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Glass cockpit failure modes

Ed_Wischmeyer

Well Known Member
Ever since my RV-9A went all glass, I've struggled to learn all the system failure modes, a task much more involved than in the days of steam gauges and manual calculations.

One level of analysis is what happens if a single component fails. In my airplane, if the ADHRS fails, the G5 still provides basic air data to the screens and the autopilot, but I lose AOA, and lose OAT, which means losing true air speed and winds, and I lose magnetic heading, so everything reverts to GPS track, that last being not that big a deal.

Similarly, if I lose the CANbus, I lose pitch servo and elevator trim speed adjustment, the flight director and autopilot lose access to the IFR navigator and are restricted to the VFR only GPS located in the display screens, and I can display lateral GPS and VOR/ILS guidance only on the IFR navigator with no vertical guidance available. I think.

These failures are the kind that an examiner could, IMHO, reasonably ask on an Instrument Rating checkride, or that a conscientious CFI could ask on a Flight Review.

But there's more. Suppose on the ADHRS, the barometric pressure sensor fails. Sure, the air data will all be compromised, but will the system self-monitor only shut off those outputs, or will it fail the entire system, including the attitude indicator? Will the gyros get rate aiding over the CANbus and keep working? Documentation doesn't say, and I have no idea. And there are other partial failure scenarios to wonder about.

On the flight home from Oshkosh, the IFR GTN650 gave me a Cooling Fan Failure warning. At the next gas stop, I called Garmin and they told me that the cooling fan only cooled the display, but that the unit would keep working. I set the backlight to minimum, not knowing any better and hoping it would help.

Of course, if the display fails, you don't know what frequencies you're set to and can't program the flight plan. I called ATC before takeoff -- I would be going through the St. Louis Class B, other things being equal -- and described a contingency plan that they agreed to.

On the next leg, I got a Display Overheat warning. I didn't realize at the time that I could still tune the comm radio and the VOR/ILS from the G3X displays.

But I didn't realize until much later that if the display failed, I couldn't switch the navigator's output from GPS to VOR/ILS. (I want to add an external switch to do this, but until this flight, I'd not considered its use in a failure mode).

Bottom line is that there's lots to consider!

My IFR fallback is that my CFII once had me fly a non-precision approach using only the G5 standby flight display and a handhelp Garmin aera 660, simulating a total electrical failure. Sort of. I cheated and used the electric trim.

Lots to think about and wonder about, and this is without considering multiple failures (aside from total electrical failure). And with most systems being custom installations...
 
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Ed,

You are doing what all builders should do;
- Identify each risk
- Evaluate the outcome if that risk takes place
- If the outcome can be managed, move on
- If the outcome cannot be managed, add the needed risk mitigation to your system design

For modern IFR installs this generally means two EFIS displays, two ADAHRS modules, two GPS navigation sources (not necessarily TSO GPS Navigators), two radios, and redundant power feeds. The power feeds must include backup modes that have been identified, tested and added to the POH. Here the old school “Essential Buss” and “Avionic Buss” approaches fall short of the mark.

Carl
 
All good points Ed. Complicated by the fact that everyone's installation is slightly different.

Having just gone through the training and checkride myself there is a lot to consider. However, thought out responses were fairly easy in my book (most of the time).

First, one thing we are lucky with is the the modern EFIS does such a good job of monitoring itself and providing a warning, especially if you have dual ADAHRS built in. (not saying you don't have to still monitor to identify problems though)

In any sense, if I had a failure of just about anything on my PFD I'm no longer trusting/using it and reverting to my G5 backup to get me on the ground. I guess deciding what a failure mode on my EFIS would do wasn't a worthy thing to delve in because after all there is a full PFD as my backup, mind as well use it in the situation.

Total power failure, well hopefully one has some backup batteries installed on the EFIS as well as the backup if you are doing IFR work.

In my case if I had a total power failure everything still functions except my navigator. In that case an IFR pilot better always know where the nearest VFR conditions exist. An important part of flight planning in my book.

