Looks like there are four wires connecting the ELT to the outside world.

• Four 22AWG wires to transmitter unit
provide full functionality

Battery cells.
 

Something could indicate that some aircraft batteries are experiencing more frequent thermal events than consumer batteries. Why!, Size difference?

As others have already mentioned, protection of a battery cell against over current and over-voltage does not help if there is an internal fault in the cell.

As I see it, small cells have a significant lower probability of experiencing a thermal event, since they are more likely discharged before a fault in the foil (hotspot) reaches the autoignition temperature. And since they are small they can better conduct the heat from the hot-spot to the case of the cell.

A larger cell have:
1) More foil area, higher risk of faults in the foil → Higher probability of fire.
2) More foil area, more electrical energy to disparate in a fault. → Higher probability of fire.
3) Larger thermal resistance from fault location, to case. → Higher probability of fire.

In the transmission business, a good ground plane is essential in getting a solid signal off the antenna, while using the least amount of power. In a traditional metal fuselage, that ground plane would obviously be the fuselage itself. In this bird however, a large piece of conductor for the antenna to work against may be more difficult to come by, especially at the ELT's required frequencies of operation (121.5 and 406 MHz).

If the ELT was activated some how, there is a possibility that a highly inefficient transmission scenario was occurring. This usually results in higher transmitter temperatures and/or higher battery drain.

I don't have a Honeywell ELT battery, but here is a picture of the lithium pack for an Artex C-406

[IMG]http://i1365.photobucket.com/albums/...psc3dfebb9.jpg[/IMG]

The 4 green cells are each about the size of a D battery. The small circuit board has 2 fuses and a small ic for monitoring the condition of the battery.

You can see 4 small black parts, 1 across each cell. They are diodes so if a cell goes flat it won't see reverse polarity.

But if one of those diodes short, you will have a dead short across that cell.

I am trying to work up the nerve to short a diode out and see what happens, but I'm a little chicken at this point. :} And the boss said if I burn down the hanger I'll be in trouble. :eek:

ABOUT HUMIDITY ON 787
 

From a few friends involved in 787, and a few not well publicized comments/documents about the 787. One selling feature was to be able to have higher humidity in the 787 than on a regular aluminum bodied airliner due to less or virtually zero corrosion issues resulting from passenger breath and temperature deltas in flight, along with a lower cabin altitude.

But during early flight tests, some wags considered renaming it the rainliner, due gto unexpected amounts of condensation in the cabin. Which admittedly was not in a well finished cabin configuration, but filled with instrumentation racks, exposed insturmentation wiring, etc.

Business & Technology | Dreamliner's woes pile up | Seattle Times Newspaper

. . .
The latest delay will at least give engineers more time to test design fixes, including some for less consequential troubles, not uncommon on new jets, such as the maddening drip, drip, drip of "rain in the plane." On 787 flight tests, drip trays padded with squares of absorbent cloth are positioned to collect the condensation.
Fancher said "a good design fix" to dehumidify the interior is being installed and will be tested when the Dreamliners resume flying.
Employees working on the 787 complain about insufficient oversight of suppliers and a management system that the senior engineer called "totally broken."
"This program is not like anything we've seen," said the veteran 787 employee. "It's a screwed-up mess." . . .




Now add to that the wonderous decision to use predominately aluminum wiring to save weight instead of copper or copper/silver alloy or similar.

http://www.safran-group.com/IMG/pdf/EN_mag2_29-31-2.pdf

Now for flight data and computer data lines with low voltage and low current, aluminum ** might ** be an acceptable choice.

But years ago, the push for house wiring of aluminum met eventually with corrosion or similar problems due to buildup of non conductive films on aluminum, resulting in hot spots and IF I RECALL a few fires.

Check your local replacement power plug or switch in the us and the warning re cu aluminum and the availability of a cu-al grease/coating to preven oxidation.

Surely BA would have considered that ???????????

With 6000 similar ELT units in service it would be hard to believe that this was the first one to sufffer a battery problem.

It would be interesting to know how many batteries have been replaced, per qouted 10 year life any replacement would be a failure not end of life.

The really ineresting question would be the condition of the replaced cells,
what if failure is "rare but happens" but on metal aircraft "thermal event" energy is insufficient due to heat sinking etc to light a fire or even be noticed?

md80fanatic
Large ground planes are a requirement with transmitters operating on much lower frequencies. That's why cellphones don't use the 27MHz band ;)

The 121.5MHz transmitter in the ELT works at very low power, a mere 150mW. I wouldn't worry too much about power drain, even with a not-so-efficient antenna configuration, unless I'd want to power the thing from a 9V transistor battery.
The 406MHz transmitter has a rated output of 5W, but at such a high frequency you really do not need a large ground plane - a piece of alu foil a foot square would be quite OK. Also, the 406MHz signal is made up of short bursts, with a low duty cycle: overall power drain is fairly modest.

