Learn to Fly
7 day IFR Rating
Transponder Operation - AIM
Reporting (MODE C)
F. Mode C
Operation Under VFR
Codes on Flight Plans
should be aware that proper application of transponder operating
will provide both VFR and IFR aircraft with a higher degree of safety
the environment where high-speed closure rates are possible.
substantially increase the capability of radar to see an aircraft and
MODE C feature enables the controller to quickly determine where
traffic conflicts may exist. Even VFR pilots who are not in contact
ATC will be afforded greater protection from IFR aircraft and VFR
which are receiving traffic advisories. Nevertheless, pilots should
relax their visual scanning vigilance for other aircraft.
Traffic Control Radar Beacon System (ATCRBS) is similar to and
with military coded radar beacon equipment. Civil MODE A is identical
military MODE 3.
and military transponders should be adjusted to the "on" or normal
position prior to moving on the airport surface to ensure the aircraft
is visible to ATC surveillance systems. IN
ALL CASES, WHILE IN CONTROLLED AIRSPACE EACH PILOT OPERATING AN
EQUIPPED WITH AN OPERABLE ATC TRANSPONDER MAINTAINED IN ACCORDANCE WITH
FAR PART 91.413 SHALL OPERATE THE TRANSPONDER, INCLUDING MODE C IF
ON THE APPROPRIATE CODE OR AS ASSIGNED BY ATC. IN CLASS G AIRSPACE, THE
TRANSPONDER SHOULD BE OPERATING WHILE AIRBORNE UNLESS OTHERWISE
on an IFR flight who elects to cancel the IFR flight plan prior to
destination, should adjust the transponder according to VFR operations.
a U.S. OFFSHORE AIRSPACE AREA from outside the U.S., the pilot should
on first radio contact with a U.S. radar ATC facility that such
is available by adding "transponder" to the aircraft identification.
be noted by all users of ATC transponders that the coverage they can
is limited to "line of sight." Low altitude or aircraft antenna
by the aircraft itself may result in reduced range. Range can be
by climbing to a higher altitude. It may be possible to minimize
shielding by locating the antenna where dead spots are only noticed
abnormal flight attitudes.
ATC to utilize one or a combination of the 4096 discrete codes FOUR
CODE DESIGNATION will be used, e.g., code 2100 will be expressed as TWO
ONE ZERO ZERO. Due to the operational characteristics of the rapidly
automated air traffic control system, THE LAST TWO DIGITS OF THE
TRANSPONDER CODE SHOULD ALWAYS READ "00" UNLESS SPECIFICALLY REQUESTED
BY ATC TO BE OTHERWISE.
Altitude Reporting (MODE C)
transponders are equipped with a MODE C automatic altitude reporting
This system converts aircraft altitude in 100 foot increments to coded
digital information which is transmitted together with MODE C framing
to the interrogating radar facility. The manner in which transponder
are designed differs, therefore, a pilot should be thoroughly familiar
with the operation of the transponder so that ATC may realize its full
transponder to reply on the MODE A/3 code specified by ATC and, if
to reply on MODE C with altitude reporting capability activated unless
deactivation is directed by ATC or unless the installed aircraft
has not been tested and calibrated as required by FAR Part 91.217. If
is required by ATC, turn off the altitude reporting feature of your
An instruction by ATC to "STOP ALTITUDE SQUAWK, ALTITUDE DIFFERS
of feet) FEET," may be an indication that your transponder is
incorrect altitude information or that you have an incorrect altimeter
setting. While an incorrect altimeter setting has no effect on the MODE
C altitude information transmitted by your transponder (transponders
preset at 29.92), it would cause you to fly at an actual altitude
from your assigned altitude. When a controller indicates that an
readout is invalid, the pilot should initiate a check to verify that
aircraft altimeter is set correctly.
of aircraft with operating MODE C altitude reporting transponders
report exact altitude or flight level to the nearest hundred foot
when establishing initial contact with an ATC facility. Exact altitude
or flight level reports on initial contact provide ATC with information
that is required prior to using MODE C altitude information for
purposes. This will significantly reduce altitude verification requests.
transponder shall be operated only as specified by ATC. Activate the
feature only upon request of the ATC controller.
making routine code changes, pilots should avoid inadvertent selection
of codes 7500, 7600 or 7700 thereby causing momentary false alarms at
ground facilities. For example, when switching from code 2700 to code
switch first to 2200 then to 7200, NOT to 7700 and then 7200. This
applies to nondiscrete code 7500 and all discrete codes in the 7600 and
7700 series (i.e. 7600-7677, 7700-7777) which will trigger special
in automated facilities. Only nondiscrete code 7500 will be decoded as
the hijack code.
