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AIM

10/12/17

4−1−16

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., 7600−7677, 7700−7777) 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;