ELT is an interesting device, each of us wants to have it functioning inboard, but never wants to use it at any time.
Long story short, I will explain to you this important device, what does ELT stands for in aviation, and how it actually saves lives of many pilots in general aviation aircraft, as airliners use additional more advanced systems see this article for more see Aircraft Black Box Colour, and let’s get started.
The ELT in aviation stands for the Emergency locator transmitter, a device that’s used to transmit and broadcast a special signal, depending on the application, on a specific frequency. These signals help the rescue team to locate the aircraft’s crash location and rescue onboard crew and passengers. The ELT can be activated by shock from a crash or manually by any crew member.
ELT working principle
In modern days, ELT is an integral component of the international satellite system, Civil Aviation authorities and detectives use it for search and rescue (SAR) after a loss of connection with aircraft probably after it crashed, this locator device can be activated manually from the cockpit – or automatically after the device immersed in water or as a result of high ‘g’ forces on impact.
ELTs transmit a distress signal which can be detected by non-geostationary satellites on orbit and then located precisely by either or both GPS trilateration and Doppler triangulation.
Aircraft ELT Issues
Latest studies on why ELTs stop functioning in some aircraft crashes, concluded the below reasons evolved post the impact;
- damage and/or removal of the antenna during impact
- Forget to selecting the ELT activation to armed before flight
- incorrect installation of the locating device
- flat dead batteries
- lack of water proofing
- lack of fire protection
- disconnection of the co-axial antenna cable from the unit during impact
- An aircraft coming to rest inverted after impact which cause the signal to be blocked
ELT frequency and regulations
So, what frequency does an ELT transmit on? According to ICAO Annex 10, Volume V, which requires that ELTs carried in compliance with the Standards of Annex 6, Parts I, II, and III shall operate on both 406 MHz and 121.5 MHz Although the SAR satellite systems are no longer able to use 121.5 MHz signals, this frequency is considered necessary to allow homing.
All ELTs capable of transmitting on 406 MHz must be coded in accordance with ICAO Annex 10 and registered with the national agency responsible for initiating Search and Rescue or another nominated agency.
In ICAO Annex 6, Part IIA, a Recommendation is made that all airplanes operated on extended flights over water and when operated on flights over designated land areas shall be equipped with an automatic ELT.
There is an identical Recommendation in respect of certain Classes of helicopters when conducting overwater operations. – From Skybrary
Types of ELTs
According to the AC 91-44A CHG 1, ELTs may come in any of these forms:
TYPES OF ELTs. There are five basic types of ELTs: automatic fixed (ELT-AF), automatic portable (ELT-AP), survival (ELT-S), automatic deployable (ELT-AD), and distress triggered (ELT-DT).
- Automatic Fixed (ELT-AF). ELT-AF devices are permanently attached to the aircraft and designed to stay attached even after a crash to aid Search and Rescue (SAR) teams in locating a crash site.
- Automatic Portable (ELT-AP). While attached to an aircraft, ELT-AP devices can be removed from the aircraft and continue to function. These devices act as an ELT-AF and can be activated by a crash, but can also be removed and tethered to a life raft or person, or carried to a safe location away from the crash site. This type of device is designed to bring SAR teams to the survivors (e.g., on a life raft in the ocean), rather than the wreckage.
- Survival (ELT-S). ELT-S type devices are portable in nature, are manually activated, and are the type typically carried by backpackers.
- Automatic Deployable (ELT-AD). ELT-AD devices are designed to be attached to an aircraft, but deploy (detach) automatically after a crash event has been detected. ELT-AD devices must be capable of floating on water and are designed to help crash investigators locate the crash site.
- Distress Triggered (ELT-DT). ELT-DT devices are designed to be activated by the aircrew, or automatically by an internal or external trigger. In-flight events and detection criteria are defined in European Organization for Civil Aviation Equipment (EUROCAE) Specification ED-237, MASPS for Criteria to Detect In-Flight Aircraft Distress Events to Trigger Transmission of Flight Information.
How to Test ELT
- Care should be taken to prevent accidentally triggering a SAR response. Accidental activation of an ELT will generate an emergency signal that cannot be distinguished from that of an actual emergency and could lead to expensive and frustrating searches.
- Moreover, the unwarranted ELT signal could tie up the emergency frequencies such that a genuine emergency signal would not be picked up. In addition, if an ELT signal is transmitted on or near an airport, it may render some radio communications channels unusable.
- Regardless of where the ELT is, or the duration of activation, a 406 MHz beacon broadcast will be detected by at least one Geostationary Local User Terminal (GEOLUT) and possibly every Low Earth Orbit Local User Terminal (LEOLUT) in the Cospas-Sarsat System.
- Alert messages will be routed to every Mission Control Centers (MCC) in the Cospas-Sarsat System for coordination around the world and a response will be made (unless prior coordination is made with Cospas-Sarsat and local authorities).
- Direct connect testing is preferred to prevent inadvertent activation of the SAR response system. Over-air testing should always be avoided if possible. Use of an antenna boot or a direct connection from test equipment to the antenna port is preferred.
- Testing an ELT system in a metal hangar will not guarantee the radiated signal will not be detected by the Cospas-Sarsat System. Technicians testing ELT devices in a hangar should treat the test as if they were testing outside. When testing an ELT, a 50-ohm dummy load or antenna boot should be used to prevent the signal from being radiated into space. The signal must be attenuated to less than -51 dBW (a power flux density of -37.4 dB (W/m2 ) or a field intensity of -11.6 dB (V/m).
- Owners of ELTs should carefully follow owner manual instructions and when possible limit testing to the “Self-Test” function.
- If over-air testing must be accomplished, technicians should carefully follow Cospas-Sarsat instructions and use the built-in test message on the ELT device. The ELT test message is different from messages transmitted during an emergency, but is still detectable by the Cospas-Sarsat System. Cospas-Sarsat should be contacted prior to performing over-air testing. Cospas-Sarsat can be contacted at https://www.cospas-sarsat.int/en/.
- If over-air testing must be accomplished, the local air traffic control (ATC) facility should be contacted in advance. 8.2.9 Follow test set instructions or place the test set a minimum of 12 meters, (39.4 feet) from the ELT antenna. Test in each mode and frequency the ELT unit transmits.
How Often Must ELT Be Inspected
- Each ELT required by § 91.207(a) must be inspected within 12 calendar-months after the last inspection for proper installation, battery corrosion, operation of controls and sensors, and radiated signal strength.
- Due to the variety of ELTs and the different ways they can be installed, the ELT owner’s manual should be followed when performing a system inspection. With this in mind, inspections of an ELT system should consider the following:
- Proper Installation. Verify the ELT is not installed in an area prone to being bumped or damaged, such as a luggage compartment.
- Verify the ELT is installed on a rigid airframe component not subject to vibrations that may trip the ELT system.
- Verify the ELT or associated antenna cables are not installed near moving parts, such as flight control rods or cables, that may come in contact with the ELT system.
- For fixed-wing aircraft, verify the ELT device is mounted as parallel to the longitudinal axis of the aircraft, as practical. For rotorcraft, ensure antenna is mounted per manufacturer’s instructions.
Note: On some helicopters, this may mean the ELT device is forward-facing and maybe pointing downward at a 45-degree angle, ± 15 degrees.