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Would you have two GPSs?

With the Stec GPSS, the feature is considered a minor modification. Adding a switch to it would also be a minor modification. Using this an example, I don’t see how adding a 1/2 relay to the Arinc 429 signal to the KFC225 would be anymore than a minor modification and not require an AFMS. Aspen has such a feature on its built in GPSS.

KUZA, United States

Not a myth at all.

The starter motor produces massive transients on the power bus when the starter relay is energised.

Whether this blows avionics up depends on what they are.

I know someone who blew up most of his centre stack, in a 1980s TB10. The insurance company paid for it, he said.

On later TBs, Socata put an extra relay on the avionics relay board which de-energises the three avionics bus relays when there is power applied to the starter motor. Those TBs have 4 relays instead of 3. So yeah you should be able to start with the avionics master switch left ON, but it doesn’t do you any good because the power to everything on the 3 avionics buses gets interrupted while the ignition key is in the Start position, so the GPSs all restart and have to re-acquire, etc! It’s the same in just about every car that’s ever been made – they interrupt power to the radios etc during starting.

I have designed automotive equipment (e.g. a little gadget I designed ended up on all 3000 (?) buses in Berlin) and there is an ISO spec on the transients which such equipment is supposed to withstand, but avionics would not have been designed to such a spec until recently. Even my relatively modern panel is mostly 1990s stuff. Also meeting that spec is no assurance it won’t get blown up, because so much depends on how the wiring is done.

A few items are expressly designed to go onto the raw power bus. For example Sandel’s SG102 AHRS gyro is, because it needs to be kept still for ~ 2 mins after power-up. Mine is actually wired to an avionics bus (because I didn’t want to tempt fate with a $5k box) and the 2 min wait is a non-issue because there is other stuff to do anyway.

The starting systems on turboprops (what JasonC flies) are very different to the very primitive car (12V) or commercial-vehicle (24V) systems used in piston GA.

Everybody is throwing in the GTN750 today. In the hangar today I saw a 750 going into a turboprop, with a 650 under it. I have lost the IFR GPS just twice in 12 years, plus the screen got damaged once when somebody thought it was a touch screen

Last Edited by Peter at 26 Mar 20:59
Administrator
Shoreham EGKA, United Kingdom

I was considering one day replacing my two GNS430 (non wass) to one GTN750. I really dont take advantage of the dual thing other than tapping a waypoint in one and cross filling it to the other but I really could do without that. I have also only ever once put in a full route and tend to just enter the route one waypoint at a time. Actually I tend to put the route into SkyDemon on my ipad and ignore the waypoints on the GNS :)

EGNH

Primary reason why the dual GNS430 and GNS430/530 combos were and are quite popular is that they crossfeed.

That is true, but only for either two non WAAS version, or two WAAS versions. A non WAAS version can not crossfill with a WAAS version, which is something to take in account if you have non WAAS GNS’s and want to upgrade to WAAS. Either upgrade one, and loose crossfill, or upgrade both.

I wonder if this turn off avionics thing is an urban myth.

No start starting, and releasing the starter knob can cause spikes on the bus. Nowadays their are more fast surge protection devices then in the old days.

JP-Avionics
EHMZ

Primary reason why the dual GNS430 and GNS430/530 combos were and are quite popular is that they crossfeed. What I have seen is that particularly in the case of the 530/430 combo the 530 is used in a moving map configuration while the 430 shows the navigation screen. The same is usually done with two 430’s. Any change done on either box will reflect on the other one if they are copuled that way. What can be done as well is to keep different flight plans so as to switch boxes in case of a diversion. In that case of course, the boxes will not crossfeed.

Another advantage of this configuration is that it will give 2 8.33 radios (which according to some countries interpretation is a possible source of irritation in the future, if all of a sudden 2 × 8.33 would be required) and of course redundancy also in NAV/ILS capability.

Most EFIS allow the selection of a Nav Source for GNSS. So the Aspen as well as the Sandel, I reckon the Garmin PFD’s should also be capable. I’ve also seen NAV1/2 selectors for the analog channel switching for VOR/LOC/GS Modes in certain installations.

LSZH(work) LSZF (GA base), Switzerland

and because you have to turn off the avionics when starting, so the GPS power up sequence is thrown away.

Interesting. Not on G1000. Why on yours? I start on battery which has PFD, GPS1 and COM1 running. I wonder if this turn off avionics thing is an urban myth.

Last Edited by JasonC at 25 Mar 22:42
EGTK Oxford

I did wonder if the GNS radio can be somehow used before the screen interface appears. Obviously the practical answer is NO even though it possibly “works”. In that sense a normal radio is a lot better (for stuff like requesting start) because it is usable almost instantly and because you have to turn off the avionics when starting, so the GPS power up sequence is thrown away.

My Q on two GPSs was in the context of already having a second NAV/LOC/GS receiver. If I didn’t have that, then two GNS430 boxes make perfect sense.

Yes the SN3500 can select GPS1 or GPS2 but that works with the autopilot only if you use its FCS (analog) outputs to drive the autopilot (which is what I would do, having configured its roll steering option). If the autopilot is fed directly from the GPS via ARINC429 roll steering then you can’t do that unless you install a switch, which then needs a new AFMS (which is impractical to do legally).

Administrator
Shoreham EGKA, United Kingdom
I am 99% sure only one of them could drive the autopilot – at least there was ever only one “NAV/GPS” switch. Having both capable of driving the AP would have meant a new KFC225 AFMS…

Nowadays, with WAAS/EGNOS, you would need two “WAAS grade” GPS antennae which are quite pricey. But that’s only if the 2nd GPS is to be used “officially” for LPV, I would think.

The main limitation is the KFC225 has a single HSI. With your Sandel, the Nav source can selected for the autopilot.

The WAAS antenna is part of the purchase price for the GPS and comes with the install kit. Only approved GPS antennas can be used with the STC although some of the early systems have antennas that limit the usability for sole source of IFR navigation in the US (91.205) and the AFMS ends up having similar limitations as a non WAAS GPS. A non WAAS antenna will function, but would not be an acceptable installation on a US certified aircraft.

KUZA, United States

Dual GNS430’s provide redundancy of GPS, MAP, VOR/ILS, and COM. In my experience, many use the second system as a dedicated map for things such as traffic. There are some times when one unit can’t both display a waypoint of interest while conducting a procedure. Ultimately it is the backup that fully redundant systems provide.

KUZA, United States

Oceanic navigation requires the use of two GPS units to be an acceptable navigation source.

KUZA, United States
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