Search & Rescue Systems

Search and Rescue System

By Jim Sparks

November 2000

ImagesWords that each of us in the aviation maintenance industry dread to hear are "An aircraft went down." However, with the technology and safety measures currently employed, accident survivability is realistic. The response time of emergency equipment can have a dramatic affect on the ultimate outcome. The ability to pinpoint a crash site is an advantage when dispatching crash and rescue crews because the less time spent searching can often be translated into more lives saved.
Search and Rescue (SAR) systems are devices that most of us hate to think about, yet it is a method to bring together experts from every walk of life whose main mission is to save lives. This not only involves the pilots and passengers in the aircraft concerned, but the people monitoring the distress signals, government, civil, and military forces responding to a crash, and even the Emergency Medical Technicians who are dispatched once an accident confirmation occurs.
The Emergency Locator Transmitter (ELT) is the device tasked with providing the dreaded signal that relays that a crash has occurred. These devices have evolved over the years, but still require periodic maintenance as well as functional and operational testing.
Four types of ELTs exist and are classified as:
1. Automatic Fixed (AF)
2. Automatic Portable (AP)
3. Survival (S)
4. Automatic Deployable (AD)
Operation of the system all revolves around the proper triggering of the ELT unit on board the aircraft. Originally the radio beacon radiated outward with the intent of being received by an orbiting Search And Rescue Satellite (SARSAT). Communication of the signal then goes from the satellite to a ground-based Local User Terminal (LUT). There are just less than 40 of these stations around the world with three providing the coverage across the continental United States. Once a signal is received by the LUT, investigation as to the signal origin begins. A VHF frequency of 121.5 Megahertz (MHz) was the first frequency of choice for ELT communications. Unfortunately, reception of this type of signal is based on line of sight transmission. So, the possibility exists that if an active ELT is not within sight of a SARSAT the emergency transmission will not be heard. In fact, with this type technology, only about 60 percent of the Earth can be physically monitored. Not only this, in order for the satellite to advise the LUT, it must also have "a line of sight." That means the satellite will only relay ELT information to the LUT when it has both the transmitting ELT and receiving LUT within view. There are other drawbacks to the 121.5 MHz units including exact location determination. Tracking this type of signal will generally get within a 15-mile radius of the point of origin. Unfortunately, scanning a 30-mile circular area will absorb significant resources. The 121.5 MHz range is also subject to many false signal transmissions. It is estimated that something less than five percent of all 121.5 MHz distress signals are actual ELT distress broadcasts. In fact, of all the signals received, about 20 percent are coming from an ELT and the majority are false transmissions.

Multiple channel ELTs
One solution to this dilemma is an ELT with multiple channels. There are currently two- and three-band devices in current operation. In addition to transmission on the 121.5 MHz frequency, the two-band unit will also transmit a distress signal on 243.0 MHz. This is considered an Ultra-High Frequency and does not follow the line of sight transmission rule and is also monitored by military forces. The likelihood of detection of this signal is significantly greater than the stand-alone 121.5 MHz call.
Technical Standard Order (TSO) C91 addresses the minimum performance standards for the 121.5 /243.0 MHz ELT. The TSO was based on Document number DO-183, issued by the Radio Technical Commission for Aeronautics (RTCA) in the year 1983. It is important to remember that FAR 91.207 states that an ELT meeting the requirements of TSO C91 may not be used for new installations. This applies to installation on older aircraft, and the replacement of existing ELTs as they become unusable or unserviceable. The requirements of TSO C91a must be met in these cases.
A further evolution appeared in 1989 with the development of a third channel. In addition to transmitting a Search and Rescue Beacon on the 121.5 and 243.0 MHz bands, a digitized, coded signal could also be sent out on the 406 MHz frequency. This may be accomplished by installing a stand-alone 406 MHz transmitter or by installing a three-band ELT. The significance of this third channel is that it is monitored by NOAA, the National Oceanic and Atmospheric Administration. Weather satellites can closely pinpoint this signal and the coded message will give aircraft identification. This type of device must be registered with NOAA and registration information should include ELT data, Aircraft Owner, Aircraft Registration and Emergency Contacts.

