B737
MAINTENANCE MANUAL
EFFECTIVITY
Post ELT STC ST03081AT
23-24-00
PAGE 002
JAN 02/07
G.
One advantage of the 406.025 MHz transmitter is that it will produce a much more accurate
position, typically 1 to 2 kilometers as compared to 15 to 20 kilometers for 121.5/243.0 Mhz
transmitters. When coupled with the aircraft’s navigation system the position accuracy improves
to approximately 100 meters. The ELT also transmits a digital message which allows the search
and rescue authorities to contact the owner/operator of the aircraft through a database.
Information contained in the database that may be useful in the event of a crash is shown below:
• Type of aircraft
• Address of Owner
• Telephone Number of Owner
• Aircraft Registration Number
• Alternate Emergency Contact
H.
Once the ELT is activated and the 406.025 MHz signal is detected from the satellite and a
position is calculated, the 121.5/243.0 MHz transmissions are used to home in on the crash site.
Because aircraft communication radios are not capable of receiving 406.025 MHz transmissions,
the only method of monitoring the ELT is the blinking cockpit light, the buzzer or the 121.5/243.0
MHz transmissions which may be monitored on the aircraft communication transceiver.
I.
The switch below the ELT cockpit light allows you to turn the unit on for testing and to reset the
unit. You cannot disarm or disable the unit from the cockpit. Cockpit operation is limited to
deactivating the ELT after it has been activated, or manually activating the ELT.
J.
The C406-1 series ELT’s are housed in a high impact fire resistant polycarbonate plastic case.
The ELT unit is able to withstand extremely harsh environments. Units, like yours, have been
subjected to numerous 500g shock pulses, 1000 pound crush weights and severe penetration
tests, and continue to operate normally. Continued operation in a temperature range of -20
degrees C through +55 degrees C is assured.
2. Component Details
A.
ELT Transmitter
The ELT transmitter rests in a mounting tray that is attached to the aft fuselage of the airplane.
The ELT transmitter consists of these components:
(1) The printed circuit assembly (PCA) of the C406-1 series ELT is unique in that the PCA has
no RF function but rather only control functions for the RF module. The PCA contains a
one time programmable microprocessor that controls all the ELT functions(ON/OFF,
modulation, 406 MHz digital message and RESET).
(2) All RF functions are contained in a separate module. The transmitter operates
simultaneously on 121.5 MHz and 243.0 MHz with the carrier frequencies remaining within
0.0005% under all environmental operating conditions. Approximately every 50 seconds
the 406.025 MHz transmitter transmits. During this time the 121.5 and 243.0 MHz
transmitter is momentarily turned off.
(3) The AM modulation at 121.5 and 243.0 MHz is designed as A9 with the carrier amplitude
modulated with an audio frequency sweeping downward or a range of not less than 700
Hz, within the range of 1,600 to 600 Hz. The sweep repetition rate is between 2 and 4 Hz
with a modulation factor of at least 0.85. The modulation applied to the carrier frequency
has a minimum duty cycle of 33% and a maximum duty cycle of 55%.
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