This is old and is a common problem which many European owners have encountered.
There is no logic to it. Reportedly, EASA asked Garmin for proof that one GNS box cannot affect the other GNS box, and Garmin couldn’t be bothered to supply the proof. They probably fell over backwards laughing!
One proposed solution, put forward by a pilot I know, was to not have the crossfill connection. This removes the EASA objection, but EASA’s reply (coming no doubt from some failed Brit ISO9000 inspector who emigrated to feed on the Cologne gravy train) was “we known damn well you will connect it back up afterwards” so they would not approve it either.
A lot of airframe manufacturers have done the dual installation under their Type Certificate. Socata did this in 2000 (2xGNS430 and possibly also a GNS430+530) for the TB20 GT (airframe serials 2000+ only, unfortunately) and e.g. Cirrus was one of many others. How they achieved it I don’t know. I guess they used their EASA 21 authority and threw some money to the Cologne people, because there is nothing special about those installations, which are straight out of the Garmin IMs.
Of course, no problem under N-reg.
The GTN is the way to go in most cases if no excisting GNS approvals are available. It will be less expensive to upgrade to GTN then to have a custom made STC.
Installation of a GNS is still possible, but only sensible if excisting approvals can be used.
Presumably there are Part 21 shops which have generated EASA STCs for the dual installations. The two shops which have been most prolific in this game over the years are Lees (GAMA now) and Avionics Straubing. The sort of prices charged for the paperwork tend to be around €2000.
Some avionics installers are reluctant to buy in these paperwork packages however (i.e. not offering the option to the client) because it is such a bad (nonexistent, actually) value for money. That didn’t stop one Part 21 shop wanting to charge me €2500 for an FAA DER 8110 paperwork package for mounting the Avidyne TAS605 box (which itself has an STC anyway) as that package would enable them to obtain an EASA STC for the whole thing, notwithstanding the fact that the TAS605 is covered by an EASA approval anyway, grandfathered from some old UK CAA approvals.
No, EASA also has issued dual GNS approvals themselfs. I do hold a few of them, not for this aircraft though.
If the issue is the crossfill interface, who does EASA require an airframe specific approval, once they have issued an STC for one type of airframe?
I would have thought the issue (if one exists, which IMHO it does not) would be the same for all airframe types.
I have always heard that the complexity was the problem, not the crossfill.
Where exactly is the complexity?
Never found out, I don’t see the problem either, neither do most avionics shops. Having a Part 21 DOA copying the installation manual schematics doesn’t make it less complex.
Antenna’s can be a pain in the ass as well. I also had issues onces with a G-reg, it required a backup avionics master switch, which was difficult to get approved, as “they” didn’t see the use for it, even though it is a UK CAA requirement.
Peter,
This is the reply I received from the CAA to clarify why a dual GNS430 is a major mod.
The classification of a dual GNS 430/530 is (typically) classified as a Major Change because of the potential to lose both Nav and Comms capability through a common mode failure. Loss of all Nav and Comms in IMC is considered to be a Hazardous failure condition – see extract from AC 23.1309
Any change that introduces a Hazardous failure condition is generally classified as a Major Change – AMC to 21.A.91 refers
The consideration that a dual GNS 430/530 system may be vulnerable to common mode problems that can affect navigation and communications functions simultaneously (such as through software or interference issues) is what has driven the potential for a potentially hazardous failure effect and thus Major Change classification. This is also consistent with what EASA has published on its GA FAQ pages and the table intended as guidance for change classifications:
http://easa.europa.eu/certification/faq/docs/FAQ%20table%20of%20design%20change%20classification.pdf
However, in discussions with EASA there has been an agreement to allow a dual installation to be covered under a Minor Change where there the aircraft has further navigation and or comms capability (thereby reducing the effect of any dual GNS430/530 failure). This then typically results in a Limitation to the Minor Change approval along the lines of:
“Approval of this change is limited to a single GNS430/530() installation only, except where the final configuration of the modified aircraft includes additional and dissimilar (not GNS430/530()) radio navigation and/or communications systems.”
The Limitation is written this way because what (allegedly) happened is that owners would simply embody the mod to install a single GNS twice.
An installation of one GNS430/530() and one GTN series would not have the same common mode issue.
In my opinion that is not a valid argument, as it is only used on this GNS installations.
Two KX-155’s could have the same hardware issues. The same is true for the new GTN series, which are allowed (through STC) In a lot of countries the use of a backup avionics master switch is not required. This creates a potential single point failure for all avionics, and is allowed.
It just doesn’t make sense, IMHO.
