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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There are more than 600 airfields within the flying range of the missing MH370.
An Indian Navy boat patrols in the waters of the Andaman Sea near Port Blair, the capital of India's Andaman and Nicobar islands, March 16, 2014.
The Royal Malaysian Navy corvette KD Terengganu and a U.S. Navy MH-60R Sea Hawk helicopter from the US Navy guided-missile destroyer USS Pinckney conduct a coordinated air and sea search for a missing Malaysian Airlines jet in the Gulf of Thailand
A Royal Malaysian Navy Fennec helicopter preparing to depart to aid in the search and rescue efforts for the missing Malaysia Airlines plane over the Straits of Malacca.
Crew members from the Royal Malaysian Air Force prepare a Malaysian Air Force CN235 aircraft for a Search and Rescue operation to find the missing Malaysia Airlines flight MH370, in the Straits of Malacca
A crew member from the Royal Malaysian Air Force uses binoculars onboard a Malaysian Air Force CN235 aircraft during a Search and Rescue operation to find the missing Malaysia Airlines flight MH370, in the Straits of Malacca
Boeing 777 Malaysian Airlines
Ground workers push a Vietnam Coast Guard Casa aircraft after a search operation for the missing Malaysia Airlines flight MH370 plane over the southern sea, at a military airport in Ho Chi Minh city on March 13, 2014.
Indonesian Air Force personnel aboard an Indonesian Air Force military surveillance aircraft over the Malacca Strait, a sea passageway between Indonesia and Malaysia, while searching for the missing Malaysia Airlines flight MH370 plane.
Muslims perform a special prayer for passengers of the missing Malaysia Airlines MH370 plane at the departure hall of the Kuala Lumpur International Airport
Malaysian Muslim children offer prayers for passengers of the missing Malaysia Airlines flight MH370 plane at the Kuala Lumpur International Airport
Malaysian Muslims read the Koran during a special prayer for passengers of the missing Malaysia Airlines flight MH370 plane at Kuala Lumpur International Airport (KLIA)
Muslims perform a special prayer for passengers of the missing Malaysia Airlines MH370 plane at Kuala Lumpur International Airport
Students light candles as they hold placards in solidarity with families of the passengers of the missing Malaysia Airlines flight MH370 plane during a candle light vigil at the university belt in Manila on March 13, 2014.
A satellite image taken from space, illustrating objects in a "suspected crash sea area" in the South China Sea, thought to possibly be from the missing Malaysia Airlines flight MH370
China said its satellites have detected three large floating objects in a suspected crash site near where the missing Malaysian jet lost contact
The 13-nation search for MH370At least 43 ships and 39 aircraft from Malaysia, Vietnam, Singapore, Thailand, China, the US, Indonesia, the Philippines, Australia and New Zealand have joined the search for the missing Boeing 777-200ER. India, Japan and Brunei joined the search operations on March 12.
A map with a pin at 105.63 degrees longitude East and 6.7 degrees latitude North, where images of suspected debris from the missing Malaysia Airlines flight MH370 was captured by a Chinese satellite
Republic of Singapore Navy personnel participating in the search and rescue operations, approximately 380 nautical miles (700 kms) north of Singapore, in the South China Sea for the missing Malaysia Airlines flight MH370.
Singapore Navy personnel participating in the search and rescue operations, approximately 380 nautical miles (700 kms) north of Singapore, in the South China Sea for the missing Malaysia Airlines flight MH370.
Philippine Air Force crew aboard a C-130 plane scour the seas while flying over the western Philippines in the South China Sea as they help search efforts for the missing Malaysia Airlines flight MH370.
A Philippine Air Force crew member aboard a C-130 plane scours the seas while flying over the western Philippines in the South China Sea as they help search efforts for the missing Malaysia Airlines flight MH370.
A C-130 Hercules transport plane belonging to Japan's Air Self-Defense Force (ASDF) is seen at the ASDF base in Naha on Japan's southern island of Okinawa, before departing to help in the search for the missing Malaysian Airlines Boeing 777-200 plane
An Indonesian National Search and Rescue Agency boat patrols the Malacca Strait off Aceh province located in the area of northern Sumatra island in search for the missing Malaysia Airlines flight MH370.
The search for a missing Malaysian jet swung northwest towards the Andaman Sea on March 12, far from its intended flight path, exposing Malaysia to mounting criticism that its response was in disarray.
