This advisory circular provides information and guidance
concerning the importance of transponder operation and altitude reporting in
the National Airspace System (NAS).

a. The forerunner of today’s beacon system was developed during
World War 11 to enable radar operators to identify targets as.either friendly
or enemy. It was originally called “IFF” (identification friend or foe) and
the system used both ground and airborne equipment. The ground transmitter
sent a signal to the aircraft transceiver which, in turn, replied in a set
code, depending upon how the pilot had set the “black box.” Only a few codes
could be used and these were changed daily, if not more often.

b. Following the war, there seemed to be no civilian requirement for this
identification system until radar itself was phased into the Air Traffic
Control (ATC) system. At first, the number of aircraft presented few problems.
The normal “skin paint” or echo return on a scope was sufficient to identify
an aircraft for the controller. But heavier traffic , particularly in high
density terminal areas, highlighted the need for some rapid, positive means of
identifying each target; eliminating the need for identifying turns.

c. Subsequently, the Air Traffic Control Radar Beacon System (ATCRBS) was
developed which provided the controller with continuous, reliable, and accurate
information concerning the position and identity of all transponder-equipped
aircraft within an area by a series of coded replies which appeared on the
radar scope. The first ATC transponder had a capability of only 64 different
codes. Now, the NAS uses 4,096 individual codes in addition to Mode C replies
from aircraft transponders which report an aircraft’s altitude in 1000foot
increments. With this system, the controller is able to identify any ATC
transponder-equipped aircraft up to a distance of 200 nautical miles (slant
range) and at an altitude limited only by the service ceiling of the aircraft.

Pilots desiring to fly in Group I Terminal Control Areas (TCA) or at altitudes above
12,500 MSL, excluding airspace at and below 2,500 AGL, are required to have
a transponder with 4096-code capability on Mode 3/A and altitude reporting
Mode C. Pilots desiring to fly in a Group II TCA are required to have a
transponder with 40960code capability on Mode 3/A. Specific details can be
found in FAR 91.90. Operators of aircraft that are transponder-equipped can
expect safer and more efficient handling by ATC when flying in VFR or IFR
conditions.

Certification rules (FAR 23.1301 and 25.1301) require equipment to function properly when installed. The rules apply to transponder equipment and include the TSO requirement as specified in FAR 91.24. The owner/operator is primarily responsible for maintaining an aircraft in an airworthy condition. Maintenance facilities are responsible for maintaining transponders at least equal to their original or properly
altered condition (FAR 43.13). Fur&&, manufacturers’ maintenance instructions should be followed. The maintenance record entry should indicate that the system has been properly tested and meets the requirements of the applicable regulations (FAR 43, Appendix F). Each person approving the aircraft for return to service should comply with the provisions of FAR 43.9 regarding content, form, and disposition of the record.

/s/
R.P. Skully
Director, Flight Standards Service