Shop Talk: Mode S transponders

February 12, 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)(2) allow for the exchange of Mode A/C transponders in Part 135 aircraft?

Answer: No. Section 135.143 (c)(2) allows for an operator to continue the use of his or her Mode A/C transponder as long as it can be maintained. However, if it can no longer be maintained it must be replaced with a Mode S transponder. The rule does not allow for exchanges of the transponder as a method of maintaining equipment except when maintenance is being accomplished.

What does Section 135.143 (c)(2) actually state?

Section 135.143 General requirements.

(c) ATC transponder equipment installed within the time periods indicated below must meet the performance and environmental requirements of the following TSOs [Technical Standard Orders]:

(2) After January 1, 1992: The appropriate class of TSO-C112 (Mode S). For purposes of paragraph (c)(2) of this section, "installation" does not include -

  • (i) Temporary installation of TSO-C74b or TSO-C74c substitute equipment, as appropriate, during maintenance of the permanent equipment;
  • (ii) Reinstallation of equipment after temporary removal for maintenance; or
  • (iii) 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

To answer the exchange as a function of maintenance question, you have to go back to the original Feb. 3, 1987 rule. In the Federal Register notice (52 FR 3280) under the “Summary of FAA Actions” section, the FAA states that “The requirement to meet the Mode S TSO after January 1, 1992, does not apply to – (a) A transponder which met the requirements of the rule when originally installed, and which is removed from an aircraft for maintenance and then reinstalled on the aircraft from which it was removed.”

The rule does allow for a Mode A/C transponder to be exchanged temporarily when the permanent transponder is removed for maintenance as long as the permanent equipment is reinstalled following maintenance.