LASJayhawk
A silicon diode like the one in the picture shorting out spontaneously is a very unlikely event. More so when, as in this type of circuit, they are not under any significant stress.
Well, if you want to do a little experiment, here's how I'd do it:
Solder a few feet of heavy gauge wire (12AWG) to both cell terminals, connect the other ends to a heavy duty switch (make sure the switch is open!).
Close the switch and get away real quick.
The total circuit resistance will be in the order of 0.03 ohms or so if the connections are nice and solid and you use a good quality, 30A rated switch.

Of course you'd only ever do this out in the open, away from any flammable materials.

DEJA VU?
 

Is this another Swissair MD11 accident, except it happened on ground, without crew & pax being involved and too far from professional firefighters with proper kit?

I reword the question I posed in the original 787 grounding discussion:

Was there one or more grumpy old f**t with a long flying/line-engineering pedigree on the Boeing design team, and if not, then why not?

Perhaps such an animal would ask questions and quote incident/accident reports which would make the rest of the "Children of Magenta" team squirm uneasily in their seats and possibly think long and hard before venturing down the Li-On and other innovative but risky solutions?!

Other postings here imply that all is not harmonious sunshine and happiness in the Boeing family, so maybe those at the top of the food chain there had better review their choice of whizz-kids who drool at the thought of new and unproven technology and get back to K I S S basics whilst still producing good kit, or Monsieur HiTecq in Toulouse may win the war in the end? :eek:

Presume?
 

They presume it was the ELT, it hasn't been confirmed, why let this dangerous aircraft fly, again they should all be grounded until the cause of the fire is established. The C of A should be revoked!
I wouldn't fly in one of these death traps! :=

Alumium wiring
 

Aluminium wiring has been used in cabins for years to save weight. Back when I was an avo I dealt with it regularly in AB cabins. Just wack up the voltage and it works fine. Only area you MUST use copper is around the engines and APU due to temperatures (melting point of copper being higher). I havn't worked in an MRO for years so I'm sure somebody else who has can elaborate.....

While we stretch for ever more remote possibilities, just wondering if they had flown through an electrical storm on their last flight and if so whether lightning dispersal across the 787 skin could have set off a slow reaction within the ELT batteries?

Presume!
 

GD Slacker, get a life!!

Transportation of goods or people, by whatever form, involves risk. The trick is to be able to assess probabilities dispassionately.

You clearly cannot!l You probably take bigger risks when you cross the street!

:D:D

olandese:

Seems like a diode shorting or a major defect in the cell are the only reasons I can think of for an ELT to decide to self immolate.

More curious to me is why we need a 50+ hour transmit time on a 406 ELT. The Satellites should get a very good position fix in under an hour, even if the ELT isn't reporting last known position. It's not like we're using a loop antenna on 121.5 to find the position anymore. Maybe go back to alkaline batteries and accept an under 12 hour transmit time??

So, should we pull these same units out of the 777's now?

Isn't the presumption that it didn't activate as the first alert would have been from the monitoring system rather than a vigilant ATC seeing smoke from the aircraft?

The AAIB report would also have mentioned if an alert had been generated by the transmitter.

I don't think that's what will end up happening. The three paragraphs in the section Emergency Locator Transmitter (ELT), on page 3, read to me as:

The device is not a new design and has a proven history. The AAIB are looking into the possibility that a short, external to this device, caused the problem with this device, on this plane.

By the way... and just putting it out there... but before everyone got focused on the main battery/apu battery catching fire... I think there were reports of these batteries being flat/failed and having to be replaced...

More than 50 million flight hours without a fire
 

According to the report in today's NY Times:

Either Boeing has extremely bad luck to have this happen on a 787, or there is something else going on here.

re. integration with aircraft wiring.
 

The Honeywell datasheet claims that this unit is connected by 4 x 22AWG wires. This picture shows at least 33 connections as well as the antenna.


http://i1280.photobucket.com/albums/...ps40310599.jpg

Does anyone know what the other 29 or more connections are, and if any of them are likely to be carrying anything other than data/control signals which will most likely be no more than a few milliamps?

Yea, I suspect as much else all these things sitting around on shelves as spares would be popping off right and left.

So what kind of summer sunlight heat load can they take at the top of a composite aircraft ?

Don't think this link has been posted here before, they seem to be eeping a pretty good track of events.

Airchive Blog re 787 Fire incident