All good points though Ed and something worth thinking about!

P.S. Carl I liked your post above and agree with almost all of it. If a guy truly needs a totally redundant power source to power everything they are either failing in flight planning or take waaaay more risk in widespread IFR conditions than I would. One of the hardest parts about a thought process that Ed brings up is the fact that everyone's level of acceptable risk makes differences in installation. I have always found this a fun topic to think about.
 
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LRU Failure Scenarios

Ed,

Good thoughts here, a few points that may be helpful included below.

One level of analysis is what happens if a single component fails. In my airplane, if the ADHRS fails, the G5 still provides basic air data to the screens and the autopilot, but I lose AOA, and lose OAT, which means losing true air speed and winds, and I lose magnetic heading

This is true of a GMU 22 magnetometer, as it is connected directly to the ADAHRS. The GMU 11 is advantageous in this situation, as it is connected via CAN, and if the GSU 25 were to fail, magnetic field data would still be available to any remaining ADAHRS LRU or G5 on the CAN bus.

But there's more. Suppose on the ADHRS, the barometric pressure sensor fails. Sure, the air data will all be compromised, but will the system self-monitor only shut off those outputs, or will it fail the entire system, including the attitude indicator? Will the gyros get rate aiding over the CANbus and keep working? Documentation doesn't say, and I have no idea. And there are other partial failure scenarios to wonder about.

You can find the answer to this question on page 29 of the G3X Touch Pilots Guide.

Lots to think about and wonder about, and this is without considering multiple failures (aside from total electrical failure). And with most systems being custom installations...

This is a good point and is why we consider the G5 to be such an important part of any IFR installation. Outside of a total component failure of the G5, for attitude to fail, a combination of inputs to the attitude algorithm must fail at the same time. The G5's hardware and software are different from the GSU 25, and the same string of input failures that is required to invalidate attitude on one, is less likely to invalidate attitude on the other.

Here are a few ideas on how you can design your system to protect against certain types of failures.
  • With a backup battery connected directly to the unit, it is less reliant on the aircraft electrical system to continue running in the case of an external electrical system failure.
  • GPS is one of the inputs to the G5 attitude algorithm. For this reason, it is best to make use of the GPS receiver internal to the G5 for that data. This makes the unit less reliant on an external piece of equipment being operational to receive this important data, and puts it on the battery backup power bus of the G5. Again, you can lose GPS data and the unit will continue providing attitude data just fine. The problems arise when you have multiple failures, and 2 or more inputs to the attitude algorithm fail. By protecting GPS in this way, you expand your margin of safety a bit.

Thanks,

Justin
 
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But I didn't realize until much later that if the display failed, I couldn't switch the navigator's output from GPS to VOR/ILS. (I want to add an external switch to do this, but until this flight, I'd not considered its use in a failure mode).

Bottom line is that there's lots to consider!

My IFR fallback is that my CFII once had me fly a non-precision approach using only the G5 standby flight display and a handhelp Garmin aera 660, simulating a total electrical failure. Sort of. I cheated and used the electric trim.
.

Perhaps think about following our Canadian friends, and use two separate boxes for vor/ILS and gps systems.
I’ve tried (under the hood) shooting a VOR approach using nothing but my D6 battery powered efis and an old Sportys handheld VOR receiver. The Sportys is so old, there is no cdi - just a digital readout of the to/from bearing. I found that if I squinted my eyes and visualized it as an NDB approach, it worked acceptably well. Like you, I think I cheated and used electric trim. If it had failed, I’d have to fly the approach at the trimmed airspeed, which would be okay if there was descent visibility underneath. I’ve also used an iPad mini and wingX to fly a gps approach to LNAV minimums. Again, it was acceptable for a non precision approach.
For those that still have a whisky compass, it can be useful. If your main efis shows wings level but your backup efis does not, a quick glance at the compass (Is it moving?) will tell you which efis is lying.
 
Perhaps think about following our Canadian friends, and use two separate boxes for vor/ILS and gps systems.

