time the Dynon GPS puck) is saying it has a lock, we use that and ignore all others.

For GPS, being 1000 miles off is obvious to the user, but it would also be easy for us to detect. Being 1 mile off is a lot more insidious, but also much, much harder for us to detect. When you have two GPS units, they will never agree perfectly, so you can't just see if they are the same or not. Giving false positive reports is really bad since a user will get used to them and ignore them, so it's a challenge to do something like that right.

When you fly a glidepath approach from a GPS, that is fully sourced by the certified GPS driving us, so you are trusting that GPS to not tell us that it's OK when it's not, which is part of the reason certified GPS units cost so much.

As for AHRS units, today, you choose one, and the system uses that until it totally disappears from the system, and then the system will switch to a backup unit if you have one. However, one of our software guys is currently working on AHRS cross-checking and we expect this will be in our next software release. This will be an automated, background check that will then warn the user if the two AHRS units disagree and allow the user to see both at once and vote for the one they trust. Given that there are almost always only two AHRS units for us to look at, we can't vote 2 against 1, so the pilot will need to be the tiebreaker.

Since there has been discussion around these parts about documentation, I'd like to point out that this is all in our SkyView documentation already. Dynon firmly believes that you don't release software without documentation, and this type "how does the system behave" data is critical to safe operation of an EFIS system.

--Ian
 
Totally confused

About how loss of pitot (dynamic pressure) input and/or loss of static pressure input can affect attitude indication.
My confusion stems from my assumption that the AHRS functions as and replaces the mechanical gyros. Once the mechanical gyros are spinning at speed there is nothing (never was) linked to the pitot/static system. Roll/pitch/yaw/rate of turn information has nothing to do with pitot/static.
Furthermore, it is my understanding that the AHRS is basically three electronic gyros placed orthogonally to each other that supply pitch/roll/rate of turn and yaw information, and linked to the magnetometer functions as a HSI.
I understand the loss of pitot/static will affect airspeed/altitude and VSI, and the mode of failure will determine how these indications will vary. In the six-pach there is no link between the gyros and pressure. Maybe the Air Data computer uses and interfases the pressure indication???
So please help me out!!
Cheers.
Johan
 
Almost all AHRS systems need some sort of aiding to cancel out the errors that are present in these gyro's.

Much is in the archives about it and the long story gets deep fast.
 
The GSU 73 ADAHRS with GMU 44 magnetometer that is used in the G3X system uses GPS, magnetometer, and air data inputs, as needed, to achieve excellent accuracy and integrity.

As shown in Table 10-1 in the G3X installation manual, and also described in some detail in the AHRS Operation section of the G3X pilot?s guide, the G3X system never uses air data to assist in assuring attitude accuracy unless the GPS data is lost.

Since each G3X display has a built-in high integrity GPS sensor, an external source of GPS data is not required. In a multi-display G3X system, only a single GPS antenna is required, although each display can be equipped with its own antenna if GPS redundancy is desired.

Since air data is only used in very rare cases when GPS data is not available, the system is generally not susceptible to attitude error should icing conditions affect pitot and static sources.

One additional note on using GPS data. The AHRS in Garmin systems do not use GPS groundspeed as a direct approximation of aircraft airspeed. There is other data available from our GPS sensors that make our proprietary methods very accurate and robust. These techniques have been proven in thousands of piston and turbine aircraft with millions of flight hours and we are pleased to be able to offer these advantages to experimental aircraft pilots.
 
Dorfie said:
... it is my understanding that the AHRS is basically three electronic gyros placed orthogonally to each other that supply pitch/roll/rate of turn and yaw information...
Click to expand...

I *hope*, but don't actually expect, that it would be *four* electronic gyros :) for redundancy.

But in any case...aircraft nomenclature may be different than what I'm used to, but three gyros normally form an Inertial Reference Unit (IRU), which gives you attitude (well, change in attitude, actually, but that gets used to update the current attitude). Add 3 accelerometers, and you have an Inertial Measurement Unit (IMU), with the accelerometers giving you acceleration around 3 orthogonal axes (not necessarily the same ones as the gyros), which are then used in the computation of both attitude and attitude rates.
 
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