The 411 On 406s


Most aircraft owners and operators are keenly aware of the rapidly approaching deadline to comply with the federal mandate on ADS-B installations. However, there is an additional aircraft technology wave upon us that also offers potentially significant advancements in aviation safety. This involves new digital 406 MHz emergency locator transmitters and since this new technology could also become a federal mandate (in additional to saving your life), it is important to understand the technology, its advantages and its costs.

In the early 70s, particularly in Alaska, there were a series of unfortunate accidents and other events where downed aircraft turned out to be difficult or impossible to find. That prompted the U.S. federal government to mandate that all aircraft operating in the country (with some published exceptions) must be equipped with an emergency locator transmitter (ELT), to assist in search and rescue efforts. Most other countries followed suit if they didn’t have such a mandate already. The standards were pretty simple. The ELT device needed to be relatively durable in order to survive a high-G impact and to have a self-activating shock triggering mechanism, its own antenna and an internal power supply sufficient for an extended broadcast period.

Those early designs had severe faults and limitations. Most of their battery packs were highly specialized and expensive with limited shelf lives. The units themselves were notorious for going off when they weren’t supposed to and not triggering when they were actually needed. Their pulsating analog homing signal was little more than dumb noise with no supplemental information. False alarms were difficult to screen out, and rather large search areas were calculated when signals were detected. Stricken aircraft usually took several hours to locate at best.

In spite of their severe limitations, there can be no doubt that there are many pilots and passengers who are alive today because of their ELTs. Lamentably, there are also documented cases of search and rescue personnel who have been tragically lost while valiantly responding to alert missions, often in the same poor flying conditions around weather and/or terrain that caused the alerted event in the first place.

ELTs broadcasting on the traditional 121.5 MHz VHF frequency have been monitored in the past by other aircraft, ground stations and even satellites. However, in 2009 the SARSAT monitoring of the 121.5 analog signals was quietly terminated, something that a lot of operators even today may not realize. Certain ground stations and specialized search and rescue aircraft usually have the equipment available to “direction find” (DF) the signal to home in on it. Monitoring aircraft without such equipment can only detect a signal’s presence and judge whether the intensity of the signal is increasing or decreasing with no idea of the signal’s actual position or azimuth. Airliners are “requested” to monitor 121.5 on backup radios when they aren’t in use, but common airline routes don’t cover all areas where signals could originate, especially in off-peak hours. Sometimes ATC will request a certain aircraft to listen and report in an effort to track down a suspected signal or radar disappearance, but that process is akin to playing Battleship over a massive area and is woefully inefficient and time consuming.

A Digital World

Around the turn of the century, the digital advancement in radio technology brought a new, much improved product into the marketplace in the form of 406 MHz ELT units. One huge advantage of a digital signal is that in addition to a homing signal, it can send digital data that can be vital in determining the legitimacy of the alert and actual location of the transmission, be it a harmless false alarm on an airport ramp or a dire life-or-death emergency in a remote location. Each device is preregistered with the operator’s contact information and assigned a unique digital hex code, which is extremely beneficial to weeding out false alarms prior to sending out expensive search and rescue crews on a potential folly. The old 121.5 units could only be located to within a 12- to 25-mile range. That might look tiny on a map but it is huge on snowshoes.

The new 406 units refine the area down to 2 to 3 miles by signal alone; however, most units also digitally broadcast precise lat/long GPS data that pinpoints the target to mere yards/meters, effectively taking the “search” out of search and rescue. Unlike the 121.5 units whose satellite monitoring was terminated, the 406 units continue to be monitored by GEOSAR satellites 24/7 over most of the world. Additional 406 design upgrades include better shock, fire and water-resistance standards. Also, since the digital signal can be pulsed a few seconds every minute rather than continuously broadcast like the analog units, the internal batteries, usually modern li-ion, can sustain the alert broadcast for a much longer period of time. The “transmission power” of new 406 units is advertised by one vendor as 50 times greater than previous units.

Most aviation advocacy groups have lobbied the U.S. government to not mandate a transition to the 406 units, at least for a while, so that those who want to continue the use of their previous generation 121.5 units can continue to do so and so far, the United States hasn’t made their use mandatory. Other countries have mandated the changeover, most notably for U.S. pilots is our neighbor to the south, Mexico, which after several extensions went with a hard mandate this past July that non-exempt aircraft operating in their airspace must be equipped with a 406 MHz unit. Canada and some Caribbean countries could be following Mexico’s example shortly. At some point the U.S. likely will as well.

Like all bleeding-edge technological advances, the initial units to hit the market were much more expensive than their predecessors, especially for those who don’t have the capability to install the units themselves, as they require an instrument panel control head and associated cabling installed that most older generation units did not. Unless the unit has an internal GPS as some do, they also require a GPS signal feed from onboard avionics. Those wiring issues can get complicated and expensive in certain applications, especially for certified aircraft owners. The GPS signal isn’t required for FAR compliance, but its absence negates arguably the largest safety advantage of the technology upgrade.

As is often the case with new technology, the passage of time and market presence has dramatically reduced the purchase price of the new units to about the same level that their predecessors were back in the day. Some manufacturers build new units that are plug and play replacements of their old designs using the same mounts, control heads and antennae. A quick perusal of aviation online sites shows complete 406 ELT packages available at less than $500 USD. Since appointment availability at avionics shops is already tight with the ADS-B mandate, it may make sense for some to bite the bullet and have both projects accomplished during the same visit.

While the advantages of the new digital units are head and shoulders above the old technology and will literally save lives, obviously there are those whose flying activities simply don’t justify the investment to change, especially with the already mandated deadline of ADS-B compliance which in its own right has search and rescue advantages imbedded within. The risk envelopes of an all-weather country crosser are different than a fair-weather burger runner who usually short hops around with several others from the air park. Enough is enough some will say, and that is certainly a justifiable position. Choice is good.

Obviously, for someone building an experimental aircraft, now or in the future, installing a 406 unit is an absolute no brainer. For those experimental or certified who will need to convert over, a serious assessment of the value of getting found quicker (or at all) when the landings column no longer equals the takeoffs column will need to be made. Sometimes federal employees can actually come up with worthwhile ideas (cue laugh track).

Since the advent of seat belt and shoulder harnesses, rarely in our industry has such a relatively minor investment offered so many potentially lifesaving capabilities as a new digital 406 MHz emergency locator transmitter.

Myron Nelson has flown professionally for over 30 years. He and his son both fly for Southwest Airlines. Myron built and flies an award winning RV-10 based in Arizona.