no circumstances should a pilot of a civil aircraft operate the
on Code 7777. This code is reserved for military interceptor operations.
pilots operating VFR or IFR within restricted/warning areas should
their transponders to code 4000 unless another code has been assigned
Mode C Transponder
details concerning requirements to carry and operate Mode C
as well as exceptions and ATC authorized deviations from the
are found in FAR Part 91.215 and FAR Part 99.12.
the FAR requires aircraft to be equipped with Mode C transponders when
or above 10,000 feet MSL over the 48 contiguous states or the District
of Columbia, excluding that airspace below 2,500 feet AGL;
30 miles of a Class B airspace primary airport, below 10,000 feet MSL.
Balloons, gliders, and aircraft not equipped with an engine driven
system are excepted from the above requirements when operating below
floor of Class A airspace and/or; outside of a Class B airspace and
the ceiling of the Class B Airspace (or 10,000 feet MSL, whichever is
and above all Class C airspace, up to 10,000 feet MSL;
10 miles of certain designated airports, excluding that airspace which
is both outside the Class D surface area and below 1,200 feet AGL.
gliders and aircraft not equipped with an engine driven electrical
are excepted from this requirement.
99.12 requires all aircraft flying into, within, or across the
U.S. ADIZ be equipped with a Mode C or Mode S transponder. Balloons,
and aircraft not equipped with an engine driven electrical system are
from this requirement.
shall ensure that their aircraft transponder is operating on an
ATC assigned VFR/IFR code and MODE C when operating in such airspace.
in doubt about the operational status of either feature of your
while airborne, contact the nearest ATC facility or FSS and they will
you what facility you should contact for determining the status of your
requests for "immediate" deviation from the transponder requirement may
be approved by controllers only when the flight will continue IFR or
weather conditions prevent VFR descent and continued VFR flight in
not affected by the FAR. All other requests for deviation should be
by contacting the nearest Flight Service or Air Traffic facility in
or by telephone. The nearest ARTCC will normally be the controlling
and is responsible for coordinating requests involving deviations in
Operation Under Visual Flight Rules (VFR)
otherwise instructed by an Air Traffic Control Facility, adjust
to reply on MODE 3/A code 1200 regardless of altitude.
transponder to reply on MODE C, with altitude reporting capability
if the aircraft is so equipped, unless deactivation is directed by ATC
or unless the installed equipment has not been tested and calibrated as
required by FAR Part 91.217. If deactivation is required and your
is so designed, turn off the altitude reporting switch and continue to
transmit MODE C framing pulses. If this capability does not exist, turn
off MODE C.
Codes on Flight Plans
both civil and military, will use the following phraseology when
to operation of the Air Traffic Control Radar Beacon System (ATCRBS).
by ATC refer only to MODE A/3 or MODE C operation and do not affect the
operation of the transponder on other MODE's.
Operate radar beacon transponder on designated code in MODE A/3.
Engage the "IDENT" feature (military I/P) of the transponder.
(number) and IDENT:
Operate transponder on specified code in MODE A/3 and engage the
(military I/P) feature.
Switch transponder to standby position.
Operate transponder on low or normal sensitivity as specified.
is operated in "NORMAL" position unless ATC specifies "LOW" ("ON" is
instead of "NORMAL" as a master control label on some types of
Activate MODE C with automatic altitude reporting.
Turn off altitude reporting switch and continue transmitting MODE C
pulses. If your equipment does not have this capability, turn off MODE
SQUAWK (mode in use):
Switch off specified mode. (Used for military aircraft when the
is unaware of military service requirements for the aircraft to
operation on another MODE.)
Switch off transponder.
Operate transponder in the emergency position (MODE A Code 7700 for
transponder. MODE 3 Code 7700 and emergency feature for military
Operate radar beacon transponder on code 1200 in the MODE A/3, or other
appropriate VFR code.
with Mode C
no Mode C
with Mode C
no Mode C
with Mode C
no Mode C
VOR/DME, or INS, Transponder with Mode C
or INS, Transponder
with no Mode C
or INS with
Positioning System (GPS)/Global navigation Satellite System (GNSS)
aircraft with en route, terminal, and GPS approach capability.
Date: Saturday, 18 February, 2012 11:19 Subject: Change to
From: Nick Modders, CFI/CFII
Comments: Darren, Reference your material about transponders
[and] the change in transponder operations as stated in AIM 4-1-20.
This has recently been changed in the AIM. The procedure is now to turn
on the transponder and altitude reporting before moving from parking
and don't turn it off until you are parked. Yours, Nick