The Emergency Locator Transmitter (ELT) is a battery-powered transmitter installed in aircraft to help locate the aircraft if it crashes. The ELT emits a signal on two emergency frequencies that can be detected by search and rescue teams. It is designed to automatically activate upon crash impact and transmit continuously for 48 hours. Proper installation, periodic testing, and battery replacement are required to ensure the ELT functions properly in an emergency.

1. Emergency Locator
Transmitter
AVIONICS
TECHNOLOGY
The Emergency Locator Transmitter (ELT) is a self‑contained battery operated
transmitter that is designed to help locate an airplane after a crash.
An ELT has an inertia switch that closes in the event of a crash and starts the
transmitter emitting a series of down-sweeping tones simultaneously on two
emergency frequencies, 121.5 MHz in the VHF band and 243.0 MHz in the UHF
band.
The battery in an ELT has a design life long enough to operate the transmitter
continuously for 48 hours.
An emergency locator transmitter (ELT), also referred to as a locator beacon, is
required on aircraft:


to provide a signal or signals that will enable search aircraft or ground stations
to find aircraft that have made crash landings in remote or mountainous areas.
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The ELT system is a transmitter that emits an onmidirectional signal on the
international distress signals at 121.5 and 243.0 MHz.

General-aviation aircraft, commercial aircraft, the FAA, and the CAP (Civil Air Patrol)
monitor 121.5 MHz.

The 243.0 MHz is monitored by the military services.
The unit is designed:

to automatically activate under emergency condition

or may be operated manually from the cabin to summon assistance in other than a
disabling emergency condition,
the distress signal can be detected by Search and Rescue Satellite (SARSAT).

ELTs are installed as far aft in the fuselage as it is practical to place them, and they
are connected to a flexible whip antenna.
The installation must be such that orients the inertia switch so that it is sensitive to a
force of approximately 5G along the longitudinal axis of the aircraft.
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When an ELT is properly installed, it requires little maintenance other than ensuring
that it remains securely mounted and connected to its antenna.

There must be no evidence of corrosion, and the battery must be replaced
according to a specific schedule.
Non-rechargeable batteries must be replaced or chargeable batteries recharged:

when the transmitter has been used for more than one cumulative hour, or

when it has reached 50% of its usable life, or

if it is rechargeable 50% of its useful life of charge.
The date required for its replacement must be legibly marked on the outside of the
transmitter case and recorded in the aircraft maintenance records.
An ELT can be tested by removing it and taking it into a shielded or screened room to
prevent its radiation from causing a false alert.
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An operational check may be made with the ELT in the aircraft by removing the
antenna and connecting a dummy load.
If it is not possible to use a dummy load:

the antenna may be left in place

the ELT operated for no more than three audible sweeps

the test must be conducted within the first five minutes after any hour
If the ELT must be operated outside of this time frame:

the nearest FAA control tower must be contacted and the test coordinated with
them
The pilot should check at the end of each flight to be sure that the ELT has not been
triggered.

This is done by tuning the VHF receiver to 121.5 MHz and listening for the tone. If
no tone is heard, the ELT is not operating.
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System Components
The fixed ELT must be installed
securely in the aircraft at a location
where crash damage will be minimal.
The location selected is usually in the
area of the tail cone;

In some cabin-type aircraft, the
unit is installed in the aft, top part
of the cabin.
Access is provided in either case so the
unit can be controlled manually.
ELT system block diagram
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System Components (cont’d)
An ELT transmitter for small airplanes with the antenna and coaxial cable
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System Components (cont’d)
The ELT system components consist of:
 Transmitter including adapter plate
and battery
 Remote control
 Antenna
 Wire kit for remote control
 Antenna cable
ELT system components location
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8. AVIONICS
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System Operation
The ELT is battery powered and is automatically turned on by crash forces. It will
transmit a special swept tone for 48 hours on two different emergency frequencies.
They are 121.5 MHz and 243.0 MHz;


121.5 is the civilian emergency frequency
243.0 is the military emergency frequency.
It is designed to activate automatically and no preliminary procedures other than
proper installation, periodic testing, and specified battery replacement are required.
The transmitter is activated by an acceleration operated switch when a rapid
deceleration force is applied along the longitudinal axis of the aircraft.
The ELT must be installed as far aft as possible but in front of the tail surfaces
since this area has been shown to remain intact in most airplane crashes.
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System Operation (cont’d)
The batteries in the ELT must be replaced or recharged at specific intervals as
required by the FARs.
A typical ELT consists of a self-contained dual-frequency radio transmitter and
battery power-supply with a suitable whip antenna.

