actually the ELT is designed for an impact on land. It's activated by a g-switch (no values given) and/or a mercury switch when position is changed into vertical (>70deg) . The ELT can be removed from his fixed position and is than able to float in upright position. Battery pack holds 48-60 hrs only. when no one removed an ELT from the fixed position, I think it sunk, and is destroyed by water pressure. Couldn't find an info for underwater ops, and it makes also no sense. The ELT has to contact a satellite to narrow down the position by 1.8 km accurance.
The battery is kept in the other half of the housing and supplies power to the transmitter. The battery operates when immersed in water (salt or fresh) or any other water based fluids (tea, coffee, soup or urine). Holes in the end cover of the battery housing let the water in. The displaced air is dispersed through vent holes in the top of the battery housing. External test points (3) on the battery housing let you check the battery condition and test the ELT .
anyhow, to contact the satellite , the antenna must be out of the water.
Yes but if it has not been crushed by the sea depth pressure, it will still be transmitting on 121.5. To correct myself the CVR or DFDR should have ULB's installed on them that to ping sound that sonar can pick up, while the ELT transmits on RF that is not much use under water.
As far as I can remember, these "portable" ELTs float (or are semi-submerged). I seem to remember a swimming pool exercise with a life-raft and having to tie the ELT to the raft with a long-ish rope, then throwing it into the pool. Been a while, so stand to be corrected.
An ULB is directly attached to the front panel of the DFDR. The beacon actuates on immersion in water down to a depth of 6000 meters.It has a detection range of 1800 to 3600 meters.You can service the ULB without disassembly of the DFDR. Maintenance has to be performed at determined time intervals to replace the battery of the ULB .
Same ULB is on the CVR.
But no position sent by ELT, so the ULB has a really small range to get find easily.
To answer the original poster's question, yes, ELT's should function when submerged, and many function because they're submerged.
While 121.5 was the VHF frequency established internationally, it's now 406 MHz, with many broadcasting on both. Many ELT units establish a voice position relaying the lat long, and identification of the indivudal radio station.
Portable ELT's, often attached to life rafts and emergency equipment, are often referred to as EPIRBS; Emergency Position Indicating Radio Beacons, rather than ELT's (Emergency Locator Transmitters).
Some ELT's today also come with voice transmission capability as well as simply a beacon.
ELT operates 21.5/243/406 mhzs The last freq contacts a satellite monitoring system. It uplinks the serial No of the Box which the Sat authority use to look up the Regulatory authority which keeps a list of serial no's versus tail no.s
Some ELT's are tied into the Nav sys to uplink last position. Many rely on the Sat system to direction find the signal
ELB operates when in contact with water to output Ping to allow the CVR & FDR to be recovered
An ELT due to the frequencies involved is not able to contact the satellite from underwater. It is also not a waterproofed box
It's important to differentiate between the different types of beacons.
The ULB (Ultrasonic Locator Beacons) are fitted to the front of the DFDR and (optionally) on the CVR. These are specifically designed for underwater operation and will only activate on contact with water. These are powered by replaceable, not rechargeable batteries. The range varies with the water condition. The beacon is only certified down to a certain depth (I guess it will crush at lower depths). Ours are certified to 20,000'. The range, for some reason, is quoted in meters (1800~3000)
There are portable ELB's stowed in the cabin at various locations. These are usually activated by water, but are not designed to transmit underwater. These are designed to float on the surface of the water. The water usually dissolves a paper/cardboard collar holding a folded antenna in place. When the collar releases, the antenna springs up and the transmitter starts operating. If there is no water at the crash site, the collar can be broken manually.
Finally.. the latest and greatest ... are the fixed ELT's mounted on top of the fuselage (the blade-type antenna is just forward of the vertical stabiliser/fin on the outside of the aircraft and the transmitter/battery is mounted just below it). These are g-force activated. A PPRuNer quotes 5g's for 11milliseconds in any plane/direction. These can also be manually activated and reset by a switch on the cockpit overhead panel. These are also not designed to operate underwater. These send signals on VHF and UHF. The UHF, I imagine, will be picked up by satellite. The UHF signal contains the aircraft ID, but not a GPS position.
Other than a typo on 121.5, Avspook has it mostly.
NSEU: "These are also not designed to operate underwater. These send signals on VHF and UHF. The UHF, I imagine, will be picked up by satellite. The UHF signal contains the aircraft ID, but not a GPS position."
The 406 MHz signal is data burst, and Lat/Long is optionally and usually wired from the Nav system. The 406 transmitted to a satellite constellation provides much more accuracy and reliablity in finding the aircraft.
Satellite monitoring of 121.5 was discontinued in February.
In operation the transmitter sends:
A swept-tone-modulated homing-signal simultaneously on frequencies 121.5 MHz (Civil) and 243 MHz (Military).
If a 406MHz transmitter is installed, a signal on frequency 406 MHz to the COSPAS-SARSAT satellite system.
The satellite system transmits the 406 MHz distress signal to a Local User Terminal (LUT), when the LUT is in range. The LUT receiving range is a radius of approximately 2,500 km (1367.00 NM). When the LUT is not in receiving range, the satellite system stores the distress signal until transmission is possible. The LUT automatically processes the signal to identify and show the position of the aircraft to a radius of approximately 1.8 km (5900.00 ft.). The processed data is transmitted to a Mission Control Center (MCC). The MCC sends the data to an applicable Rescue Coordination Center (RCC), where Search And Rescue (SAR) operations are started. The 121.5 MHz and 243 MHz signals are used to find the aircraft in the final stage of SAR operation.
So and now someone explain me pls how UHF radio waves are working in water ?? High frequenced get absorbed, so thats why subs used LW or had to get an antenna to the surface.
I dont know how cold it is 2 miles deep, but those batteries wont be lasting to the upper limit of battery life Iwould imagine...
like I remember from school (long time ago) water temp on highest density 2 deg C (39 deg F)
RF is useless under water. ELT's are designed for surface recovery. This thread is not worth anythig as concerned to a submerged aircraft unless there was a sucessfull water landing to recover live floating people.
ELT's are designed to recover life, not a sunken aircraft.