Shop Talk: Mode S Transponders

April 9, 2010

Wish you had a better understanding of the regulations when talking to your mechanic or the avionics shop? Aircraft Electronics Association Vice President of Government/Industry Affairs Ric Peri answers your frequently asked questions.


Question: Does 135.143(c) require that Part 135 aircraft with operational Mode C transponders installed replace them with Mode S transponders before March 1, 2007?

Answer: No, not until the installed Mode C transponder can no longer be repaired.

Under paragraphs (c) of Sec. 121.345 and Sec. 135.143, after Jan. 1, 1992, only Mode S transponders may be newly installed in U.S. registered civil aircraft.

Since May 1996, the FAA has regularly granted exemptions to Part 135 operators for this requirement.

On Oct. 7, 2005, the FAA issued a policy on the installation of Mode S transponder requirements. This policy announced that the FAA would discontinue issuing exemptions from the Mode S transponder equipment requirements under Title 14 of the Code of Federal Regulations Sec. 121.345(c) and Sec.135.143(c).

Therefore, Sec. 135.143(c) is the regulatory requirement for the installation of a transponder for aircraft operating under Part 135.

Under these regulations, the term “installation” does not include:

(1) Temporary installation of a Mode C transponder or substitute equipment as appropriate, during maintenance of the permanent equipment;

(2) Reinstallation of equipment after temporary removal for maintenance; or

(3) For fleet operations, installation of equipment in a fleet aircraft after removal of the equipment for maintenance from another aircraft in the same operator's fleet.

The FAA proposed that effective March 1, 2007, if a transponder needs to be permanently replaced it must be replaced with a Mode S transponder.

The policy is very clear that this does not mean that effective March 1, 2007, operators are required to install Mode S transponders, if they have an operable and appropriate Mode C or Mode A transponder in the aircraft. The current regulation only requires the replacement of a Mode A or C transponder with Mode S when the existing transponder can no longer be repaired.

In addition, the FAA noted that if the operator was issued an exemption before March 1, 2007, allowing the operator to install a Mode C transponder on their aircraft, they may continue to use that transponder until it no longer can be repaired and only then must it be replaced with a Mode S transponder.