This popped into my email the other day and with the sender’s consent I thought it would make an interesting thread…
He installed, in a TB20, an EDM930
which is certified to be installed as a replacement for primary instruments e.g. fuel gauges, tachometer, CHT gauge(s), etc.
As a result, his fuel gauges
are now dead and have been taped over with black insulating tape 
I realise this is probably legal but it raises some questions:
In the specific case of the TB20, one could extract the two fuel gauges (they are moving coil edge-style meters) and make a new blanking plate, perhaps using the blank space for some switches or connectors. But the JPI STC isn’t going to cover that additional mod, and it is not a trivial mod because that whole indicator cluster is driven from one PCB which sits behind it, so it would need an intelligent approach.
The same issue arises all the time with replacing a single factory CHT probe with one of these instruments (even the old EDM700) which need (per the airframe MM) to use that probe location. You end up, at best, with the factory gauge reading some less than accurate value, or you end up with another dead gauge or a hole in the panel… One could solve this easily because a single thermocouple can drive two indicating instruments (this is not true for any other sensor type, and for those it is possible but more complicated) but nobody seems to.
To me it seems really cynical to bring out a “primary” instrument like this whose installation creates such a mess elsewhere.
I took a Chieftain to Straubing for the EDM930 to be fitted and it was made quite clear to me in advance that the original gauges had to be disconnected and blanked. There was no choice. I don’t know why, but the advice was unequivocal.
I suppose my main drift is: how much effort is the installer supposed to put in to do a neat job, when the STC is not authorising any mods other than completely trivial like removing a whole instrument.
Take the case of having two of these “display-everything” boxes and the second one duplicates some of what the first one shows. Are you going to stick bits of black insulating tape over parts of the LCD of the first one? I do realise some of these products will cleanly delete a parameter from the display if the respective probe is not connected.
We have EDM930s in our Chieftains. The only system parameters that have been blanked are the fuel quantity gauges. There rest are still there; rather handy when the EDM tells you this halfway between Funchal and Ponta Delgada. (Oh, the engine was most definitely working)
So, what has replaced the blanked-out gauges? I think this scenario is the opposite of the one I was describing. Or is it the case that the EDM930 was not STCd primary for the Chieftain for the fuel gauge replacement so they had to be blanked out while you keep the old ones? However the two crossed-out ones are confusing too.
What you are seeing is an EDM failure. The system has lost information on MP, RPM, Oil P, Fuel P and Fuel Quantity for the Left engine. Fortunately all these instruments (apart from the fuel quantity gauges) remained in their original form. I don’t know why Timothy’s Chieftain had all the original instruments blanked.
In the specific case of the TB20, one could extract the two fuel gauges (they are moving coil edge-style meters) and make a new blanking plate, perhaps using the blank space for some switches or connectors.
I guess this would make the most sense. Recently I accompanied someone on a D-registered Bonanza (for his 3 landings within 90 days) which had been retrofitted with Garmin 1000 avionics. The installers had of course redone the entire panel for that and removed every engine related instrument and most others as well apart from a standby AI, ASI and altimeter.
re. The system has lost information on MP, RPM, Oil P, Fuel P and Fuel Quantity for the Left engine.
That’s a bit different because most of those will not use the original sensor when a “glass” instrument is installed. RPM will probably be a wire wrapped around one of the ignition leads. MP, oil pressure and fuel pressure will be new pressure sensors. So there is no technical reason to remove the existing instruments and a legal requirement for doing so would be a charade.
Maybe… having two oil pressure sensors would mean installing an extra T-piece somewhere (I have that on mine – have two independent electronic oil P indicators). I know for a fact a lot of shops are not keen on installing a t-piece.
Fuel tank (fuel quantity) gauges are a different thing however. Nobody is going to install two lots of sensors in the wing tanks.
I know somebody who had two fuel totalisers – one Shadin and one “glass-something”. They were fed from the same transducer, which works except a power failure on either instrument will remove power from the sensor. He wasn’t aware of this and clearly neither was the installer (one of UK’s biggest) but there is a simple (probably unapprovable, so has to be off the books) fix using two 1p diodes. You can get the same situation in other scenarios e.g. here (search for “two diodes”).
what_next wrote:
had been retrofitted with Garmin 1000 avionics
Is that possible? I thought that G1000 was only available as factory install and that the G950 was used for retrofit. But perhaps there is no difference between the G950 and G1000 except that one is for retrofit and the other for factory installations.
Hello!
Is that possible? I thought that G1000 was only available as factory install and that the G950 was used for retrofit.
It looked and felt like a G1000 to me, so I wrote “G1000”. But actually I am not familiar enough with these avionics to note (subtle) differences between different systems… I was happy enough that I could assist the pilot by doing the radio and setting up the map and course.
Personally I relally prefer separate engine instuments and PFD/MFDs. Probably that’s because of flying like that for many years and a wish for redundancy. I don’t like the idea that the failure of a single screen or electrics bus will leave me with no instruments at all.