Location, location, location
Within the aircraft, the ELT is a stand-alone device - that is, it does not depend on the aircraft electrical power systems for its operation, nor does it require a queue from an aircraft switch. In fact, in many aircraft, the unit is self-contained and only requires a mechanical means of attachment to the aircraft structure.
In some situations, the ELT unit is buried within the bowels of the airframe and some additional external components are essential to insure operation. Many aircraft will have provisions for a remote ELT antenna. Locations will vary depending on airframe; however, most components associated with this emergency system are mounted in the area of an aircraft most likely to survive a crash, which is often in the tail section. In addition to a remote antenna, a flight deck mounted switch is another common option. This will provide a means of disarming the ELT during routine maintenance plus enables reset of an activation or even initiation of a test. In some cases, the switch will also provide a means of manual activation of the emergency beacon. Aircraft not utilizing a flight compartment switch, depend upon accessibility to the ELT unit for system isolation or test. Some aircraft also use a remotely located "G" switch or impact sensor to trigger a transmission. Most ELT units contain an internal impact switch that will also command automatic transmission. Cockpit-mounted switches generally have an override capability that will enable the pilot to manually activate a distress signal if a crash landing is imminent.

The power of ELTs

ELT antenna

As an Emergency Locator Transmitter is a stand-alone component, it must provide itself with an adequate power source and typically, a lithium battery is used. If the battery has adequate capacity, ELT transmission should be uninterrupted and continuous for at least 48 hours. These batteries are generally not replenished by the aircraft electrical system, therefore any activation of the ELT for more than several hours may require battery replacement.
There is also a calendar time placed on these lithium batteries. Federal Aviation Regulation 91.207, Emergency locator transmitters states that when 50 percent of a batteryƕs useful life expires, or when the ELT cumulative operation exceeds one hour, the battery should be replaced. This life limit is to be established by the ELT manufacturer. Generally, the limitation is 48 months and can be determined by a date stamp on the ELT unit. Battery modules are considered field replaceable and most manufacturers provide detailed instructions to accomplish this operation. Once a new battery is installed, it is the responsibility of the installing technician to revise the date for the next replacement.

Cautionary measures
The lithium battery has been the subject of significant concern. In the past, there have been incidents of explosion, burning, violent ventilation and subsequent corrosion.
Technical Standard Order C97 provides a minimum performance standard for one type of lithium sulfide dioxide battery. In fact, the FAA issued three Airworthiness Directives against lithium batteries. The final one requires that batteries in use today meet the requirements of TSO C97. This AD was issued in February 1980 and the RTCA is developing a minimum performance standard for lithium batteries that will become the basis for FAA certification of these power supplies. Explosive results can be expected if these batteries are exposed to fire. It is therefore necessary to take precautionary procedures when disposing of spent or out of date batteries.
Once a battery supply is replaced, a functional test of the unit is required. Official standards are in place to accommodate ELT testing. Transmission on any of the three designated frequencies is considered acceptable for the first five minutes of each hour and should not exceed ten seconds duration. This can be accomplished by selecting a switch, either installed on the ELT or remotely in the flight deck, to a manual transmit position and either observe a "Transmit" light or tune the Communication Radio to 121.5 MHz and listen for the broadcast.

ELT controls

Testing this emergency system for proper operation is another essential part of normal maintenance. The Federal Aviation Administration in the United States requires that a visual inspection as well as a functional test be performed at least once every year. The visual check will verify attachment of equipment to the supporting structure as well as electrical connection. In addition, observation of corrosion may indicate battery problems. The functional test will include verification of the controls including the "Impact Switch" as well as presence of sufficient radiated signal from the antenna. This is defined by Technical Standard Order C-91 as on the 121.5 MHz band at least 30 percent of the transmitted energy will be within a plus or minus 30 Hz. The 243 MHz transmission should have at least 30 percent of its transmitted energy within a range of plus or minus 60 Hz.
Many of the new generation systems include a remote switch in the flight deck, which will allow system arming, test and deactivation. In some cases a "Transmitting" indicator light is also available to the flight crew. Once the test mode is selected, the transmit indicator will illuminate. It should, of course, extinguish at the conclusion of the test. During a test, the aircraft VHF Communication radio should be tuned to 121.5 and the ELT signal can be monitored for presence and of course termination at the conclusion of the test. In the event of test malfunction, the most common recourse is to replace the ELT unit. Once again, attention should be paid to the type of unit installed. If the 406 MHz device is used, registration of the unit with NOAA is an essential part of the installation.
Government airworthiness authorities determine which aircraft need to have an Emergency Locator Transmitter. In the United States, the Federal Aviation Administration provides this information in Federal Air Regulation 91.207. There is also a criteria contained here for aircraft operation with an inoperative ELT.
Search and Rescue operations involve a wide scope of participants including the National Oceanic and Atmospheric Administration, United States Coast Guard, world military forces, local civilian services such as police and fire departments, as well as Emergency Medical Response Teams both airborne and ground based. The response of all of these groups is primarily based on the proper operation of one little box. In the reality of an aircraft accident, the thin band between life and death may have a lot to do with a certain percentage of energy being transmitted in a specific frequency spectrum and the ability of a battery to insure this signal can be maintained until rescuers arrive.