Especially as, IIRC from a few years ago, the build quality of the CGR-30P is not exactly stellar. I saw one close-up.
But it is this insistence of removing instruments which were previously classed as “primary”, which I don’t get, and nobody is talking.
It could be that they were previously “primary” under an STC (or a TC) and if you replace such, you are breaking that STC. The former head of Sandel once told me that this is why it is so hard to replace the KI256. You end up breaking the TC of the aircraft, or at the very least the STC of the autopilot, and that needs muscle.
You’d have to ask the specific STC holders about their particular requirements and how these limitations were imposed. I haven’t written an EIS STC, but my guess would be that it relates to mandatory markings. These are depicted in the Flight Manual/POH and so how would you determine which indicator is the master if both display limitation markings and a discrepancy appears between them? You can’t vote out the bad one if there are two conflicting displays without an annunciated failure of either.
On the previous comment about retained fuel quantity indicators. How can this be achieved and maintain total system separation unless a second set of fuel quantity transducers are fitted? If both a new EIS and existing analogue gauges share the same sensor data, you have immediately defeated the objective of an independent back-up.
wigglyamp wrote:
How can this be achieved and maintain total system separation unless a second set of fuel quantity transducers are fitted? If both a new EIS and existing analogue gauges share the same sensor data, you have immediately defeated the objective of an independent back-up.
A backup does not require that every single thing is backed up. A backup of the instrument only, is also a backup. You seldom backup the pitot tube, yet a backup of the ASI is deemed “good backup”.
Peter wrote:
But it is this insistence of removing instruments which were previously classed as “primary”, which I don’t get, and nobody is talking.
Take a Cub. It has sight glasses for fuel quantity. So if a new system is installed, then these sight glasses must be removed or painted? I am sure that not a single person would do that. Surely if no one can find a reference to the appropriate regs, then removing “de-primary” instruments is more of a common practice than an actual requirement ?
I can understand this if the old instrument is wired or connected in such a way that a failure in that instrument will also make the new instrument fail. But then again, a failure in the non primary ASI casing it to leak air, will also fail the primary ASI in the exact same way.
There could be other cases where fiddling with the sensor (connecting extra wires etc) is not according to spec for the old instrument. It is therefore no longer certified. Isn’t there a “nazi” regulation saying every instrument in a certified aircraft must be certified?
I don’t know. I take it as yet another reason why non certified is better than certified 
On the previous comment about retained fuel quantity indicators. How can this be achieved and maintain total system separation unless a second set of fuel quantity transducers are fitted? If both a new EIS and existing analogue gauges share the same sensor data, you have immediately defeated the objective of an independent back-up.
Sure, but that is a completely different issue.
Capacitive fuel level sensors probably cannot be shared. These need AC excitation. The rest would depend on detail but at least the double loading of the sensor could affect the reading.
Fuel totalisers (the usual Floscan 201B sensor) can usually be shared if you do it right (feed the +5V into the sensor with 2 diodes, so the failure of one instrument doesn’t bring down the other) but almost nobody will know this. One owner claimed he had this done but when I asked the shop directly they said they had never heard of it
my guess would be that it relates to mandatory markings. These are depicted in the Flight Manual/POH
So, you need to get an AFMS approved for yours which deals with this (in FAA-land an AFMS needs an ACO but even I got that done for the SN3500 installation).
and so how would you determine which indicator is the master if both display limitation markings and a discrepancy appears between them
I am not trying to be cocky but the pilot has one of these (well, most have)
Correctly used, it is a useful tool.
Looking at it differently, how can one have a plane with two AIs? One electric one vacuum? And even a third electric one – the TC. Gosh, in my plane I have even more; I have five attitude displays (2xSN3500 rev AI mode)! Not all independent of course.
I just find that rule incredibly patronising, hence wonder if there is some real reason for it.
Peter wrote:
I just find that rule incredibly patronising, hence wonder if there is some real reason for it.
What does “the rule” actually say? The legal text I mean.
Amazing that we are allowed (under EASA obliged) to manage 2 (two!) altimeters connected to the same tubing system.
Sounds like an alliance of lawyers and marketing folks: we do not like them to see the regular inaccuracies of our “modern” instruments. Bad for us and the whole industry, if “glass” is not reliable. Let’s make them remove the previous instruments – “they would not be able to handle the differences…”
If an STC requires unreasonable steps, one need to find a reasonable shop for installation and ARC :-)
Why not mark the fuel gauges as inop and fit a fuel flow meter?
Or if the aircraft is used for training just mark them as inop.
The answer will lie in knowing who demanded these restrictions and why.
So far nobody has posted anything concrete.
And I don’t believe the holder of say an engine monitor STC would demand the removal of other engine monitoring instruments, in order to make more money. How will he make more money? He obviously won’t. In fact he is likely to get way more reports of failures published – like engine failures in twins are way under-reported.
He might be doing it to avoid having to produce an amended AFMS for the other instruments, in some cases. But I don’t really get that either.
The guidance material I used for writing Garmin G500/600 and Aspen EFD1000 glass cockpit STCs is FAA AC23-1311-1C and it includes a section on engine indication. Section 9.2b does allow secondary back-up instruments, but 9.2a makes clear that no single failure shall cause a display of misleading data. I would agree that a mechanical tacho or direct-reading Manifold Pressure gauge should be an acceptable back-up if the System Safety Assessment needs it to meet the required safety levels in AC23-1309-1E, but not any parameter which needs to share a sensor such as a fuel quantity indicator whereby a float failure could cause loss of, or misleading data on both primary and back-up displays.
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23_1311-1C.pdf
does allow secondary back-up instruments
And so it should.
However, AIUI, the STC(s) in question here require the removal of instruments which were previously primary for that parameter(s).
And this is when not sharing the sensor.
That is the bit I don’t get.
Is there something in the STC of a “primary” instrument which states that it must be “primary”, or must be removed?
And it is not always an STC. Loads of avionics never had an STC for Type X; the factory (being a 145 company, etc, etc) got a box certified under its factory TC holder authority. I think the original TB stuff was done thus: GNS430, EDM700… all well before the days of AML. Back then the stuff “had” to be TSO, which is pretty easy (loads of stuff is TSOd but has no usable STCs for anything) but later even that is not needed.
