AIM
4/3/14
4−1−15
Services Available to Pilots
4
−
1
−
20. Transponder Operation
a. General
1.
Pilots should be aware that proper application
of transponder operating procedures will provide
both VFR and IFR aircraft with a higher degree of
safety in the environment where high-speed closure
rates are possible. Transponders substantially in-
crease the capability of radar to see an aircraft and the
Mode C feature enables the controller to quickly
determine where potential traffic conflicts may exist.
Even VFR pilots who are not in contact with ATC will
be afforded greater protection from IFR aircraft and
VFR aircraft which are receiving traffic advisories.
Nevertheless, pilots should never relax their visual
scanning vigilance for other aircraft.
2.
Air Traffic Control Radar Beacon System
(ATCRBS) is similar to and compatible with military
coded radar beacon equipment. Civil Mode A is
identical to military Mode 3.
3.
Civil and military transponders should be
turned to the “on” or normal altitude reporting
position prior to moving on the airport surface to
ensure the aircraft is visible to ATC surveillance
systems. IN ALL CASES, WHILE IN CON-
TROLLED AIRSPACE EACH PILOT
OPERATING AN AIRCRAFT EQUIPPED WITH
AN OPERABLE ATC TRANSPONDER MAIN-
TAINED IN ACCORDANCE WITH 14 CFR
SECTION 91.413 MUST OPERATE THE TRANS-
PONDER, INCLUDING MODE C IF INSTALLED,
ON THE APPROPRIATE CODE OR AS AS-
SIGNED BY ATC. IN CLASS G AIRSPACE, THE
TRANSPONDER SHOULD BE OPERATING
WHILE AIRBORNE UNLESS OTHERWISE RE-
QUESTED BY ATC.
4.
A pilot on an IFR flight who elects to cancel
the IFR flight plan prior to reaching destination,
should adjust the transponder according to VFR
operations.
5.
If entering a U.S. OFFSHORE AIRSPACE
AREA from outside the U.S., the pilot should advise
on first radio contact with a U.S. radar ATC facility
that such equipment is available by adding
“transponder” to the aircraft identification.
6.
It should be noted by all users of ATC
transponders that the coverage they can expect is
limited to “line of sight.” Low altitude or aircraft
antenna shielding by the aircraft itself may result in
reduced range. Range can be improved by climbing
to a higher altitude. It may be possible to minimize
antenna shielding by locating the antenna where dead
spots are only noticed during abnormal flight
attitudes.
7.
Aircraft equipped with ADS−B (1090 ES or
UAT) must operate the equipment in the transmit
mode (on position) at all times while on any airport
surface.
NOTE
−
Pilots of aircraft equipped with ADS
−B should refer to
AIM, Automatic Dependant Surveillance
− Broadcast
Services, Paragraph 4
−5−7, for a complete description of
operating limitations and procedures.
b. Transponder Code Designation
1.
For ATC to utilize one or a combination of the
4096 discrete codes FOUR DIGIT CODE DES-
IGNATION will be used, e.g., code 2100 will be
expressed as TWO ONE ZERO ZERO. Due to the
operational characteristics of the rapidly expanding
automated ATC system, THE LAST TWO DIGITS
OF THE SELECTED TRANSPONDER CODE
SHOULD ALWAYS READ “00” UNLESS SPECIF-
ICALLY REQUESTED BY ATC TO BE
OTHERWISE.
c. Automatic Altitude Reporting (Mode C)
1.
Some transponders are equipped with a
Mode C automatic altitude reporting capability. This
system converts aircraft altitude in 100 foot
increments to coded digital information which is
transmitted together with Mode C framing pulses to
the interrogating radar facility. The manner in which
transponder panels are designed differs, therefore, a
pilot should be thoroughly familiar with the operation
of the transponder so that ATC may realize its full
capabilities.
2.
Adjust transponder to reply on the Mode A/3
code specified by ATC and, if equipped, to reply on
Mode C with altitude reporting capability activated
unless deactivation is directed by ATC or unless the
installed aircraft equipment has not been tested and
calibrated as required by 14 CFR Section 91.217. If
deactivation is required by ATC, turn off the altitude
reporting feature of your transponder. An instruction
by ATC to “STOP ALTITUDE SQUAWK, ALTI-
TUDE DIFFERS (number of feet) FEET,” may be an
indication that your transponder is transmitting
incorrect altitude information or that you have an