A321
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural FAR
General Maintenance Action
Procedure; Aircraft
We were preparing to fly and the display on the transponder was inoperative. A typical digital display for a single number is made up of seven dashes in the form of a boxy figure eight. The middle dash was not working and one could not read the number that would be put into the first digit of a four digit transponder code. So a number 3 would look like a backwards C.We contacted dispatch who contacted Maintenance and a mechanic came to the airplane. He applied MEL 34-XX-XXX. The MEL says that zero transponders are required as long as certain conditions are met: May be inoperative provided: a) Operations do not require its use; b) Prior to flight; approval is obtained from ATC facilities having jurisdiction over planned route of flight; and c) Apply MEL 02-XX-XXX.We avoided operations requiring its use in A. We met the standards of section B by having dispatch contact the appropriate ATC's. And C applies to avoiding RVSM airspace so we filed and flew at FL280.  At the bottom of the procedure was the following NOTE: 14 CFR 91.215(b) requires [the company] to have one serviceable ATC Transponder operative for dispatch at all times; unless otherwise specified per 14 CFR 91.215(d) or other appropriated regulations for authorized deviationsI am not sure we were in compliance with the note as we did not have a serviceable transponder due to the MEL. And we had no way to read the language of 14 CFR 91.215(d) as that is not referenced in any of our guidance. Further; we had no guidance as to what we were supposed to do with the transponder during flight. Keep it in Standby; TA; or TA/RA. There is also the question about what we would do if we had to squawk an emergency code such as 7500; 7600; or 7700.Pilots need to have a copy of the CFR's in our compliance reference materials. The [company] safety team should discuss whether or not it is safe to fly without an operable transponder on a transport category aircraft in revenue service and disseminate those findings to the pilot group. We should have written guidance on what mode to operate the transponder in that has been MEL'd and what to do in case of needing to squawk an emergency type code.
A321 pilot reported that the transponder was deferred due to a missing digit.