I am sorry, but I can’t understand why two GNS units would less safe than one.
NCYankee wrote:
I am sorry, but I can’t understand why two GNS units would less safe than one.
No one can, except pencil pushers at EASA.
It’s a totally mad regulation, with no technical basis. Insiders tell me EASA does it because they employ a load of ex Part 21 DOA people and they in turn are looking after their old mates who make nice money generating STCs which do exactly nothing except generate “legal” paperwork.
Welcome to the EU…
One other aspect of this project is whether both GPSs can drive the autopilot. There might be significant certification (€€€) issues there.
One possible area to investigate is whether 2×GNS is easier if only one can drive the autopilot. In FAA-land, a GPS with no AP connection is a Minor mod (in general terms) and you can have 10 of them installed….
But if say just the 530 can drive the AP (not the 430), then one can save a bit of money by not having a current database in the 430. Crossfill will work only if both have the same database date (doesn’t have to be current but has to be the same) anyway. We have multiple threads here on the database costs (which have incredibly confusing purchase options; some from Garmin and some from Jepp, and that’s before you get onto the “what is Europe” debate) but IIRC the cost of two is about 1.5x the cost of one.
It’s a totally mad regulation, with no technical basis. Insiders tell me EASA does it because they employ a load of ex Part 21 DOA people and they in turn are looking after their old mates who make nice money generating STCs which do exactly nothing except generate “legal” paperwork.
This is so far from reality! There’s definitely no ‘looking after their old mates who make nice money….! Those of us who write general aviation Avionic STCs would make far more money by just selling equipment and doing installations using existing FAA data if validation was automatic. As validation isn’t currently as simple as the bi-lateral might suggest, there is a business need for new EASA STCs, so some mug has to write them, and it’s certainly not very profitable!
@wigglyamp can you illuminate why EASA so badly hates 2x GPS installations?
I think I know a bit of the background e.g. they said if one fails it could bring down the other one via the crossfill interface. They asked Garmin for proof that this cannot happen. Garmin didn’t, ahem, take the question seriously. So it was left like this.
However airframe mfgs did manage to find ways around it e.g. Socata certified 2×430 and 430+530. But only for certain airframe S/N ranges. For all the others, the TC is worth zero even though the official wiring diagram is the same.
One pilot I know personally tried to get a 2×GNS approval without the crossfill connection and the EASA guy just laughed at him, saying he knows perfectly well the connection will go in as soon as the plane leaves the shop (which BTW was the famous Straubing). EASA seems to be treating this as a schoolboy challenge.
I’ve never heard about the supposed crossfill issue being a problem in STC certification and this is probably just another myth. We’ve done quite a few dual GNS STCs before we produced the Part 23 AML and this issue has never arisen.
So probably the question is: Why an STC in the first place?
Well it all comes down to an FAA document! FAA AC-23-1309-1( ) appendix 1 states that total loss of all Nav and Com in IMC is a hazardous failure condition and consequently the required probability of failure must exceed 1E-7. Any failure case of hazardous or above requires an STC.
Due to a potential common-mode failure, this can’t be achieved by a dual GNS installation so some alternative means of Nav or Com is required, Some EASA experts have interpreted that as meaning a 3rd Com had to be installed be many approvals have been issued in that basis.
Personally, I argued that the transponder provided a back-up means of Com (setting a discrete squawk for radio failure) and having either a DME or ADF addressed the needs for an alternate Nav. Provided that including the failure rate of the alternate equipment in my safety assessment meant I achieved better that the 1E-7, I could claim compliance. In all cases this argument has been accepted, by UK CAA, LBA, DGAC and directly by EASA.
So in reality, it’s not an EASA problem, but down to providing a correct interpretation of an FAA rule.
What was the evidence provided to assess a common mode failure for a dual GNS installation?
Cobalt asked “What was the evidence provided to assess a common mode failure for a dual GNS installation?”
The data I have from Garmin to support STC certification is protected under NDA so I’m afraid I’m not in a position to disclose any more than I’ve stated above.
In that case why doesn’t the FAA require the STC?
It looks like EASA just picked up a “useful” document (useful for work creation) and ran with it.
I argued that the transponder provided a back-up means of Com (setting a discrete squawk for radio failure) and having either a DME or ADF addressed the needs for an alternate Nav. Provided that including the failure rate of the alternate equipment in my safety assessment meant I achieved better that the 1E-7, I could claim compliance. In all cases this argument has been accepted, by UK CAA, LBA, DGAC and directly by EASA.
That this is needed to satisfy EASA, that’s totally perverted!
1×GNS = OK
2×GNS = not OK (“less reliable”)
DME and ADF = alternate nav for GA???
About the only practical way to use DME for nav is to fly a DME arc. And as for NDBs, there are nowhere near enough of them about.
The world has gone totally mad.
The crossfill objection is real in as much as it was given by somebody in EASA. It was given (via their avionics installer) by EASA to a number of people I know personally, as the reason for objecting to a dual installation. To the extent that any of this makes sense, that does make sense because it is the only interconnection (well, apart from the power bus) between the two and thus the only means of a dual installation potentially reducing reliability.
Using 3rd radio as a means to get approval would be an easy route for many. But somebody I know tried exactly that (with a KX155 or 165 already installed) a few years ago and was refused. He was the guy who was told that “they know” he will reinstate the crossfill connection when out of the shop.
How would one use a 3rd radio to get approval and is it expensive?
I can’t see any unique common mode failure of dual GNS units. They can interconnect via the crossfill, but that can be disabled at installation time or by the pilot during flight. The VOR/LOC/GS and Com units are as independent of each other as any other second Nav/Com. They are independently powered with separate circuit breakers and have their own on/off switch. They can connect to independent CDI/HSI. The GPS units don’t share an antenna. The Nav units share to the same extent in any other dual Nav installation does. The Com units don’t share antennas. A GPS antenna can fail and block all GPS reception on the aircraft, but turning the offending unit allows the non failing unit to continue to operate.
