All glass cockpit redundancy

Any Central maintenance computer system and fault data handling method is only as good as the BITE software in each LRU…in normal English.
The faults you see will only be as good as the design has foreseen. History is full of unexpected unexpected cases … The more electronics to enhance the performance of a system the more sophisticated the back up electronics have to be to keep the same level of performance in case something doesn’t work . So the questions is what kind of performance do you expect from a small aircraft? Electronics do fail even on the big jets. Having single point of failure is never good.. Displays, relays, switches, sensors, antenna…even the cockpit door locking switch…Full redundancy has a meaning in avionics….The G1000 schematic as shown doesn’t show the full picture I think.

loco wrote:

As long as there’s no data connection and no source code shared between g3000 and standby, I’m happy to fly all glass.

Experience from software development

Peter wrote:

That’s why one must always have a completely standalone backup for flying the plane in a basic way.

That’s why I miss one classic VOR/ILS/GS instrument as backup in G1000 cockpit besides AI, altimeter and ASI.

achimha wrote:

Then the question is what did his second PFD show? G1000 has 2 separate AHRS.

The avionics units (GPS/NAV/COM) are duplicated, but the AHRS is not. (At least not in a SEP installation.)

That’s why I miss one classic VOR/ILS/GS instrument as backup in G1000 cockpit besides AI, altimeter and ASI.

from a separate power generating source and NAV/COM radio

from a separate power generating source and NAV/COM radio

Of course

The avionics units (GPS/NAV/COM) are duplicated, but the AHRS is not. (At least not in a SEP installation.)

A lot of newer SR22 actually have dual AHRS.

Airborne_Again wrote:

The avionics units (GPS/NAV/COM) are duplicated, but the AHRS is not. (At least not in a SEP installation.)

Depends on the installation – there are SEP with dual AHRS (later SR22s, later Cessna TTX come to mind)

But that is not the point.

I think the real question is – in what situation do you want a backup independent from the redundancy the G1000 already supplies. With a dual AHRS G1000, there is only ONE single point of failure for the G1000 system – a total electrical failure, e.g., electrical fire. The MD302 standby instrument in the Cirrus has (at least) 60 minutes of self-contained battery power, so it will keep going. G1000 dead, MD302 alive.

There are of course other failures that can take out your primary attitude indication, but they are all DUAL failure. Dual AHRS failure. Or GPS failure combined with loss of either the airspeed or magnetometer input. Or a dual G1000 display failure.

It is less clear what the MD302 attitude indicator needs to keep going. The pilot’s guide appears to imply that it needs airspeed, so if you get a combined GPS & pitot failure (the SR22 certainly has only one pitot system), so in that case it will likely not provide backup.

Compared to that, a simple mechanical attitude indicator, either backup battery driven or vacuum driven, provides better back-up, but this is only relevant in dual failure scenarios, which will be rare.

So objectively – if it is true that the MD302 needs pitot to keep going – the comparison goes as this
– MD302 is better if the likelihood of a simultaneous Pitot-GPS failure is higher than the likelihood of a simultaneous total electrical failure and mechanical AI failure
– Mechanical AI is better if the reverse is true.

I would argue that electrical fires / total electrical failures in dual-alternator electrical systems are rarer than mechanical AI failures, so objectively the MD302 would be safer, but in any case if it comes to risks to worry about when flying in IFR, both of them would be low on my list.

I just though of one more scenario.

Total electrical failure in icing —> G1000 dead, pitot heat off. —> Pitot head ices up —> MD302 goes tits-up.

A pitot head (pointy and thin) ices up more quickly than the wing (round and fat), so you might be in trouble with no attitude when the wing is barely showing any sings of icing, especially if still wet from the TKS before the pumps cut out.

Peter wrote:

You can’t just use gravity alone

You certainly can, but eventually there will be a need to correct the attitude, and that means to fly straight and level, which perhaps is the main issue to start with (in IMC). A 3D magnetometer will solve this. But I don’t see a need for GPS except to increase the accuracy. Looking at that diagram, a GPS is not strictly needed for the Garmin either.