When armed, it will be activated by an impact force of 5g or more, as may be
experienced in a crash landing.
The ELT is powered by the alkaline battery pack.
After a crash landing, the ELT will provide line-of-sight transmission up to 100 mi
[161 km] at a receiver altitude of 10,000 ft [3050 m].
The ELT transmits on both distress frequencies simultaneously at 75-mW rated
power output for 50 continuous hours in the temperature range of -4 to +131oF
[20 to + 55oC].
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System Operation (cont’d)
There are times when an aircraft technician may need to test an ELT so he
should be familiar with the procedure.

If possible, the ELT should be tested with the antenna disconnected or shielded to
prevent the transmission of emergency signals into the air.

If this cannot be done, it is still permissible to test the ELT, but only during the
first five minutes of any hour and for three audio sweeps maximum.

A VHF communication radio is turned on and tuned to 121.5 MHz.

The ELT is then switched on manually until the signal is heard on the receiver and
then switched off again.
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Automatic Operation
Manual Operation
The ELT function switch, located in the
ELT transmitter must be set in ARM
position.
The ELT remote control switch, located
in the cockpit must be set in ARM
position.
The ARM position is the "normal"
position for the control switch.



Chapter 3
Set the ELT function switch to ON
position, or
Set the ELT remote control switch to
ON position.
Manually operate the ELT only when:

In this position, the transmitter will
automatically commence transmitting
if the aircraft stops abruptly within
defined specification.
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There are two forms of manual
operations of ELT:

A "soft” crash does not activate the
ELT.
For very brief periods during
authorized test of transmitter.
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12. AVIONICS
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Test Operation
The control panel on an ELT unit
contains a switch with three positions:
AUTO, OFF, and ON.
The unit may be tested by tuning the
VHF COMM receiver to 121.5 MHz and
then placing the ELT switch in the ON
position.
The emergency tone will be heard if
the ELT is operating.
Immediately after the test, the switch
should be returned to the AUTO
position.
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Set the VHF No. 1 or VHF No. 2 aircraft
transceiver to 121.5 MHz with the
volume turned up.
Turn on the transmitter for 1½
seconds.

A repeating downward swept audio
signal should be heard on VHF
communication receiver.
Turn the transmitter off after 3 audio
sweeps.
Move the transmitter switch to ARM
position (normal position).
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Test Operation (cont’d)
If an ELT is inadvertently turned on,
owing to a lightning strike or an
exceptionally hard landing,


the control switch should be placed
in the OFF position to stop the
transmission,
then the switch should be returned
to the AUTO position to arm the unit.
Testing of an ELT should be performed
within the first 5 minutes of an hour
(FAA regulation) and only three pulses
of the transmitter should be activated.
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For example, a test could be conducted
between 1:00 p.m. and 1:05 p.m. with
a maximum of three beeps being heard
on a frequency of 121.5 MHz.

Tests conducted in this manner do
not alert the FAA to a crashed
aircraft.
Before this check or test:


we should contact the airport control
tower to avoid an unnecessary
“alert" or false alarm
coordinate with search and rescue
agency since the activate ELT can be
detected by the Satellite ground
station.
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Service for an ELT
An ELT requires a minimum of service;

however, certain procedures are
necessary to assure satisfactory
operation.
The battery pack must be changed in
accordance with the date stamped on
the unit.

Typically, the batteries are replaced
every 2 years or after 20 min of
continuous use.
The replacement date must be clearly
marked on the battery's data plate;
otherwise, the battery is not airworthy.
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The ELT should be tested regularly to
assure satisfactory operation.
An inspection of the ELT mounting and
antenna should be made periodically to
ensure firm attachment to the aircraft.
Regulations regarding the operation of
ELTs are set forth in FAR Part 91.52.

Technicians involved with the
installation and service of ELTs
should be familiar with these
regulations and manufacturer's data.
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15. AVIONICS
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ELT Regulation
FAR 91.207 Emergency Locator Transmitter
No person may operate a U.S. registered civil airplane unless it meets the applicable
requirements listed below for ELTs.
Each emergency locator transmitter must be in operable condition and meet the
requirements of TSO‑C91 or TSO‑C91A and it must be installed as far aft as
practicable.
Batteries used in the ELT must be replaced or recharged as appropriate:

When the transmitter has been in use for more than one cumulative hour; or

When 50% of the useful life has expired.
The expiration date for the replacement or recharge of the battery must be legibly
marked on the outside of the transmitter and entered in the aircraft maintenance
record.
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FAR 91.207 Emergency Locator Transmitter (cont’d)
The requirements for ELT do not apply to:

A newly acquired aircraft that must be ferried to a place where the ELT will be
installed.

