GTX327

When ordering my avionics, I noted which units had integrated power
switches and which did not, so I could include power switches for those
that didnt on my panel. For example, the GRT EFIS does not have a power
switch, so I installed one on the panel. The GTX 327 transponder does have
a power switch, so I did not include a switch for that. However, after
cutting my panel, installing the avionics and proceeding to flip various
switches to see that everything lights up, I have discovered that the GTX
327 comes on when I turn on the master, regardless of whether I turned the
unit off the last time I used it. So, if I want the unit off before engine
start, I will have to reach up and turn it off, when in my mind at least,
it should already be off because thats how I left it the last time I used
it. A minor inconvenience, but irritating to me, and I dont really want to
add another panel switch.

Does anyone know whether there is something in the GTX327 instrument set up
that will allow me to change the default setting to power off instead of
power on?

thank you

Erich Weaver

No avionics buss?

You've got it wired into the Master somehow. Don't you have a dedicated Avionics Master? All radios and avionics should be on there.

Well, not quite....

RV7Guy said:

You've got it wired into the Master somehow. Don't you have a dedicated Avionics Master? All radios and avionics should be on there.

Click to expand...

Darwin's statement about Avionics Master switches is not universally accepted.
AeroElectric Bob thinks otherwise..... the switch is a single point failure.

www.aeroelectric.com/articles/avmaster.pdf

If Garmin did a good design (and they should have for the $$$ they charge) then it's no big deal if it's on when you start your engine...

gil in Tucson

avionics master

Im a loyal "Nuckoll head" when it comes to electrical systems and thus do not believe in avionics busses. So yes, it is "wired into the master". Nonetheless, when I push the switch on the GTX327 to off, I was assuming it would stay off until I turned it back on! I agree with you Gil, if the unit was built to its advertised standards, i have no reason to be concerned for its safety during start up.

Configuration

erich weaver said:

Im a loyal "Nuckoll head" when it comes to electrical systems and thus do not believe in avionics busses. So yes, it is "wired into the master". Nonetheless, when I push the switch on the GTX327 to off, I was assuming it would stay off until I turned it back on! I agree with you Gil, if the unit was built to its advertised standards, i have no reason to be concerned for its safety during start up.

Click to expand...

Erich... Interestingly enough, Garmin recommends that it not be powered during start-up....

I think you have the configuration incorrectly set-up. Page 6 of the manual tells you how to get into the Configuration Mode.

http://www.garmin.com/manuals/GTX327Transponder_PilotsGuide.pdf

Your unit is set up as if it had an Avionics Master - it comes on as soon as Power is applied. The manual implies another mode exists for non-Avionics Master installations - this mode would come up OFF when power is applied, and then pressing ON, STBY or ALT would power up the transponder.

Try the configuration mode.....

gil in Tucson

I'd rather have one avionics master then 10 little on/off switches for each device..

erich weaver said:

Does anyone know whether there is something in the GTX327 instrument set up
that will allow me to change the default setting to power off instead of
power on?

thank you

Erich Weaver

Click to expand...

Sound like you have Pin #1 and Pin #15 of the GTX-327 tied together and to your main buss. Pin #1 is the Avionics Master ON. When this is wired to power, the unit will power on each time power is applied to it. To disable this behavior, do NOT wire this pin to power, then you have to manually turn the unit on each time.

That being said, Nuckoll has a lot of good ideas, I don't particularly subscribe to this one.

One or ten items to fail

Radomir said:

I'd rather have one avionics master then 10 little on/off switches for each device..

Click to expand...

Yes, but if one little on/off switch fails, you only lose one item...
If the Avionics Master switch fails, then you lose all ten items...

I would contend that the failure rate of a switch is non-trivial....

gil in Tucson

Radio Master Switch Failure?

az_gila said:

Yes, but if one little on/off switch fails, you only lose one item... If the Avionics Master switch fails, then you lose all ten items...
I would contend that the failure rate of a switch is non-trivial....


General question. I keep hearing about the radio master switch potential failure scenario, (but not the aircraft master switch).
Will a switch that has been working properly and not overheating fail after it is turned on or do switch failures occur during the make or break sequence??
Has anyone ever heard of a radio master switch failing during flight?

Click to expand...

George in Langley BC said:

General question. I keep hearing about the radio master switch potential failure scenario, (but not the aircraft master switch).
Will a switch that has been working properly and not overheating fail after it is turned on or do switch failures occur during the make or break sequence??
Has anyone ever heard of a radio master switch failing during flight?

Click to expand...

I personally know people who have lost switches in flight, myself included. I don't recall specifically if I personally know someone who'd lost their avionics master in flight.

This whole "get rid of the avionics master" thing is somewhat misunderstood. Getting rid of the avionics master doesn't buy you much because you still have the Master and the battery contactor to worry about. What buys you A LOT is wiring a second, redundant power feed directly from the battery to the avionics bus and using a diode to prevent backfeeding the the rest of the aircraft electrics should something like the battery contactor fail, or perhaps you simply decided to turn off the master to conserve battery because of an alternator failure.

