Sorry, I wasn’t clear enough.
I thought this thread was only about fuel gauges. It was only the fuel gauges which had to be blanked.
We chose to keep almost everything else, for the reasons Dave gives.
When we had a CGR-30P fitted in our TB20, we removed the Tacho/RPM gauge and the CHT/EGT Gauge.
The original fuel gauges, oil gauges and voltmeter in the strip across the top of the centre console, remained. This is an N reg aircraft.
Keeping the majority of the old system has proved very handy during the initial break in period as you can check that the gauges agree. The old fuel gauges which work well are very handy as a quick visual reference, saving you having to scroll through to the fuel pages of the new unit.
I am sometimes tented to go to EDM as primary, which would indeed mean removing the original instrumentations.
Then I remember how often the EDM gauges fail or provide wrong info.
I wonder how you get fuel level indications in both places… especially with capacitive gauges. I need to check the IMs for any wiring hints. I know how to do it but that isn’t the same question 
Or maybe nobody has done it with capacitive gauges? The resistive ones are just a potentiometer and if the supply voltage to the pot can be taken care of, the output of the pot can go to two indicators concurrently.
I have just phoned an avionics shop I know (Bournemouth Avionics) which has done some of these installations and they said that a Primary STC does require the old gauges to be removed. That is one data point, anyway.
Somebody has just been caught out by this. He installed a GI-275 EIS and the installer left in the analog MP gauge, which is apparently illegal and it now needs to be removed.
Really bizzare and utterly stupid, since the analog gauge is completely analog and independent.
Peter wrote:
Really bizzare and utterly stupid, since the analog gauge is completely analog and independent.
Can’t beat the paper pusher – the STC says get it out, get it out.
“ Really bizzare and utterly stupid, since the analog gauge is completely analog and independent”
So in your preferred scenario, what would you do if there was a difference in reading between the retained analogue indicator and the new digital one? Which would you believe?
The analog one, obviously 
Especially given the GI275 bug history.
The true answer is that one would look at all information sources together. For example if you depart with full tanks, and after 10 mins your primary-certified fuel gauge tells you that they are empty, and your previous-certified-primary gauge has been removed, then a) the removal was obviously dumb and b) you look out of the window and unless you see a huge stream of avgas, you will know the new certified-primary gauge is duff.
I just don’t get the rationale for removing what is now a backup instrument, just because it was once certified as primary. If it was never primary then it could remain. This is surely BS logic.
I’ve been designing electronic products professionally since ~1975 and it’s very obvious that “primary” means very little when it comes to reliance. For example the old EDM700 is certified-primary for a TB20, and you have to remove the old EGT+CHT gauge. The changes of an EDM700 blowing up is far far higher than the chances of the old gauge blowing up (on any MTBF calculation method whatsoever). For some reason this is not true for the TB21 where JPI didn’t obtain the “primary” certification so the old gauge has to be left in (but obviously nobody looks at it anymore).
@wigglyamp do you know the additional certification route involved in getting “primary”, and does it involve a demonstration (by MTBF calculation, or whatever) that it will be more (or as) reliable than the original primary?
Peter wrote:
For example if you depart with full tanks, and after 10 mins your primary-certified fuel gauge tells you that they are empty, and your previous-certified-primary gauge has been removed,
Risk Management rules would say – if you depart with full tanks, and after 10 mins your primary-certified fuel gauge tells you that they are empty, and your previous-certified-primary gauge shows another, you have to assume the primary is right anyways. So what do you gain by keeping the old one?
So in your preferred scenario, what would you do if there was a difference in reading between the retained analogue indicator and the new digital one? Which would you believe?
We have 4 certified altimeters in aircraft (5 if one adds uncertified true altitude from certified GPS, actually, 10 altitude values if one adds 2×pressure altitudes plus 3×true altitudes from portable tablet, phone, dynon)
I don’t think having EFIS value (G5 primary) has resulted in INOP stickers for all other altitude display?
Also, I don’t think there are that many issues in guessing which altitude is and which one is wrong? including when your are near 200ft DA with clogged statics
Why for engine MP, this suddenly becomes a problem?