Beachgoers walk past a sand sculpture made by Indian sand artist Sudersan Pattnaik with a message of prayers for the missing Malaysian Airlines flight MH370
The China ambassador to Malaysia Dr Huang Huikang speaks to members of the media and leaves the resort in Cyberjaya after meeting with the Chinese relatives of the missing MH370 plane passengers
A group of fishermen found a life raft bearing the word 'Boarding' 10 nautical miles from Port Dickson town at 12pm on March 11.
The 11-nation search for MH370At least 40 ships and 34 aircraft from Malaysia, Vietnam, Singapore, Thailand, China, the US, Indonesia, the Philippines, Australia and New Zealand have joined the search for the missing Boeing 777-200ER. On March 12, India announced that it is joining the search operations as well. No sign of the plane has been spotted so far.
MH370, the flight number of the Malaysia Airlines plane that has been missing since Saturday, is seen projected onto an illuminated building in Shanghai
A woman writes a message of support and hope for the passengers of the missing Malaysia Airlines MH370 on a banner at Kuala Lumpur International Airport
Ibrahim Mat Zin, a local well-known "bomoh" (the Malay term for a shaman), performs a ritual to help find the missing Malaysia Airlines MH370 at Kuala Lumpur International Airport
Mat Zin, a local well-known "bomoh", arrives at Kuala Lumpur International Airport on March 12 to perform a ritual to help find the missing Malaysia Airlines MH370.
Family members of passengers aboard Malaysia Airlines flight MH370 vote to set a deadline for the company to answer their requests, including providing solatium without additional conditions, at a hotel in Beijing
Surti Dahlianese, niece of Dutch national, Surti Dahlia, a passenger on the missing Malaysia Airlines flight MH370 looks at identity documents of her missing aunt Surti Dahlia while other Indonesian relatives watch television to monitor the latest developments on the missing aircraft from their residence in Medan city, located in Sumatra island.
Interpol Secretary General Ronald Kenneth Noble (2ndR) speaks next to the Police Services Executive Director Jean-Michel Louboutin (R), the Director of Operational Police Support Michel O'Connell (L) and the Chief of staff Roraima Andriani (2ndL) during a press conference where they display an image of two suspects from the missing Malaysia Airlines MH370
Malaysian police said they had identified one of two men who boarded a missing Malaysian jet with fake passports as a 19-year-old Iranian believed to be seeking to emigrate to Germany.
Malaysian police officials display photographs of one of the two men who boarded missing Malaysia Airlines MH370 flight using stolen European passports at a press conference
Malaysia's Police Chief, Inspector General Khalid Abu Bakar told a news conference, that one of the men (L) was identified as a 19-year-old Iranian Pouria Nour Mohammad Mehrdad while the identity of the other was being investigated.
A Malaysian child joins others at a candlelight vigil for missing Malaysia Airlines passengers.
A woman holds a candle and during a vigil held for the passengers of Malaysia Airlines MH370 near Independence Square in Kuala Lumpur.
Malaysians singing songs as they hold a candlelight vigil for the passengers of Malaysia Airlines MH370.
Malaysian ethnic Chinese hold candles and a banner before a candlelight vigil for the passengers of the missing plane
Malaysian ethnic Chinese launch a Kongming lantern with well wishes written by attendees at a candlelight vigil for the passengers of Malaysia Airlines MH370
Malaysian ethnic Chinese write well wishes on "kongming" lanterns (sky lanterns) during a candlelight vigil for the passengers of Malaysia Airlines MH370
More than 200 people take part in a candlelight vigil to pray for the safe return of MH370 passengers at Taman Jubli Perak on March 10.
Specifications of a Malaysia Airlines planeAs the search for MH370 enters day three, a new theory emerged on 10 March as to why the debris from the aircraft has been so difficult to locate.
DIGITAL FLIGHT RECORDERThis solid state device records 25 hours of data, including more than 100 parameters. A sturdy, insulated housing protects the unit from impact, fire, and seawater, for recovery later.
Malaysia Airlines flight MH370 reportedly climbed to a cruise height of 35,000 feet before losing contact with air traffic controllers.