An aircraft with an inoperative ELT that must be ferried to a place for ELT repair.

Turbojet powered aircraft.

Scheduled air carrier flights.

Training flights conducted entirely within 50 nautical miles of the airport of operations.

Design and test flights.

Delivery flights of new aircraft.
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FAR 91.207 Emergency Locator Transmitter (cont’d)
The requirements for ELT do not apply to (cont’d):

Aircraft engaged in aerial application of chemicals for agricultural purposes.

Research and development aircraft.

Exhibition and air racing aircraft.

Aircraft equipped to carry only one person.

An aircraft during any period in which the ELT has been temporarily removed for
inspection, repair, modification or replacement, subject to the following:



A maintenance record entry must be made that includes the date of removal, the
serial number and the reason for removal.
A placard must be placed in view of the pilot which states "ELT not installed".
The aircraft must not be operated more than 90 days after initial ELT removal.
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18. Cockpit Voice and Flight
Data Recorder
AVIONICS
TECHNOLOGY
The cockpit voice recorder (CVR) and the flight data recorder (FDR) are designed to
automatically record information in flight that can be used during an investigation
following an accident or serious incident.
They are installed on all air carrier jets and some commuter airliners and privately
owned aircraft.
The recorders are installed in the aft fuselage as since this area is least likely to be
severely damaged in an accident.
A Cockpit Voice Recorder and Flight Data Recorder installed in the aft section of the fuselage
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The CVR is designed to record sounds in the cockpit and communications on the
intercom and radio systems.

It has a hot microphone in the cockpit which is always activated to record voices,
warning sounds, engine noise etc.
The CVR is also connected to the intercom so that conversations between the
members of the crew can be recorded.

It is also connected to radios so that communications with ATC are recorded.
The CVR has a continuous recording system that holds approximately the last 30
minutes of audio.

It is located in the aft fuselage for better survival and it is waterproof and
protected against fire and impact forces.
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The flight data recorder has many more inputs than the cockpit voice recorder.

It has a recording time of 8 hours on smaller aircraft and about 24 hours on larger
aircraft.
The CVR and FDR are located in the same area of the aft fuselage and have
similar protection from water, fire etc.
Air carrier jets have been required to carry CVRs and FDRs for some years, but
recently new regulations have gone into effect that require these devices on
smaller aircraft.
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Regulation for CVR and FDR
FAR 91.609 Flight Recorders and Cockpit Voice Recorders
 Multi‑engine turbine powered airplanes or rotorcraft with 10 passenger seats or more
manufactured after October 11, 1991 must have a digital flight data recorder with 8
hours storage.
 After October 11, 1991, multi‑engine turbine powered airplanes and rotorcraft with 6
passenger seats or more and with a required minimum flight crew of 2 pilots must
have an approved cockpit voice recorder with minimum storage of 15 minutes.
 If an accident or incident occurs, the operator must hold the data 90 days or longer if
requested.
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Regulation for CVR and FDR (cont’d)
FAR 91 Appendix E: Flight Recorder Specifications
1.
Pitch Trim Position.
2.
N1, EPR or Prop RPM and Torque.
3.
Vertical Speed.
1. Airspeed.
4.
Angle of Attack.
2. Altitude.
5.
Autopilot Engagement.
3. Magnetic Heading.
6.
TE Flap Position.
4. Vertical Acceleration.
7.
LE Flap Position.
5. Longitudinal Acceleration.
8.
Reverse Thrust.
6. Pitch Attitude.
9.
Spoiler / Speedbrake Position.
The flight recorder required for certain
aircraft under FAR 91.609 must record
the following items:
7. Roll Attitude.
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Cockpit Voice Recorder
The cockpit voice recorder, or CVR, is an important device for determining the
cause of an aircraft accident.
An endless tape allows for 30 minutes of recording, and then it is automatically
erased and recorded over.
There are four inputs to the recording heads:

