Many theories are out there on what happened to Malaysia Airlines (MAS) Flight MH370, but without the flight data recorder and cockpit voice recorder, together referred to as the "black box", little can be known.
Before the black box can be retrieved, however, the plane must be located, and that is where the missing aircraft's Emergency Locator Transmitter (ELT) comes in.
An ELT for aircraft such as MH370's Boeing 777-200 works by emitting a signal that can be picked up by satellites and relayed to search craft, say aviation experts.
However, the puzzle is that there has been no signal from MH370's ELT since the the plane went missing on March 8.
Neither has any signal been detected from its black box, which carries its own underwater locator beacon.
Such beacons transmit signals at distances of between 3km and 10km.
The Straits Times has tried to verify with MAS whether the missing aircraft was installed with an ELT, without success.
The ELT is designed to self-activate by certain triggers such as an impact or contact with water - similar to the black box's locator device - but a key factor is that nothing is absolutely guaranteed to work all the time.
"The dynamics of an impact are extremely complex, and there is always a chance that the automatic activation does not work," Mr Gerry Soejatman, an aviation consultant with Whitesky Aviation in Jakarta, told The Straits Times in a telephone interview.
Depending on the manufacturer, ELTs also have limited power - anything from 24 hours to two years - and this adds to the urgency of finding MH370 fast.
ELTs older than two years are usually analogue types that work by triangulation via radar, but newer ones can give their Global Positioning System (GPS) coordinates, making searches relatively easier.
If underwater, the ELT's signal could be blocked by debris, and even satellites may have trouble picking up the signal, said Mr Soejatman.
But a pilot who asked not to be named, who is also a safety director with a Philippines-based private charter company, said that although the International Civil Aviation Organisation recommends having an ELT onboard, it is up to each country's civil aviation authority to enforce its use.
"Private charter flights are random, we will go anywhere and we must have them on board, but a carrier with scheduled flights like MAS may not necessarily (install) them (on its aircraft)," he said.
However, the MH370 case is not the first time an ELT has failed to transmit signals.
An undetectable ELT was one of the reasons it took the authorities two years to find the wreckage - and black box - of Air France Flight AF447, which disappeared over the Atlantic Ocean in June 2009.
It is crucial to find the black box as it contains information that can give clues to what happened to the plane. It records actual flight conditions, altitude, airspeed, heading, vertical acceleration and aircraft pitch, and records cockpit conversations and engine noise.
"So far, there is no case of the (black box's underwater locator beacon) not activating, but we know it can happen," said Mr Soejatman.
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