An infographic detailing what happened before MH370 went missing
International assets deployed for search and rescue
International assets deployed for search and rescue
An aerial view of an oil spill is seen from a Vietnamese Air Force aircraft in the search area for a missing Malaysia Airlines plane, 250km from Vietnam and 190km from Malaysia, in this handout photo from Thanh Nien Newpaper taken on March 8, 2014.
Malaysian PM Najib arrives at the holding area for family and friends of passengers aboard MH370.
A signboard advises relatives of passengers of a missing Malaysia Airlines plane to go to a nearby hotel for information on the flight at Beijing International Airport in Beijing on March 8, 2014.
Singapore has sent a C130 Hercules aircraft to help in the search for the missing Malaysia Airlines flight MH370
A Malaysia Airlines spokesperson speaks to journalists during a news conference at a hotel in Beijing on March 8, 2014.
Reporters waiting outside a room reserved for next-of-kin at a Beijing Hotel
Tearful and angry, the friends and relatives of passengers on board missing Malaysia Airlines flight MH370 lashed out at the company.
According to Malaysia Airlines, 153 of the 239 people on board the missing flight - a codeshare with China Southern Airlines - are Chinese citizens.
Security at times struggled to hold back the huge throng of reporters crowding outside the door and making it difficult for relatives to enter or exit.
Scores of family members spoke to airline officials in small groups in a room on the hotel's second floor.
An information screen displays a message "Let Us Pray For Flight MH370" at Kuala Lumpur International Airport in Sepang, on March 8, 2014.
The plane lost contact with air traffic controllers at 1.30am Saturday.
Malaysia Airlines said a flight carrying 239 people from Kuala Lumpur to Beijing went missing early Saturday, and the airline was notifying next of kin in a sign it expected the worst.
The flight path from Kuala Lumpur to Beijing (screengrab from Google Maps).
Friends and family members of missing flightIt was carrying 227 passengers, including two infants, from 13 different nationalities, and 12 crew members.
MAS confirms reports which stated flight MH370 had made an emergency landing in Nanning, China is false, reported The Star.
Fuad Sharuji, VP Operation Control, said during the last reported contact at 35,000 feet, around two hours from KLIA, there was no call from the crew or notification from the tower that they were having any kind of difficulties.
Malaysia Airlines Chief Executive Ahmad Jauhari Yahya speaks to the media during a news conference on missing Flight MH370, in Sepang.
MAS Group CEO Ahmad Jauhari YahyaIn a Facebook statement, MAS confirms that flight MH370 had lost contact with Subang Air Traffic Control at 2.40am, on Mar 8.
"We deeply regret that we have lost all contacts with flight MH370 which departed Kuala Lumpur at 12.41 am earlier this morning bound for Beijing," MAS CEO Ahmad Jauhari Yahya (above) said in a statement.
The statement said the Malaysian flag carrier was working with authorities, who had launched an effort to locate the aircraft.
"Our team is currently calling the next-of-kin of passengers and crew," MAS CEO Ahmad Jauhari said.
Journalists wait in a conference room for a news conference regarding the missing Malaysian Airlines flight MH370, at a hotel in Beijing March 8, 2014.
China's state television said 158 of the passengers were Chinese.
Xinhua also quoted Chinese aviation authorities saying the plane did not enter China's air traffic control sphere.
A report by China's Xinhua news agency said contact was lost with the plane while it was over Vietnamese airspace.
A woman displays the driver's licence of a friend who may be a passenger on board the missing Malaysia Airlines flight at a hotel in Beijing on March 8, 2014
A crying woman is escorted to a bus for relatives at the Beijing Airport after news of the missing Malaysia Airlines Boeing 777-200 plane on March 8, 2014
A relative (woman in white) of a passenger onboard Malaysia Airlines flight MH370 cries as she talks on her mobile phone at the Beijing Capital International Airport March 8, 2014
Family members of those onboard the missing Malaysia Airlines flight MH370 walk into the waiting area at Kuala Lumpur International Airport in Sepang March 8, 2014
An airport worker writes on a sign board advising relatives of passengers of a missing Malaysia Airlines to go to a nearby hotel for information on the flight, at Beijing International Airport in Beijing on March 8, 2014
Family members waiting at KLIA airport in Sepang
Family members waiting at KLIA airport in Sepang
About 60 MAS Special Assistance team volunteers off to Beijing boarding at KLIA
Family members look at a wedding album of Norliakmar Hamid (R, in veil) and Razahan Zamani, passengers on a missing Malaysia Airlines Boeing 777-200 plane in Kuala Lumpur on March 8, 2014.