Now you have a redundant power path from the battery that:

a) allows you to kill power to the rest of the aircraft if you wish but still keep the avionics going. Big woop...questionable value here.

b) provides redundant power, reducing the chances of complete avionics bus failure to...heck, I don't know what it is but it's darn near close to zero.

So Nuckolls calls this second bus the "essential bus" and swears up and down that a switch is not nescessary. He's probaby right, but that's not the point. By using redundant paths the whole switch/no-switch argument becomes irrelavant. The likelyhood of TWO switches failing at the same time is really teeny weeny....even if you use the cheapest "Made in the Peoples Republic of WeMakeCheapUnreliableSwitchesIzstan" switches. Now sure, you're not going to get anymore reliable than getting rid of the switch entirely but if you have redundant feeds, a a switch on each one doesn't significantly reduce your SYSTEM reliablity anymore and it sure is convenient to have.

Single Point Failure..

jcoloccia said:

So Nuckolls calls this second bus the "essential bus" and swears up and down that a switch is not nescessary. He's probaby right, but that's not the point. By using redundant paths the whole switch/no-switch argument becomes irrelavant. The likelyhood of TWO switches failing at the same time is really teeny weeny....even if you use the cheapest "Made in the Peoples Republic of WeMakeCheapUnreliableSwitchesIzstan" switches. Now sure, you're not going to get anymore reliable than getting rid of the switch entirely but if you have redundant feeds, a switch on each one doesn't significantly reduce your SYSTEM reliablity anymore and it sure is convenient to have.

Click to expand...

And that is exactly the definition of a "Single Point Failure" that has been removed.... as mentioned in post #3.....

Most small aircraft do not use the "Essential Bus" concept, and feed all of their avionics through a single Master switch wired to the single power bus... the "single point failure".

I think AeroElectric Bob changed the terminology to "Endurance Bus".. but it seems to be the same thing.

gil in Tucson

Radio Master Switch Failure

jcoloccia said:

I personally know people who have lost switches in flight, myself included.

So Nuckolls calls this second bus the "essential bus" and swears up and down that a switch is not nescessary.

Click to expand...

(1) So help me understand this switch failure, did the switch that failed fail during activation or was the appliance in operation and just "lost power", (we are talking about a properly sized switch that is not running hot).

(2) second question, other than the extra switch etc, what is the difference between Bob's so called "essential bus" and just wiring the avionics bus straight to the battery and turning all avionics equipment off/on with their panel switches?

Old age?

George in Langley BC said:

(2) second question, other than the extra switch etc, what is the difference between Bob's so called "essential bus" and just wiring the avionics bus straight to the battery and turning all avionics equipment off/on with their panel switches?

Click to expand...

Brain farts..... leave one item of the Avionics stack ON, and no juice to start the plane when you come back the next week...

Only one switch to remember to switch off....

gil in Tucson

az_gila said:

Interestingly enough, Garmin recommends that it not be powered during start-up....

Click to expand...

Where does Garmin recommend that the GTX-327 not be powered during start up? I'm a bit baffled why they would make that recommendation, as the Installation Manual clearly states that the GTX-327 meets the DO-160 Category A qualification for voltage spikes.

An individual from the Matronics AeroElectric List forum has replied to me:

"Pin 1 is avionics power on; Pin 15 is 'power input (11-33V)'. I believe if you wish it to remain off you need to disconnect pin one and run power to pin 15."

This seems to confirm what William posted here previously. Now all of you get busy and go read the Nuckolls bible, 'Aeroelectric Connection' and follow the path to enlightenment

Thanks for the help guys.

Erich

George in Langley BC said:

(1) So help me understand this switch failure, did the switch that failed fail during activation or was the appliance in operation and just "lost power", (we are talking about a properly sized switch that is not running hot).

(2) second question, other than the extra switch etc, what is the difference between Bob's so called "essential bus" and just wiring the avionics bus straight to the battery and turning all avionics equipment off/on with their panel switches?

Click to expand...

1) I've had a couple fail in flight while they were on. Landing light once. T&B gyro, of all things, another time. Go figure...maybe I'm just unlucky.

2) Nothing. The only real debates are whether or not to put a switch on it and whether to also power the avionics from the master bus. If you do Master bus AND dedicated avionics bus (direct from the battery), and put a switch on both, you have a very reliable system.

In the Manual....

Kevin Horton said:

Where does Garmin recommend that the GTX-327 not be powered during start up? I'm a bit baffled why they would make that recommendation, as the Installation Manual clearly states that the GTX-327 meets the DO-160 Category A qualification for voltage spikes.

Click to expand...

Kevin,

In the Pilot's Manual that is on-line at the Garmin site... in the free area...

http://www.garmin.com/manuals/GTX327Transponder_PilotsGuide.pdf

gil in Tucson - and yes, I was surprised at that....

Don't worry Scott - ours will die togetehr...mine is on for engine start as well, and I don't let it bother me....

Now lightning strikes are another thing....THOSE I'll worry about....

Paul

Failure this and failure that!!

I am a huge proponent of safety and minimizing risk by taking reasonable steps to eliminate "points of failure."