Already covered with three GPS units in the panel: both display screens have WAAS GPS built in (but not approach capable), plus the external IFR GPS/VOR/ILS unit.

If I were building a new panel, there are a number of things I'd do differently, but rewiring an existing (and full) panel is quite the chore. Did that a few years back when I went all-Garmin, and ease of retrofit is a major driver for any possible new systems.
 
Preach

The power feeds must include backup modes that have been identified, tested and added to the POH. Here the old school “Essential Buss” and “Avionic Buss” approaches fall short of the mark.

THIS IS VERY GOOD ADVICE.

I'll skip the gory details for once, but some years back my whole panel lost power at night on an IFR flight. Close to a worst-case scenario with respect to avionics failures, with the very important exception that the weather was VMC.

Anyway, the combination of a G5 with battery backup, and an iPad with GPS and maps etc., actually made the whole thing pretty trivial from a safety standpoint. From a regulatory standpoint, a *working* handheld radio would have made things a lot simpler, although I confess that the FAA ultimately proved quite understanding. :) They also turned the runway lights on for me at my destination, which was hugely appreciated...

A G5 (or the equivalent) with a battery backup is a HUGE HUGE safety advantage. It just solves so many problems. On top of that, an iPad (or an iPhone, in a pinch) with Foreflight, or the equivalent, will give you everything you need to shoot a non-precision approach reasonably safely.
 
Already covered with three GPS units in the panel: both display screens have WAAS GPS built in (but not approach capable), plus the external IFR GPS/VOR/ILS unit.
.

Maybe I wasn’t clear enough: your set up would not be approved for ifr in Canada. I do understand that at this point it would be a huge undertaking to have separate boxes for different approach capabilities.
 
Already covered with three GPS units in the panel: both display screens have WAAS GPS built in (but not approach capable), plus the external IFR GPS/VOR/ILS unit.

As Bob mentioned, this wouldn't be enough in Canada. Losing your external IFR navigator wouldn't leave you with a legal way to complete an approach. Here we would need either another approach capable GPS or another VHF navigation radio. It makes the integrated units like the 650 less attractive than 2 separate units that perform the same functions.

It's an extra hurdle, but I understand the reasoning. Canada's standard is higher for this and I think it would be at the very least prudent to try to meet that standard if you plan to operate IFR in Canada. I think it is actually a requirement, if you read who I quote below for the pertinent CARs.

605.18 No person shall conduct a take-off in a power-driven aircraft for the purpose of IFR flight unless it is equipped with

...much omitted...

Sufficient radio navigation equipment to permit the pilot, in the event of the failure at any stage of the flight of any item of that equipment, including any associated flight instrument display,
(i) to proceed to the destination aerodrome or proceed to another aerodrome that is suitable for landing, and
(ii) where the aircraft is operated in IMC, to complete an instrument approach and, if necessary, conduct a missed approach procedure.

Great topic though, certainly worth sober consideration for every aircraft operator.
 
Just me

So i have a separate Panel mounted comm/nav (GNC 255) so that if the G3x goes dead, i have a comm and vor/ils and the G5.
 
Canada IFR requirements

As Bob mentioned, this wouldn't be enough in Canada. Losing your external IFR navigator wouldn't leave you with a legal way to complete an approach. Here we would need either another approach capable GPS or another VHF navigation radio. It makes the integrated units like the 650 less attractive than 2 separate units that perform the same functions.

It's an extra hurdle, but I understand the reasoning. Canada's standard is higher for this and I think it would be at the very least prudent to try to meet that standard if you plan to operate IFR in Canada. I think it is actually a requirement, if you read who I quote below for the pertinent CARs.



Great topic though, certainly worth sober consideration for every aircraft operator.

Not a requirement, but experimental airplanes are special and not covered by the bilateral agreements so it is close, part 91 is sufficient.