Fariq Abdul Hamid (left) co-pilot of Malaysia Airlines MH370 is seen here with CNN reporter Richard Quest in this picture posted on Instagram by user @boutiquebeautystyle
Prime Minister Datuk Seri Najib Tun Razak and his deputy Tan Sri Muhyiddin Yassin after meeting a relative of MAS flight crash in Vietnam
Captain Zaharie Ahmad Shah
A man (C) thought to be a relative of a passenger of the missing Malaysia Airlines flight walks through a group of reporters at a hotel in Beijing on March 8, 2014
A woman (R) breaks down while leaving the reception centre for families and friends after an airliner went missing at the Kuala Lumpur International Airport on March 8, 2014
Family members look at a wedding album of Norliakmar Hamid and Razahan Zamani, passengers on a missing Malaysia Airlines Boeing 777-200 plane in Kuala Lumpur on March 8, 2014
A relative of Norliakmar Hamid and Razahan Zamani, passengers on a missing Malaysia Airlines Boeing 777-200 plane, cries at their house in Kuala Lumpur on March 8, 2014.
Sarah Nor, 55, the mother of 34-year-old Norliakmar Hamid, a passenger on a missing Malaysia Airlines Boeing 777-200 plane, talks on a mobile phone at her house in Kuala Lumpur on March 8, 2014.
Residents look at ships anchored at a local naval base at Phu Quoc island, in the waters of southern Vietnam, where a Malaysian Airlines jet was presumed lost on March 8, 2014.
A Vietnam coast guard ship is seen anchored at a local naval base at Phu Quoc island, in the waters of southern Vietnam, where a Malaysian Airlines jet was presumed lost on March 8, 2014
Residents look at ships anchored at a local naval base at Phu Quoc island, in the waters of southern Vietnam, where a Malaysian Airlines jet was presumed lost on March 8, 2014
Malaysia's Department of Civil Aviation (DCA) Director General Datuk Azharuddin Abdul Rahman speaks at a news conference at a hotel near the Kuala Lumpur International Airport in Sepang March 9, 2014
In this picture taken on March 8, 2014 a passenger (R) walks past an empty Malaysia Airlines counter at the Kuala Lumpur International Airport in Sepang
Malaysia's Department of Civil Aviation (DCA) Director General Datuk Azharuddin Abdul Rahman (L) speaks at a news conference at a hotel near the Kuala Lumpur International Airport in Sepang March 9, 2014.
Family members of those onboard the missing Malaysia Airlines flight MH370 walk to their car outside a hotel in Putrajaya March 9, 2014
Family members of those onboard the missing Malaysia Airlines flight MH370 walk to their car outside a hotel in Putrajaya March 9, 2014
Family members of those onboard the missing Malaysia Airlines MH370 flight read the Koran inside a car outside the hotel they are staying in, in Putrajaya March 9, 2014.
Representatives of Malaysia Airlines attend a news conference about Malaysia Airlines flight MH370 at a hotel in Beijing, early March 9, 2014.
A relative (bottom R) of a passenger of Malaysia Airlines flight MH370 cries as she walks past journalists at a hotel in Beijing March 8, 2014.
A spokesman (C) of Malaysia Airlines is surrounded by journalists as he gives a briefing about Malaysia Airlines flight MH370, at a hotel in Beijing March 8, 2014
Family members of those onboard the missing Malaysia Airlines flight MH370 cry at a hotel in Putrajaya March 9, 2014
A member of a charity group, who was invited by Malaysia Airlines to comfort the relatives of passengers of Malaysia Airlines flight MH370, speaks to journalists at a hotel in Beijing March 9, 2014
A relative of a passenger of Malaysia Airlines flight MH370 cries as she walks past journalists at a hotel in Beijing March 9, 2014.
A relative (front) of a passenger of Malaysia Airlines flight MH370 cries as she walks past journalists at a hotel in Beijing March 9, 2014
The families of MH370 passengers from Beijing arrive at Kuala Lumpur International Airport on March 10
The Singapore Army sent a frigate, a missile corvette and a S-70B Naval Helicopter sent to locate the missing MH370
RSAF also deployed additional C-130 to the search area approximately 700km north of Singapore.
