I don't think it's that crazy to use analog instruments as backup considering thats what Cessna, Cirrus and many corp jet manufacturers do. There is presumably some method to their madness, and I am assuming that it is returning people to a base level of complexity when things go wrong.
From a personal standpoint, my concern about using the Dynon for backup is the length of time it takes to reset when it has been destabilized. Assuming a Murphy's Law scanario where something goes wrong with my PFD at the worst possible time -- like hard IFR in turbulence, I want to be in my personal comfort zone -- analog gauges .
Up until recently I felt exactly the same as you for IFR flight (gimmee back-up steam gauges I said
)....but then I started to think about it a bit more and now I'm not so sure. I think it comes down to what we term as "analog instruments" or "steam gauges" these days. Everyone is becoming confused by semantics in a background of rapidly changing technology.
These are my current thoughts (but I might change my mind tomorrow
).
1. The back-up AHs you see in certificated aircraft are mechanical gyros either vacuum driven (not many RV builders want to go to the expense and complexity of installing a vacuum pump these days) or electrically driven (read mega-bucks). Also IFR RV builders tend to be short of panel space so they often opt for 2.25" AH back-up guages....difficult to get in vacuum and mega mega bucks for electric.
2. So what AHs do RV builders typically instal as IFR back-ups. Inevitably it's a TruTrak ADI which is itself fundamentally a solid state device. It doesn't read like a conventional AH, it needs electrons to keep it going, and they are not immune to false readings. I wouldn't call it a "steam gauge" by any stretch of the imagination. Will it do in a jam....probably, but it may not be any more accurate, or more reliable, than a Dynon (and may even be less accurate and reliable). I certainly wouldn't call it a "return to a base level of complexity".
3. Not many RV builders want to go to the expense of a decent 2.25" altimeter (too expensive) so they put in a cheap UMA single pointer. Totally useless for IFR (totally useless for anything
). The cheapest current answer is a Winter 2.25" altimeter but they are expensive at $800 upwards and I don't see too many installed.
4. The altimeter reading on a Dynon will probably be much more accurate over the full range than even the Winter altimeter. This is because solid state pressure sensors are more accurate than mechanical ones.
5. In my opinion it is VERY demanding to fly IFR in solid IMC without a VSI. If you don't believe me then you haven't tried it. You'll reference the VSI extensively in climb and descent and the altimeter only occasionally. Personally I wouldn't want to fly a serious NPA approach in IMC without a VSI.
Very few steam gauge back-ups include a VSI. But the Dynon gives you one.
This is not a plug for Dynon...not at all. It's just that it might be time to reassess what we mean these days when we talk about "steam gauge" or "analog gauge" back-ups" for IFR.
If we mean a TruTrak ADI, a UMA single pointer altimeter, and a Chinese ASI, then it might be that a small proven glass PFD (with it's own back-up battery) might in fact offer considerable advantages in terms of accuracy and provide additional important information as a bonus.
Builders might be concerned about a PFDs single point of failure....but in reality the TruTrak ADI is in itself a single point of failure if you're down to a basic "three pack" back-up. If you're in IMC and depending only on an AH, an ASI, and an altimeter.... and you lose the AH.......you're going down.