//Snip
The aircraft shall be operated under Day VFR only, unless the operating limitations issued for the aircraft authorize night or instrument flight (IFR) operations, in which case the aircraft shall be appropriately equipped in accordance with section 605.18 of the Canadian Aviation Regulations or 14 CFR part 91.205;
//Snip.

Full Read:
https://tc.canada.ca/en/aviation/ai...ered-amateur-built-aircraft-canadian-airspace
 
Not a requirement, but experimental airplanes are special and not covered by the bilateral agreements so it is close, part 91 is sufficient.

//Snip
The aircraft shall be operated under Day VFR only, unless the operating limitations issued for the aircraft authorize night or instrument flight (IFR) operations, in which case the aircraft shall be appropriately equipped in accordance with section 605.18 of the Canadian Aviation Regulations or 14 CFR part 91.205;
//Snip.

Full Read:
https://tc.canada.ca/en/aviation/ai...ered-amateur-built-aircraft-canadian-airspace

Thanks for this, I hadn't seen it. No question, it's not a requirement, but it remains a standard in redundancy that has value.
 
For IFR I too lean toward both a TSO certified GPS navigator and VOR/LOC/ILS receiver and COMM, and a stand alone COMM #2.

On the last two planes I did this with the “I wish there was a better option” GTN-650 and the SkyView integrated remote radio (very good radio by the way).

On the current build I will go a different way:
- The new Garmin GNC-355 TSO GPS navigator and COMM
- The new TRIG TX-56A Nav/Comm (it included VOR/LOC/ILS and plays perfectly with the SkyView.

This will yield two independent GPS navigators (one certified for approach, the other being the SkyView GPS receiver) an a stand alone NAV/COMM.

I’ve been impressed with the quality of the TRIG COMM and XPDR used by the SkyView system. I can only assume the TX-56A will be just as good.

BTW - when adding up the costs for exactly the same capability, this GNC-355 plus Trig TX-56A option saves a few thousand dollars over the GTN-650 plus another radio for COMM #2.

Carl
 
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For IFR I too lean toward both a TSO certified GPS navigator and VOR/LOC/ILS receiver and COMM, and a stand alone COMM #2.

On the last two planes I did this with the “I wish there was a better option” GTN-650 and the SkyView integrated remote radio (very good radio by the way).

On the current build I will go a different way:
- The new Garmin GNC-355 TSO GPS navigator and COMM
- The new TRIG TX-56A Nav/Comm (it included VOR/LOC/ILS and plays perfectly with the SkyView.

This will yield two independent GPS navigators (one certified for approach, the other being the SkyView GPS receiver) an a stand alone NAV/COMM.

I’ve been impressed with the quality of the TRIG COMM and XPDR used by the SkyView system. I can only assume the TX-56A will be just as good.

BTW - when adding up the costs for exactly the same capability, this GNC-355 plus Trig TX-56A option saves a few thousand dollars over the GTN-650 plus another radio for COMM #2.

Carl

Carl thanks for pointing out the math there. It is in fact noticeably cheaper! Something to think about when I build the 10.
 
On the current build I will go a different way:
- The new Garmin GNC-355 TSO GPS navigator and COMM
- The new TRIG TX-56A Nav/Comm (it included VOR/LOC/ILS and plays perfectly with the SkyView.

BTW - when adding up the costs for exactly the same capability, this GNC-355 plus Trig TX-56A option saves a few thousand dollars over the GTN-650 plus another radio for COMM #2.

Carl

Hmm. I've found that I wanted more screen real estate than the GTN650, and I've to think long and hard about the lesser screen real estate of the 355 et al.
 
Hmm. I've found that I wanted more screen real estate than the GTN650, and I've to think long and hard about the lesser screen real estate of the 355 et al.

Yep - the GTN-650 screen is just one step above useless. But, the function of the GTN-650 (or GNC 355 or GTN 750) is not screen dependent - that is what an integrate EFIS system is for (like the SkyView). The 650 is fine to fat finger in the latest ATC route change and such. All the screen integration is done on the SkyView display.

Carl
 
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