Emergency Locator Transmitter (ELT)
The missing Malaysia Airlines plane, flight MH370, had 239 people on board and was en route from Kuala Lumpur to Beijing on 8 March 2014 when air traffic control staff lost contact with it. And crashed somewhere but they didn’t get an ELT signal!
What is ELT? How does it work?
An emergency locator transmitter (ELT) is an independent battery-powered transmitter activated by the excessive G-forces experienced during a crash.
It transmits a digital signal every 50 seconds on a frequency of 406.025 MHz at 5 watts for at least 24 hours.
The signal is received anywhere in the world by satellites in the COSPAS-SARSAT satellite system.
Two types of satellites, low earth orbiting (LEOSATs) and geostationary satellites (GEOSATs) are used with different, complementary capability.
The signal is partially processed and stored in the satellites and then relayed to ground stations known as local user terminals (LUTs).
READ MORE: AIRPLANES: PARTS AND THEIR FUNCTIONS
Further deciphering of a signal takes place at the LUTs, and appropriate search and rescue operations are notified through mission control centers (MCCs) set up for this purpose.
ELTs are typically installed as far aft in the fuselage of an aircraft as is practicable just forward of the empennage.
The built-in G-force sensor is aligned with the longitudinal axis of the aircraft.
Helicopter ELTs may be located elsewhere on the airframe.
They are equipped with multidirectional activation devices.
The use of Doppler technology enables the origin of the 406 MHz ELT signal to be calculated within 2 to 5 kilometres.
Second generation 406 MHz ELT digital signals are loaded with GPS location coordinates from a receiver inside the ELT unit or integrated from an outside unit.
This reduces the location accuracy of the crash site to within 100 meters. The digital signal is also loaded with unique registration information.
It identifies the aircraft, the owner, and contact information, etc.
READ MORE: HOW A HELICOPTER FLY AND CONSTRUCTED?
When a signal is received, this is used to immediately research the validity of the alert to ensure it is a true emergency transmission so that rescue resources are not deployed needlessly.
ELTs with automatic G-force activation mounted in aircraft are easily removable.
They often contain a portable antenna so that crash victims may leave the site and carry the operating ELT with them.
A flight deck mounted panel is required to alert the pilot if the ELT is activated.
It also allows the ELT to be armed, tested, and manually activated if needed.
Modern ELTs may also transmit a signal at 121.5 MHz.
This is an analog transmission that can be used for homing.
Before 2009, 121.5 MHz was a worldwide emergency frequency monitored by the CORPAS-SARSAT satellites.
However, it has been replaced by the 406 MHz standard.
Transmission on 121.5 MHz is no longer received and relayed via satellite.
Technicians are required to perform an inspection/test of 121.5 MHz ELTs within 12 months of the previous one and inspect for the same integrity as required for the 406MHz ELTs.
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However, older ELTs often lack the built-in test circuitry of modern ELTs certified to TSO C-126.
Therefore, a true operational test may include activating the signal.
This can be done by removing the antenna and installing a dummy load.
Any activation of an ELT signal is required to only be done between the top of each hour and 5 minutes after the hour.
The duration of activation must be no longer than three audible sweeps.
Contact of the local control tower or flight service station before testing is recommended.
It must be noted that older 121.5 MHz analog signal ELTs often also transmit an emergency signal on a frequency of 243.0 MHz.
This has long been the military emergency frequency.
Its use is being phased out in favor of digital ELT signals and satellite monitoring.
Improvements in coverage, location accuracy, identification of false alerts, and shortened response times are so significant with 406 MHz ELTs, they are currently the service standard worldwide.
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