Hi Ed,
Check the Yahoo forums for the MGL products...I think it's the "stratomaster user group". Lots of discussion about this over there. I think it's good for a backup, but I would still recommend using the MEMS gyros. I'm sure Rainier will chime in here and correct me if I'm wrong!
Right you are ! Let my do my chime. This is lengthy since it needs to be.
OK, what we did is experiment with the more advanced capabilities of the Swiss U-Blox GPS unit that is fitted to our systems. It is part of the system and we can get fairly low level data out of the unit, unlike what is possible if you have an external GPS via NMEA.
There is quite a bit of discussion on using GPS as a foundation for some form of attitude display and usualy opinions are to the negative. But we seldom take things at face value and we tend to do our own thing.
As background to this, consider the often desired "dual" AHRS in order to provide redundancy. I don't like this too much since in our experience, outright failures of AHRS are rare - what does happen is that a gyro becomes iffy and either just degrades the system or causes things like a slow roll or something like that. A dual AHRS system is useless in this case as the pilot cannot easily determine which AHRS to believe.
I was looking for something independent.
Anyway, to cut a long story short, we came up with a fairly simple system that uses the horizon display coupled to an interpretation of the aircrafts flight path in three dimensions, as measured by the GPS receiver.
The result is quite convincing and I have tested this myself in flight on many occasions now.
The first thing you notice if you use this is a little piece of text on the horizon display: "GPS Flightpath, no attitude".
This little sentence says it all.
What it cannot do: It cannot show aerobatic manouvers, it cannot tell if you are upside down. It does not show the aircrafts attitude in all cases, such as close to a stall or if you are upside down.
What it can do: Give you a convincing and accurate picture of your aircrafts estimated attitude in all other flight regimes. It does have the advantage of "zero drift".
I have flown this with one EFIS connected to an AHRS and one displaying the Flight path. The picture is all but identical, with only a slight lag noticable with the GPS derived picture.
During a pre-stall the picture shows nose down attitude while the nose is actually pitched up - as this is where you are going - down. This only takes a short practise and it starts making sense, but this is not what the system is about.
We did this mainly for the shortly to be released XTreme as this is a very low cost unit and a AHRS costs about the same. Yes, you can still plug in a AHRS if you like - but you can also use the GPS giving you a viable horizon display at zero cost. It makes a great backup or reference for your real AHRS display.
As this works so well, I have added this to the Enigma, Voyager and Odyssey. Here you can choose if you want to use it instead of an AHRS, as backup to an AHRS or if you don't want to use it at all. Your choice.
The big question that I get asked: Would I use this in IMC ?
Answer: In an emergency, yes absolutely. It does rely on GPS so if for some reason you don't have a fix you don't have a horizon either. Should it go wild inside a storm, it will not help you for long (unless you can keep your ship on an even keel) - but to be honest, the typical AHRS will not help you for long there either.
Like all other instruments on the EFIS, it is a tool. It is a useful one. It has good points and, like anything else it has limitations. Understand what it does and you have something you can use safely.
It is not intended to replace the AHRS but to give an alternative, a reference and a backup. For low cost systems that fly VFR and want a horizon for fun, it's great. It's free.
The system is based entirely on the GPS and does not even need an SP-2 compass.
Rainier
CEO MGL Avionics
