Transponder and Altimeter TSO Requirements

by Tim
(Lakewood, NY USA)

Can you tell me where the regulation is that states a transponder must be TSO'd?

Also for a Transponder installation with altitude encoding would the same installation require that the altimeter also be TSO'd?

ANSWER: Yes to the first question, it is FAR 91.215. No to the second question, refer to FAR 91.217. I have posted excerpts from the two regulations below:

FAR 91.215 says -

(a) All airspace: U.S.-registered civil aircraft. For operations not conducted under part 121 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate, or the appropriate class of TSO-C112 (Mode S).

(b) All airspace. Unless otherwise authorized or directed by ATC, no person may operate an aircraft in the airspace described in paragraphs (b)(1) through (b)(5) of this section, unless that aircraft is equipped with an operable coded radar beacon transponder having either Mode 3/A 4096 code capability, replying to Mode 3/A interrogations with the code specified by ATC, or a Mode S capability, replying to Mode 3/A interrogations with the code specified by ATC and intermode and Mode S interrogations in accordance with the applicable provisions specified in TSO C–112, and that aircraft is equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments.

FAR 91.217 says -

(a) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder -

(2) Unless, as installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95 percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft; or

(3) Unless the altimeters and digitizers in that equipment meet the standards of TSO-C10b and TSO-C88, respectively.

Keep in mind that even though an altimeter need not be TSO'd as mentioned above, it must have some type of production approval to be eligible for installation in a type certificated aircraft. Refer to FAR 21.303