AIM

8/15/19

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Services Available to Pilots

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
incorrect altimeter setting. While an incorrect
altimeter setting has no effect on the Mode C altitude
information transmitted by your transponder
(transponders are preset at 29.92), it would cause you
to fly at an actual altitude different from your
assigned altitude. When a controller indicates that an
altitude readout is invalid, the pilot should initiate a
check to verify that the aircraft altimeter is set
correctly.

3.

Pilots of aircraft with operating Mode C

altitude reporting transponders should report exact
altitude or flight level to the nearest hundred foot
increment 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 separation purposes. This will significantly
reduce altitude verification requests.

d. Transponder IDENT Feature

1.

The transponder must be operated only as

specified by ATC. Activate the “IDENT” feature only
upon request of the ATC controller.

e. Code Changes

1.

When making routine code changes, pilots

should avoid inadvertent selection of Codes 7500,
7600 or 7700 thereby causing momentary false
alarms at automated ground facilities. For example,
when switching from Code 2700 to Code 7200,
switch first to 2200 then to 7200, NOT to 7700 and
then 7200. This procedure applies to nondiscrete
Code 7500 and all discrete codes in the 7600 and 7700
series (i.e., 7600

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7677, 7700

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7777) which will

trigger special indicators in automated facilities.
Only nondiscrete Code 7500 will be decoded as the
hijack code.

2.

Under no circumstances should a pilot of a

civil aircraft operate the transponder on Code 7777.
This code is reserved for military interceptor
operations.

3.

Military pilots operating VFR or IFR within

restricted/warning areas should adjust their transpon-
ders to Code 4000 unless another code has been
assigned by ATC.

f. Mode C Transponder Requirements

1.

Specific details concerning requirements to

carry and operate Mode C transponders, as well as
exceptions and ATC authorized deviations from the
requirements are found in 14 CFR Section 91.215 and
14 CFR Section 99.12.

2.

In general, the CFRs require aircraft to be

equipped with Mode C transponders when operating:

(a)

At or above 10,000 feet MSL over the

48 contiguous states or the District of Columbia,
excluding that airspace below 2,500 feet AGL;

(b)

Within 30 miles of a Class B airspace

primary airport, below 10,000 feet MSL. Balloons,
gliders, and aircraft not equipped with an engine
driven electrical system are excepted from the above
requirements when operating below the floor of
Class A airspace and/or; outside of a Class B airspace
and below the ceiling of the Class B airspace (or
10,000 feet MSL, whichever is lower);

(c)

Within and above all Class C airspace, up

to 10,000 feet MSL;