The information below assumes that your airplane isn't a balloon or glider.
Mode A. Sometimes referred to as mode 3/A. Civil Mode A is identical to military Mode 3. Mode A responds to an ATC interrogation signal with the transponder code set by the pilot.
Mode C. Refers to aircraft equipped with an altitude encoder and altimeter. With Mode C, ATC will actually see the flight level altitude on their radar screen if the transponder is operating in the Mode C or "ALT" (altitude) Mode.
Mode S. Mode S is a possible platform for a variety of other applications, such as Traffic Information Service (TIS), Graphic Weather Service, and Automatic Dependent Surveillance-Broadcast (ADS-B). Under ADS-B, each aircraft periodically broadcasts its identification, position, and altitude. Overall, Mode S provides improved surveillance quality, discrete aircraft addressing function, and digital capability. Mode S is not required for general aviation aircraft.
According to the AIM, Section 4-1-19: In all cases, while in controlled airspace, each pilot operating an aircraft equipped with an operable ATC transponder maintained in accordance with 14 CFR section 91.413 shall operate the transponder, including Mode C if installed, on the appropriate code or as assigned by ATC.
The following areas require the operation of a Mode C transponder:
All aircraft are required to be equipped with a Mode C transponder when flying 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.
According to 14 CFR 99.13, no person may operate an aircraft into or out of the United States, or into, within, or across an ADIZ designated in subpart B unless operating a transponder with Mode C. Certain exemptions might apply to aircraft that were not originally certified with an engine-driven electrical system; see 99.13(d).
Aircraft not originally certificated with an engine-driven electrical system or subsequently have not been certified with such a system installed, balloons, or gliders may conduct operations:
In the airspace within 30 nautical miles of the listed airports as long as operations are conducted:
According to 14 CFR 91.215(d), requests for deviations must be made to the ATC facility having jurisdiction over the concerned airspace within the time periods specified as follows:
For flying into a Mode C veil without an operable transponder, the pilot needs to telephone the appropriate radar facility for the Class B airspace and ask for permission to make the flight. Upon agreeing to conditions (including direction of flight and altitude), the pilot will be given a code number that he will mention to the controller upon initial radio contact. This is the same procedure that a pilot with an inoperative transponder/encoder would use to fly in or out of the Mode C-veil airports for avionics repair.
The situation may be slightly different if the pilot is landing at a satellite Class D (towered controlled) airport within the veil but outside of Class B airspace. The approval is still given by the controlling radar facility via telephone. The radar facility may still issue the code number but may only require the pilot to contact the tower in the Class D airspace.
NOTE: You should not expect approvals at the busiest of Class B airports during their peak times or under difficult weather conditions, but if this telephone procedure can expand the utilization of your aircraft occasionally, then by all means, phone to find if you can "fit into" the system.
According to 14 CFR 91.413, a transponder may not be used for the above purposes unless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with appendix F of FAR Part 43.
If a transponder is installed or the maintenance of a transponder may introduce errors, the transponder must be inspected and found to comply with paragraph (c), appendix E, of FAR Part 43. The tests and inspections must be conducted by a properly equipped repair station certified in accordance with 91.415(c)(1), the holder of a continuous airworthiness maintenance program under Part 121 or 135, or the manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by the manufacturer.
Operations within 30 nm of the following airports require an operable transponder-ON (excluding exempted aircraft):
And, of course, you can always call AOPA for help or advice with any aeronautical problem:
AOPA Pilot Information Center 800/USA-AOPA (800/872-2672)
Updated Tuesday, October 24, 2006 11:09:27 AM
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