MV Swift Rescue, a Submarine Support and Rescue Vessel (SSRV), has joined search efforts for the missing flight
Supplies being loaded onto the MV Swift Rescue before its departure Mar 9 morning.
Military personnel work within the cockpit of a helicopter belonging to the Vietnamese airforce during a search and rescue mission off Vietnam's Tho Chu island
A military personnel looks out of a helicopter during a search and rescue mission off Vietnam's Tho Chu island
A general view of an oil spill is seen, from an aircraft belonging to the Vietnamese airforce, off Vietnam's Tho Chu island
Vietnamese military personnel look out of a helicopter during a search and rescue mission off Vietnam's Tho Chu island
A map of a flight plan is seen on a computer screen during a meeting before a mission to find the Malaysia Airlines flight MH370 that disappeared from radar screens
A military officer takes notes during a search and rescue mission, onboard an aircraft belonging to the Vietnamese airforce, off Vietnam's Tho Chu island
Clouds hover outside the window of a Vietnam Air Force search and rescue aircraft An-26 on a mission to find the missing Malaysia Airlines flight MH370, off Vietnam's Tho Chu island
A military personnel communicates with his colleagues by radio during a search and rescue mission onboard an aircraft belonging to the Vietnamese airforce off Vietnam's Tho Chu island
A military personnel takes photos from the window of an aircraft belonging to the Vietnamese airforce during a search and rescue mission off Vietnam's Tho Chu island
Military officers work within the cockpit of an aircraft belonging to the Vietnam Air Force during a search and rescue mission off Vietnam's Tho Chu island
A giant screen showing the hours since Malaysia Airlines flight MH370 went missing and the number of passengers onboard, at a shopping mall, in Beijing
A man writes on a poster carrying messages for the passengers of the missing Malaysia Airlines Boeing 777-200 plane at the Kuala Lumpur International Airport
Visitors write on a poster carrying messages for the passengers of the missing Malaysia Airlines Boeing 777-200 plane at the Kuala Lumpur International Airport
A man puts up a poster carrying a message for the passengers of the missing Malaysia Airlines Boeing 777-200 plane at the Kuala Lumpur International Airport
Deputy commander of Vietnam Air Force Do Minh Tuan (seated), looks at a map on a TV screen during a news conference about their mission to find missing Malaysia Airlines flight MH370 at Phu Quoc Airport on Phu Quoc Island
Deputy commander of Vietnam Air Force Do Minh Tuan points to a map during a news conference after their mission to find the missing Malaysia Airlines flight MH370 at Phu Quoc Airport on Phu Quoc Island
A helicopter takes off from Jinggangshan warship to search the waters suspected to be the site of the missing Beijing-bound Malaysia Airlines flight MH370
A search boat carrying rescue workers is set down from Mianyang warship on the waters suspected to be the site of the missing Beijing-bound Malaysia Airlines flight MH370
A woman leaves a message of support and hope for the passengers of the missing Malaysia Airlines flight MH370 at the Kuala Lumpur International Airport
Indonesian national search and rescue agency personel watch over high seas during a search operation for missing Malaysia Airlines flight MH370 in the Andaman Sea
Indonesian national search and rescue agency personel watch over high seas during a search operation for missing Malaysia Airlines flight MH370 in the Andaman Sea
A Malaysian Maritime Enforcement Agency pilot studies the map onboard a Japan Coast Guard Gulfstream V Jet aircraft, customized for search and rescue operations, as they search for the missing Malaysia Airlines MH370 plane
A Malaysian Air Force CN235 aircraft are seen on the tarmac during a search and rescue (SAR) operation to find the missing Malaysia Airlines flight MH370 plane over the Strait of Malacca in Subang.
Crew members looking outside windows from a Malaysian Air Force CN235 aircraft working during a search and rescue (SAR) operation to find the missing Malaysia Airlines flight MH370 plane over the Strait of Malacca.
Crew member from a Malaysian Air Force CN235 aircraft working during a search and rescue (SAR) operation to find the missing Malaysia Airlines flight MH370 plane over the Strait of Malacca.
AFP, Reuters, Bloomberg, The Straits Times, The Star/ANN, Mindef, New Straits Times, Twitter
