For many years, glass cockpits like G1000 used to have a traditional CDI + ASI, typically electrically driven with a backup battery. A 2012 TBM 850:
This is very redundant, there is basically no failure mode that takes out everything. However, newer aircraft went all glass, even for the standby instrument. A 2017 TBM 930:
This is a L3 backup instrument with its own battery. An AI needs an erection mechanism, traditionally using gravity in the mechanical instruments. Glass instruments either use air data like Aspen for which a simple blocked pitot can take out the whole lot in no time:
Garmin instruments use GPS for erection instead of air data. They require constant GPS updates to integrate over time. Without GPS eventually they will lose the AI display. Now consider GPS jamming or — more likely — a duff GPS antenna (I had that with a factory new GMA34) which starts radiating like crazy, basically taking out all equipment. I believe you would lose your G1000/G3000 and also your glass backup display. Maybe the L3 instrument is different from the G3000 and not affected but I assume newer TBMs will use the Garmin G5 because an all Garmin cockpit will surely get them a better deal.
Isn’t this a rather unsafe setup that is miles behind the old mechanical backup setup in terms of safety? Having no instruments at all in adverse conditions (such things never happen at CAVOK!) would be a complete nightmare.
The YouTube link does not work :(
[hopefully fixed – searching youtube.com for the token KpM1aO2OhIc yields two videos on this topic – Peter]
Isn’t this a rather unsafe setup that is miles behind the old mechanical backup setup in terms of safety?
I would say so too. But there are solutions for it:
1. Use a separate source for pitot and static pressure for the standby instruments (standard in almost eveything with more than one engine that I ever flew).
2. Use a separate GPS antenna for the standby instruments if they rely on it.
3. Use standby instruments which don’t rely on GPS. For example the “Meggitt” instrument which is very common in my work environment has it’s own solid state attitude reference and also it’s own air data computer. How the attitude reference works is still classified, rumour has it that the same hardware is used in cruise missiles and other smart weapons. Getting the things serviced is always a major act because it can only be done in the States and the amount of paperwork is terrifying. But I guess that by now it’s interiors should be common knowledge to other instrument manufacturers as well.
Edit: Out of curiosity I googled and found this link ( https://www.meggitt-avionics.co.uk/wp-content/uploads/2017/10/Meggitt-Avionics-Data-Sheet-iSFD-Sept-2017.pdf ) to a recent Meggitt standby instrument which does not rely on classified technology. And no GPS either.
Garmin instruments use GPS for erection instead of air data. They require constant GPS updates to integrate over time. Without GPS eventually they will lose the AI display.
Is this really true, for all Garmins, or is it just correct for (very) old G1000? An AI can use whatever is available, the only thing that happens is the accuracy and sensitivity increases with added measurements. MGL use gravity by default, then just add sensors when they are available (installed and powered up). Typically this is air data, electronic compass and GPS. Gravity never fails, and an AI in default mode, will not function without accelerometers and gyroes in any case. An AI works perfectly well with just gravity alone in 99.9% of all circumstances, it will function like an old fashioned gyro.
MGL can also use air data and GPS (no gyros or accelerometers), but then a red X is displayed all over. The main reason for that is there is no way of knowing what is up or down (from the accelerometers) and the update is much slower. But it’s more than good enough for steady flight with not too hard maneuvers.
It’s hard to believe that Garmin units will just stop working when the GPS is gone. It makes no sense.
Do a search here for
background AND erection
for the, ahem, background to this. You can’t just use gravity alone. A traditional vacuum or electric AI does exactly that (the famous “pendulous vanes” loved by the ATPL ground school instructors, etc) but for some reason no “primary” pitch and roll source for certified aircraft gets certified with just gravity vector sensing.
Aspens pack up if airdata is lost (e.g. pitot iced up). It may be that an all-Garmin cockpit will lose all AI functionality in the case of GPS loss (jamming, or a defective GPS antenna which radiates on 1.575GHz and wipes out the others).
I wonder if this is why the G5 is not permitted to back up a glass cockpit?
This could get really interesting…
So to lose the AI on a G1000 you need to lose both GPS data (from two receivers) and either the magnetometer or air data. That really does seem rather unlikely.
Yep. The avionics side has significant redundancy, especially if you have the MD302 SAM as fitted in the DA62.
I consider the total loss of GPS quite likely to happen right when you don’t expect it and then all you need is an iced up pitot tube… of course you will be in IMC when this happens otherwise the pitot would not have iced up
Also I distinctly (wrongly?) remember this coming up with the G500 and that reportedly didn’t have the airdata backup.
My (experimental version) G5 worked fine with no GPS signal (it has an internal GPS antenna, I wanted to see if this worked before buying and fitting an external one – it didn’t, and an external one was needed and was fitted).
Section 2.4.2 of the G5 manual states “The G5 will also use GPS and airspeed data to provide the most accurate attitude information. If none of these additional sources of information are available, attitude calculations will still be valid but accuracy may be slightly affected.” which at least suggests to me it’ll give a “good enough” indication to keep flying should you lose both GPS and air data at the same time.