There seems to be a general hysteria on many lists and forums about the concern over various component failures especially in the area of switches. However on the same forums you rarely, if ever, hear of these actually occuring. Redundancy this and that, also adds to the complexity and to the opportunity for something else to fail.

My aircraft is equipped with the EXP Buss switch panel. It has a dedicated "Radio Master" switch. I have all avionics including the Dynon 10A on this master. It is completely isolated from the start process and therefore protected from spikes etc... All of the power is going to the start process. When the engine is stabilized at 1000 RPM and I have proper oil pressure and fuel pressure, I hit the Av Master.

If I were to be concerned about switch failures I be just as concerned about the Master switch failing along with the Radio Master since they are acuated virtually the same number of times. Neither seem to be an issue.

If doing all again, I would not use the EXP Buss system in favor of conventional switches but I'd definitely use an Avionics Master.

Now watch, now that I've talked about switch failures, I'll have one tomorrow when I fly to Big Bear to have lunch with CA boys tomorrow.

Great discussion.

AeroElectric Bob

RV7Guy said:

I am a huge proponent of safety and minimizing risk by taking reasonable steps to eliminate "points of failure."

There seems to be a general hysteria on many lists and forums about the concern over various component failures especially in the area of switches. However on the same forums you rarely, if ever, hear of these actually occuring. Redundancy this and that, also adds to the complexity and to the opportunity for something else to fail.

.............................

If I were to be concerned about switch failures I be just as concerned about the Master switch failing along with the Radio Master since they are acuated virtually the same number of times. Neither seem to be an issue.

.....................

Great discussion.

Click to expand...

Darwin, I agree about your comment on complexity, and adding more stuff to allow for failures. My personal observation is the IFR pilot/builders are the most guilty of this, adding extra equipment to allow for other equipment to fail....

However, following AeroElectric Bob's bus architecture adds little complexity. Using his generic light aircraft example here...

http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z11K.pdf

The only extra "stuff" added is a diode and an alternate feed switch, and splitting a single power bus into 3 smaller busses. Very little extra complexity, with a definite payoff.

You will notice that the Alternate Bus feed comes directly off the battery, so if there is a Master Switch failure, or a Battery Contactor failure (more likely), you will have enough electronics able to be powered to make a safe landing at a nearby airport, as opposed to a complete electrical failure, and loss of all electronics.

As a member of the Grumman Gang mailing list, I see regular postings on switch and contactor failures, and some pilots even carry extra switches in their travel tool kit. They are cheap switches though....

The AeroElectric sample diagrams are worth their weight in gold, and if implemented really do not add to the system complexity, but give good redundancy to make a safe landing, even if not at the intended destination. Sounds like a good safety system to me.

I don't know what redundancy and/or safety measures are built into the EXP Buss.... do you know of any?

gil in Tucson - I think we have less electrical problems here in AZ due to a lack of corrosion effects.... This doesn't remove mechanical type problems, such as vibration though...

Redundancy

Hi Gil,

I guess I have it then with the EXP Buss. Not only is it wired with the battery, but a back up battery as well. The only problem would be if the entire panel took a shiite. Not likely since it is protected with the poly fuses.

I'd probably use Bob's system or something quite similar if doing it again.

Another product

RV7Guy said:

Hi Gil,

I guess I have it then with the EXP Buss. Not only is it wired with the battery, but a back up battery as well. The only problem would be if the entire panel took a shiite. Not likely since it is protected with the poly fuses.

I'd probably use Bob's system or something quite similar if doing it again.

Click to expand...

Darwin... sounds like the back-up battery is the built-in redundancy.... should be OK.

I'm retiring soon (17 more work days, but who's counting... ...) and sounds like there could be a market for a pre-built panel that implements the AeroElectric system.....hmmmm...

gil in Tucson - the RV-6A is #1 on the honey-do list...

The endurance bus (ala Nuckolls) not only provides a 2nd path to glory for your electrons but, it also creates the endurance mode by performing 2 critical functions when faced with "alternation interuptus" it allows rapid reconfiguration (flip 1 switch) to power only those systems you consider to be critical to safe continuance of flight and it takes the battery conntactor, and its 1 amp of draw, out of the load.

It takes all of 10-15 seconds to turn on the individual radios however, the single avionics master can take down your whole system and you know Murphy.....

Jekyll

switch failures

FWIW, in the last 12 years I have had one failed battery master and one failed avionics master. I now subscribe to the Nuckolls "endurance" bus philosophy and wired my RV7A accordingly. YMMV

Bob Brown said:

FWIW, in the last 12 years I have had one failed battery master and one failed avionics master. I now subscribe to the Nuckolls "endurance" bus philosophy and wired my RV7A accordingly. YMMV

Click to expand...

Bob... was that the "master switch" or the "master solenoid"??

gil in Tucson

switches

Gil, I had the battery master switch fail in my '79 182 last year, the avionics master switch failed in flight the year before that. Now that plane is sold and gone to the warm moist coastal air of Florida to live out the rest of its life...and I'm FREEEE of Spam.