There has been a number of posts made recently that mention ELT (Emergency Locator Transmitters). For the benefit of those who are not familiar with aircraft electronics, there are two main types of ELTs: those which are intended for use on land, and those intended for use on (or in) water.
Virtually all civil aircraft are fitted with a fixed ELT that is located towards the aft end of the fuselage (this to minimize damage in the event of a crash). This ELT is connected to a small antenna on the outside of the aircraft. The fixed ELT is typically activated automatically by G force, although it is usually possible to turn it on manually using a switch in the flight compartment. This type of fixed ELT is of little or no value if the aircraft lands (or crashes) in water and then sinks, because the radio signal emitted is attenuated by the water above it.
The second type of ELT is one that is designed for use if the aircraft lands (or crashes) on water. It transmits the same type of signal as the fixed ELT described above, but it is normally stowed in a quick-release bracket within the cabin, the idea being that it will be manually removed from its mount and deployed by the crew in the event of ditching. This type of ELT activates automatically when it comes in contact with water, and like the other type, can also be activated manually. But, if it is not removed from the mounting bracket and deployed manually after a ditching, the signal it emits will likely not be detected. Same as with the fixed ELT, if this maritime ELT sinks with the aircraft, the radio signal that it emits will be attenuated by the water above it.
Below is a photo showing a marine-type ELT as installed in an Air Canada aircraft. It is stowed in one of the overhead baggage bins, close to the forward left cabin door.
Here is a link to a document published by Honeywell (a manufacturer of ELTs) that provides additional information about the various types of ELTs available.
Honeywell 406 MHz ELTs
Maritime ELT, as installed in an Air Canada aircraft
