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Sunday, July 02, 2017

Ch 22 - Electrical/Avionics - Part 3

Panel design

I chose to place the two GRT Mini-AP on top of each other in a pretty standard PFD/MFD configuration (Primary Flight Display and Multi-Function Display), they fit just about perfectly in the panel this way.

Checking for fit

More important and interesting are the many ways to connect all the pieces of this new system together. There is no right or wrong here, just tradeoffs and personal preferences, and what I chose for myself might not be the way someone else might have liked it.

Had the Minis only had more than three serial port available, I would have been able to design a totally redundant panel, as it is, I had to compromise a little as you shall see. 

While this is still much a work in progress, let’s look at a general block diagram of what I have in mind.

Block diagram (initial draft)

First of all, you should notice that the two EFIS are connected to each other via serial port #2. This allow them to share data so that the changes made on one Mini will automatically reflect on the other. Needless to say this reduces workload and prevents input errors, unfortunately this much needed feature eats up one of the precious few serial ports.

Here is a list of the information the two Minis can share with each other over the Display-Unit Link (the blue ones will not be implemented in my case):

GPS data
EIS data (Engine Information System)
SL30 VOR/ILS data and frequency commands
SL40 frequency commands
ADS-B Traffic (weather does not transfer)
Navigation modes
Lateral/vertical/approach modes
Heading and course selects
Altitude select
Altimeter setting
Fuel Flow Fuel Remaining (when set by user)
Flight Plans and User Waypoints
Remote Analog Inputs from a Horizon EFIS
ARINC-429 VOR/ILS and GPS data

Now, there might a way to augment the serial ports via USB, but I haven’t researched it enough at this time. The bottom line is most people use the Mini as a backup to a larger screen with many more serial ports, so lack of ports is not usually an issue as it is in my case, hence the compromises I will talk about below.

Lucky for me, there are dedicated OAT (Outside Air Temperature) and Trig TT22 (Transponder) ports, which I plan on connecting on both EFIS.

Outside Air Temperature probe
Trig TT22 ADSB OUT remotely mounted transponder

I obviously want to connect autopilot, Garmin GNS480, and magnetometers as well, but this is where I had to chose due to a lack of ports. I decided to connect serial port #1 on both units to the autopilot servos through a source select switch. This will allow me to continue using the autopilot in the event of failure of one of the two Mini by flipping the switch.

Now, with only serial port #3 left, I connected one of the two magnetometers that came with the Mini-AP to the PFD, and the NAV/Com radio to the MFD via the necessary ARINC module.

Needless to say, this setup reduces redundancy a little because I’ll be relying on the data transfer connection to feed heading and GPS data to both displays.

What are the failure modes of this configuration?

Case #1: Let’s say the PFD dies. 

I would need to switch the MFD to PFD mode, then transfer the autopilot to the remaining screen. 

PFD failure mode

No significant losses here, except for the Magnetometer input. 

GRT supplied magnetometer

The Magnetometer is involved in heading generation, and calculation of TAS (True Air Speed) and WV (Wind Vector), none of which are of vital importance. The Heading ribbon on the PFD would change to display GPS Track numbers, and I will only have lost the ability to follow precise heading commands from ATC (though technically the magnetic compass is still available for use).

Overall, this is a fairly benign failure mode.

Case #2: What happens if the MFD dies instead?

MFD failure mode

Well, in this situation the only loss would be the IFR certified GPS input to the PFD. 

Let’s think about this for a minute…

First of all, both Minis have their own backup VFR GPS antennas, so we are not completely in the weeds here. Next, the GNS480 would still be working (though not connected to the PFD), and because it has its own CDI (Course Deviation Indicator) for lateral and vertical navigation, approaches can still be flown. Furthermore the Mini still has synthetic vision and the ability to fly a coupled synthetic approach to the same runway that I would tune the GNS480 to. Obviously one would have to closely monitor the Garmin’s CDI indications, as it would still be the only legal IFR navigation source, but certainly the additional help from the Mini would not be wasted. 

I would not even classify this as a real emergency, perhaps a reduced redundancy scenario.

Neither of these situations are optimal of course, but very much doable without too much stress.

So, why would a fourth serial connection have been so useful? 

With an additional serial connection, each Mini could have had its own dedicated Magnetometer and GNS480 connection, negating the drawbacks of the two failure modes we discussed, for a completely redundant system.

Building, modifying, and flying airplanes is full of compromises, and one has to make safe choices he can live with. 

Next time I will get into more panel layout specifics, then start building it.

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