The microphones of the captain

The microphones of first officer

The microphones of flight engineer

A microphone that picks up received audio and cockpit conversations
These microphones are always "hot" and do not require any type of keying.
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Cockpit Voice Recorder (cont’d)
The pickups are all in the cockpit, but the actual tape recorder is in a fire resistant box
usually located near the tail of the aircraft.
The CVR is painted bright orange so that it is easily identified among the wreckage.
Cockpit voice recorders (CVRs) are very similar to flight data recorders; they look
nearly identical and operate in almost the same way.
CVRs monitor the last 30 min of flight deck conversations and radio communications.
The flight deck conversations are recorded via the microphone monitor panel located
on the flight deck.
This panel is also used to test the system and erase the tape if so desired.
The erase mode of the CVR can be operated only after the aircraft has landed and the
parking brake set.
Playback is possible only after the recorder is removed from the aircraft.
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Cockpit Voice Recorder (cont’d)
Microphone monitor panel for a cockpit voice recorder
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Cockpit Voice Recorder (cont’d)
In Boeing 737:

The cockpit voice recorder uses four independent channels to record cockpit audio
on a 30 minute continuous-loop tape.

Recordings older than 30 minutes are automatically erased.

One channel records cockpit area conversations using the area microphone.

The other channels record any individual audio and transmissions from the pilots
and first observer.
In Boeing 757:

The cockpit voice recorder records any transmissions from the cockpit made
through the audio selector panels.

It also records cockpit area conversations using an area microphone.
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Cockpit Voice Recorder (cont’d)
In Boeing 777:

It has four-channel solid state voice recorder with flight deck area
microphone records the most recent 30 minutes of flight crew
communications.

It erases automatically so that only the last 30 minutes are on the memory
tape.

Inputs from the voice recorder are from the area microphone that picks up
any conversations on the flight deck, the captains, first officers, and first
observer's audio panel and their hot microphones (oxygen mask).
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Cockpit Voice Recorder (cont’d)
Block diagram of Boeing 777 voice recorder system
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Cockpit Voice Recorder (cont’d)
Input to the voice recorder is from:




the cockpit voice recorder
microphone,
the captain's,
first officer's, and
first observer's audio from their hot
microphone inputs to the AMU.
The cockpit voice recorder panel has
test and erase buttons and is on the
maintenance panel in the flight deck.
The cockpit voice recorder microphone
is on the overhead panel in the flight
deck.
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There is a voice recorder jack on the
service and APU shutdown panel that
permits a ground crew member to
monitor flight deck conversation.
The recorder unit is in the aft
equipment center.

It includes an underwater locator
beacon (ULB).
To bulk erase the cockpit voice
recorder, the airplane must be on the
ground and the parking brake set.
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30. AVIONICS
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Flight Data Recorder
Since 1958 any commercial
passenger aircraft flying in the
United States has been required to
be equipped with an automatic flight
data recording system.
The FAA changed the regulations to
include virtually all turboprop
aircraft.
Today flight data recorders are found
on most corporate and all transportcategory aircraft.
The system must monitor both flight
parameters and flight deck voice
activities.
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Recorded flight parameters include:









Aircraft's altitude
Airspeed, pitch attitude
Roll attitude
Magnetic heading
Vertical acceleration
Flap position
Gear position
Engine power
Greenwich Mean Time
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31. AVIONICS
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Flight Data Recorder (cont’d)
A flight data recorder is housed in a
crushproof container located near
the tail section of the aircraft.
The tape unit is fire-resistant and
contains a radio transmitter to help
crash investigators locate the unit
under water.
A data recorder is typically used for
accident investigation;

however, some airlines also use
recorded data to aid in
troubleshooting recurring
mechanical defects.
A flight data recorder
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Flight Data Recorder (cont’d)
A modern flight data recorder uses a
magnetic recording tape to store digital
data of flight parameters of the past 25
flight hours.
The recorder receives the majority of
its input signals from existing sensors
located throughout the aircraft.
The information is sent to the recording
unit, which stores up to 900 bits of
information on 1 in. of tape.
The tape unit employs:


two 4-channel record heads
two 4-channel erase heads
The tape unit from a flight data recorder
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Flight Data Recorder (cont’d)
One record and one erase head are used
when the tape travels from left to right.
The other pair of heads is used when the
tape travels from right to left.
The two pairs of erase-record heads are
set at different levels (tracks) along the
tape, thus producing an 8-track, singlechannel format.
This format allows for 25 h of data
storage at 3.125 h per track.
The tape reversal and track switching are
performed automatically when the tape
reaches its limit of travel.
The tape unit from a flight data recorder
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