Avionics News May 2018 - 23

an approved 121.5 MHz ELT unit for which
satellites stopped listening almost 10 years ago.
In the majority of respects, the new-tech ELTs
outstrip the capabilities and utility of the old
units first required 40 years ago.
Let's look at the genesis of the FAA's ELT
requirement, differences in technologies and
capabilities, and some of the numerous options
available to bring an aircraft's ELT technology
firmly into the 21st century and why it could be a
matter of life and death.
The two ELT technologies share little,
certainly where accuracy and expediency come
into play. Look at the differences between the
two, as detailed by the National Oceanic and
Atmospheric Administration in its comparison of
the 121.5 MHz and the 406 MHz technologies at
sarsat.noaa.gov/406vs121.pdf.
121.5 MHz
When the FAA mandated 121.5 MHz ELT
detection of an emergency signal, it hinged on
the passage of a satellite, once every two hours.
Miss a cycle and the wait clock started anew
with another two hours before the next satellite
passage.
Once detected, the search-and-rescue process
started, with a search area defined at a minimum
of 500 square miles. Narrowing further requires
CAP and other searchers to fly plotted sequences
in their effort to pinpoint the source of the signal.
By analyzing Doppler shift, the search area
can be narrowed to 12 to 15 nautical miles.
Once narrowed to a few square miles, the search
intensified - but still depended on homing in
on what by then would be a weakened 121.5
MHz signal. If luck ran in favor of the downed
aviator, searchers might find the aircraft by air;
otherwise, ground searches would start.
And all this would depend on the old 121.5
MHz ELT triggering properly, remaining in its
mount and retaining antenna integrity. A lot of
variables, a lot of real estate to cover.
In the best of circumstances, however, hours
still would pass before any help arrived at the
Continued on following page

The EPIRB
Option
Smart, just not legal
The 406 MHz frequency is to the world what
121.5 once was: a frequency for emergencylocator beacon broadcasting - but it's not
exclusively aviation's.
Thanks to something called an Emergency
Position Indicating Radio Beacon, or EPIRB, a
host of other users enjoy the ability to signal
their need for help, along with location, to
search-and-rescue authorities. Other users:
maritime operators; hikers; campers; hunters;
back-country guides; fishermen; off-road drivers
and motorcyclists.
These users activate their EPIRB to alert
search-and-rescue services in the event of an
emergency by broadcasting a coded message
on the 406 MHz distress frequency to alert the
nearest rescue coordination center.
Survival experts recommend them in addition
to the 406 MHz ELTs used in aircraft, boats and
ships. That's in addition to the ELT - not in place
of the ELT.
The EPIRB isn't a legal substitute for an ELT
because it doesn't meet the requirements of TSOC126b for 406 MHz ELTs.
EPIRBs are, by their nature, designed to
be portable and, for the most part, manually
activated - though some maritime models will
activate when in the water.
Some 406 MHz ELTs also are designed to be
portable, with an extra antenna to install once the
ELT is removed from its installation mount. q

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may

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http://sarsat.noaa.gov/406vs121.pdf
Table of Contents for the Digital Edition of Avionics News May 2018
