ACK 406 - Caution regarding Wiring

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.

SMO said:

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

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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....

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.

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.

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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.

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.

az_gila said:

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.

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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.

SMO said:

....
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.

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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.

SMO said:

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)

Click to expand...

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.

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

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

Bevan

704CH said:

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 ?

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It will transmit on 121.5 and 406. The only question is whether the 406 transmission will include the GPS position or not. If the 406 transmission does not include the GPS position, the satellites can still determine a pretty accurate position (within about 3 miles, supposedly). Given this accuracy without GPS data in the signal, it is understandable why they are careful to reject possibly too old GPS data, as an old fix could easily be more than 3 miles from the current aircraft position.

Kevin Horton said:

It will transmit on 121.5 and 406. The only question is whether the 406 transmission will include the GPS position or not. If the 406 transmission does not include the GPS position, the satellites can still determine a pretty accurate position (within about 3 miles, supposedly). Given this accuracy without GPS data in the signal, it is understandable why they are careful to reject possibly too old GPS data, as an old fix could easily be more than 3 miles from the current aircraft position.

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I would personally be more concerned with response time than absolute accuracy. They will find you with the 121.5 signal, but I would want the troops to come rolling as soon as possible.

From ACK....


FOR NEW GENERATION 406 MHz ELT’S WITHOUT GPS POSITION INFORMATION THE AVERAGE TIME TO PROCESS AND IDENTIFY THE AIRCRAFT LOCATION IS 1-2 HOURS WITH A SEARCH RADIUS OF ABOUT 2 MILES.

WITH GPS INFORMATION THE TIME TO LOCATE THE AIRCRAFT POSITION IS 10 MINUTES OR LESS WITH AN ACCURACY OF ABOUT 300 FEET . THE MODEL E-04 ELT MAY BE INSTALLED WITHOUT INTERFACING IT WITH THE AIRCRAFT GPS SYSTEM HOWEVER WE STRONGLY RECOMMEND THAT WHEN AVAILABLE THE GPS DATA BE SUPPLIED TO THE ELT.

just thinking out loud, maybe a simple timer circuit that would de-energize it-shelf from the battery bus after say 15 minutes or so.

bob burns
RV-4 N82RB

....and a reality check.....

all this accuracy and timing is important, but let's remember that the stat's clearly say that the ELT will FAIL TO ACTIVATE, OR SEND A SIGNAL......80% OF THE TIME!
....due to the antenna being snapped off, underwater, or a zillion other reasons that the system is less than intact and operational.
not my opinion, just the reality that seems to be well documented.
Mark, love to see your project....are you on the field at Salmon Arm? PM me?

flyboy1963 said:

all this accuracy and timing is important, but let's remember that the stat's clearly say that the ELT will FAIL TO ACTIVATE, OR SEND A SIGNAL......80% OF THE TIME!
....due to the antenna being snapped off, underwater, or a zillion other reasons that the system is less than intact and operational.
not my opinion, just the reality that seems to be well documented.
Mark, love to see your project....are you on the field at Salmon Arm? PM me?

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Isn't that 80% number for the older 121.5 ELTs certified under the original TSO.

The later 406 units have a TSO requirement that is more stringent...

TSO may not matter

Gil, I certainly respect your opinions....but in most cases, I don't think a TSO affects whether a tree snaps your antennae off.
I'd bet these are the bulk of the 'failures' of all ELT's regardless of age, type, frequency etc.
..........it's the mechanics of crashing!

flyboy1963 said:

Gil, I certainly respect your opinions....but in most cases, I don't think a TSO affects whether a tree snaps your antennae off.
I'd bet these are the bulk of the 'failures' of all ELT's regardless of age, type, frequency etc.
..........it's the mechanics of crashing!

Click to expand...

Maybe... I was actually equating the new TSO with improved required specifications. In particular the G-switch specifications. IIRC a lot of the 80% errors were G-switch related.

I also know that the 121.5 ELTs will transmit with no antenna, as a mechanic at our airpark found out the hard way.

I think the 406 antennae are pretty omnidirectional and will work updide down, but snapping on off is bad. It would be interesting to hear if any signal comes out with no cable attached.

It also points out that you should not have a nice layed out, bundled and strapped down cable from the ELT to your antenna. Leave a lot of slack in it to allow for a distorted rear fuselage.

PS ...I stopped at Salmon Arm on a flight from Red Deer to Hope. The trip through the Rockies at 8500 ft was one of the prettiest flights I have flown.

Couldn't even see Kelowna due to smoke though...

NASA did an exhaustive study of failures of the original spec ELTs (i.e. ones from the 1980s). They did find that those early ELTs failed to activate in about 80% of the crashes. The failures to activate were from a wide variety of causes, but there were a significant percentage there were expected to be addressed by the later ELT spec (g switch failure, poor installation due to inadequate installation instructions, battery failure, etc).

Based on the changes to the ELT TSO, NASA estimated that the newer ELTs would fail to activate in about 27% of crashes. The remaining failure rate would be due to things like aircraft under water, ELT destroyed in fire, impact damage to ELT, etc.

If anyone can find stats on activation rates for newer ELTs, I'd love to see them.

Kevin Horton said:

NASA did an exhaustive study of failures of the original spec ELTs (i.e. ones from the 1980s). They did find that those early ELTs failed to activate in about 80% of the crashes. The failures to activate were from a wide variety of causes, but there were a significant percentage there were expected to be addressed by the later ELT spec (g switch failure, poor installation due to inadequate installation instructions, battery failure, etc).

Based on the changes to the ELT TSO, NASA estimated that the newer ELTs would fail to activate in about 27% of crashes. The remaining failure rate would be due to things like aircraft under water, ELT destroyed in fire, impact damage to ELT, etc.

If anyone can find stats on activation rates for newer ELTs, I'd love to see them.

Click to expand...

It sounds like the NASA report you linked to created the "A" version of TSO C-91

I believe that the performance requirements got even more stringent with the new TSO-126a that covers the 406 ELTs.

This TSO calls out DO-204A - Minimum Operational Performance Standards for 406 MHz Emergency Locator Transmitters (ELT), but we are now away from free specifications on-line so I can't see actual details....

hey, if you can't trust NASA.....???

thanks Kevin for chiming in with the latest facts....I knew I could count on one of you rocket scientist types to enlighten us!

I wonder tho if the NASA tests would show a little variation ( & thus higher failure) if they encompasses only GA light aircraft, including ( perhaps especially) experimentals, with all the mix of mounting and other variables. Evidently some things can be improved by mandated specs, and others...not so much!

"It is interesting to note that 88% of the failures are crash related,
i.e., "G"switch, fire damage, impact damage and antenna broken or disconnected,"

Hey, we all just want the thing to go off, and get us found! I suppose it's been well covered here that you need more eggs in more baskets to ensure that! PLB's, Spidertrax, SPOT etc. etc.