ACK 406 - Caution regarding Wiring

[SMO]

I am installing a new ACK 406 ELT and was concerned about where to connect the wiring to ships power - the main bus, e-bus or the battery bus. So I contacted ACK as to what the purpose of this connection is anyway. Turns out that the ELT requires ships power to keep the circuits alive that process the GPS data. If the ELT loses ships power, then it will no longer process the GPS data even though the GPS is actively sending the signal.

This is a bit of a conundrum, as it is easy to envision an emergency situation where the master gets shut off well before the crash (think electrical fire). If I connect the ELT power to the main bus, then the GPS advantage is potentially lost as the ELT will not send GPS position if the master was shut off more than a minute before the crash. (edited)

I could connect the ELT to the battery bus, however this connection pulls 40 mA, and to my simple electrical mind this will kill my little 17 Ah battery in less than a couple weeks. Can somebody confirm this?

One option is to use a DP switch for my Magneto, and have one side also switch the power on from the battery bus to the ELT. Concept is to leave the mag live all the way through the crash.

Alternatively I coiuld put in a separate switch for the ELT power, and put it on the startup/runup/shutdown checklists.

Any other ideas?

I have heard all the arguments regarding whether ELTs are worthwhile or not - don't want to rehash that stuff again. Given the ELT is required I intend to make the most of whatever limited value it has.

[az_gila]

Originally Posted by SMO

.....
This is a bit of a conundrum, as it is easy to envision an emergency situation where the master gets shut off well before the crash (think electrical fire). If I connect the ELT power to the main bus, then the GPS advantage is potentially lost as the ELT will send the last GPS position it had which could be a fair distance from when the master was shut off.
......

No, as I understand it the ELT will only transmit GPS data received before a crash event if the data is quite "fresh".

If too much time elapses (I'll check the data sheets, but something like 1 minute) then the standard 406 ELT signal is transmitted with no GPS data attached.

This system logic makes sense....

[SMO]

Gil, I believe you are correct (one minute was quoted from ACK elsewhere on VAF) and this means the ELT will not send out a significantly "false" GPS fix on the crash location.

However the reason I am connecting the GPS to the ELT is so that it will send out the GPS fix of the crash location, and if the ELT is connected to the main bus and the master is shut off more than a minute before the crash occurs I lose this feature.

I have edited my post above to make it correct as per your observation. Thanks for the correction.

[Buggsy2]

From ACK via email:

The ELT will retain the last GPS coordinates for one minute when disconnected from the GPS data source before it is activated and for 4 hours after it is activated.

You pose a good question, one I'll be facing too. I like the idea of having a separate OFF/ON switch for the ELT/GPS power and just adding another item to the checklist.

[az_gila]

Are you thinking about the "wrong end" of the problem?

Power to the GPS is only part of the issue - power to your GPS source is the other end of the equation.

If you have a panel mount GPS and the Master Sw is turned off due to an electrical fire, then it doesn't matter if the ELT is powered since there would be no GPS data input anyway.

If your GPS source is EFIS based with it's own dedicated back-up battery, then the ELT could be run off aircraft power. A diode arrangement to a small 9 v. dry cell battery for the few minutes needed for keeping the ELT GPS signal valid could solve the problem if the aircraft power is switched off.

[SMO]

Originally Posted by az_gila

Are you thinking about the "wrong end" of the problem?

Power to the GPS (ELT?) is only part of the issue - power to your GPS source is the other end of the equation.

Of course there are many variations, in my case power to the GPS is already taken care of. In my -4 I have a Lowrance 2000 that has its own internal battery, and my Rocket project has a 696 which also has an internal battery. Losing the GPS feed to the ELT is a rather obvious issue with clear consequence. I am wanting to alert folks to a less obvious issue, that ships power to the ELT is required for it to accept the feed.

I was also hoping to have someone confirm/correct my calculation that a 40 mA draw will flatten the battery in a couple weeks or less.

[az_gila]

Originally Posted by SMO

....
I was also hoping to have someone confirm/correct my calculation that a 40 mA draw will flatten the battery in a couple weeks or less.

That works out to about 1 AmpHour per day.

Two weeks will drain around 1/2 the capacity of a perfect condition large aircraft battery. Just over two weeks for a PC680 battery that is 17 AH rated.


For the ELT power, run it off aircraft power with a 9 volt battery through a diode. The battery will need to be switched off when the Master is normally off.

[Kevin Horton]

Originally Posted by SMO

This is a bit of a conundrum, as it is easy to envision an emergency situation where the master gets shut off well before the crash (think electrical fire). If I connect the ELT power to the main bus, then the GPS advantage is potentially lost as the ELT will not send GPS position if the master was shut off more than a minute before the crash. (edited)

If you have turned off the master because of some major emergency, perhaps your next step should be to select the ELT to ON using the cockpit switch.

[704CH]

Also, to clarify my understanding..

After activation, no matter what the GPS functionality knows or "remembers", this ELT is still yelling at 121.5 correct? Or do the new 406 units not transmit on that frequency at all ?

I can probably look this one up, but if it can be answered here... hurray !

Thx

[Bevan]

I believe they all (most) still transmit on 121.5 for local detection and homing. Just no satelites listening on 121.5.

Bevan