Date: 202012
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Reporter organization: Air Carrier
Reporter function: Lead Technician; Technician; Inspector
Reporter qualification: Maintenance Repairman; Maintenance Avionics
Second reporter function: Inspector; Technician; Lead Technician
Anomaly: Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Contributing factors: Manuals; Human Factors; Chart Or Publication; Procedure
Primary problem: Human Factors

Narrative: On [Date 1]; I was reviewing paperwork on the current Aircraft X in the [Maintenance Visit] in ZZZ and noticed that a robbed transponder on Aircraft X was installed the previous night and did not get a signoff in the logbook releasing the aircraft for RVSM (Reduced Vertical Separation Minimum) operation. This aircraft was still scheduled to be in the [Maintenance Visit] for another 5 days so I just had the nightshift avionics tech complete the entry the next day. But I knew that the avionics tech that forgot the RVSM signoff on the [Maintenance Visit] also replaced a transponder on Aircraft Y during the overnight on [Date 2]. I pulled the paperwork from the night of [Date 2] and found that the #2 Transponder was changed to clear a MEL (Minimum Equipment List); but a RVSM release was not done in the logbook. I called Maintenance Control and told them what happened and Aircraft Y was inbound to ZZZ1 at the time so when it landed in ZZZ1; Maintenance went out and deferred the RVSM. The ZZZ Avionics tech that replaced the transponder is [Name].Found during paperwork reviewShould of; but did not realize the transponder is RVSMCalled Maintenance Control and had the system deferred (RVSM)[Parts Document] lists the PN (Part Number) of the transponders that go on our aircraft that are post ADS-B (Automatic Dependent Surveillance-Broadcast) mod. This PN [XXX] is not listed in the aircraft IPC (Illustrated Parts Catalog); so there is no statement (RVSM Critical Item) like all the other transponder PN's listed in the IPC. This statement (RVSM Critical Item) should be added to the [Parts Document]; to bring attention to it like the PN's listed in the aircraft IPC.


Second narrative: On [Date 2] I was assigned to clear a Transponder #2 MEL (Minimum Equipment List) on Aircraft Y. I determined it needed a new transponder; however; we did not have the proper transponder in stock required for ADS-B (Automatic Dependent Surveillance-Broadcast) mod aircraft. I was directed to rob the transponder from Aircraft X which was the [Maintenance Visit]; after robbing (from Aircraft X) and installing (on Aircraft Y) the transponder; I went through the operation checks and everything was good. I went to sign off the paperwork and seen; from the beginning of Chapter 34 in the AMM (Aircraft Maintenance Manual); that it was not CAT II (Category 2). I forgot that RVSM (Reduced Vertical Separation Minimum) isn't included in the beginning of ATA (Air Transport Association) chapters and since I picked the part number based on [Number] I didn't see the RVSM reminder note seen in the AIPC (Aircraft Illustrated Parts Catalog). I went to open the GMM (General Maintenance Manual) on the tech computer I was using and it wouldn't open. It was known within the base that PDF's can act up on the 2 provided tech computers. My ignorance of not remembering that RVSM wasn't included in the beginnings of ATA chapters; and not going into AIPC; and assuming that since RVSM wasn't downgraded it wasn't an RVSM component; along with my laziness of not confirming with a lead or Supervisor on their computers contributed to missing the RVSM release sign off. The plane made at least one flight with out having the RVSM release. The next night I installed new transponder in Aircraft X and Operations checked everything and since I assumed it wasn't RVSM the day before I repeated my mistake for the [Maintenance Visit on Aircraft X]; and didn't write up a RVSM release sign off. It was caught before the aircraft left and the other avionics technician went through the operation checks again and provided the RVSM release sign off.The day shift Supervisor caught the [Maintenance Visit] not having the release and knew that I did the aircraft the night before and seen that I didn't release that one either and took corrective action.I didn't put max effort into looking into seeing if the component was RVSM critical. I went off of a handful of assumptions and didn't double check with my leads or Supervisor. The [Number] didn't reiterate it was an RVSM critical item; the GMM PDF didn't want to load on the tech computers; RVSM isn't located at the beginning of ATA chapters like CAT II and RII (Required Inspection Item) are.Put RVSM in beginning ATA chapters like CAT II and RII. Add a reminder in [Number] that the transponder is RVSM critical component. Remove old transponders from stock that can't be used with ADS-B aircraft seeing that our whole fleet has ADS-B mod. Leadership verified wrong part number in stock the night before contributing to this issue being on 2 aircraft due to robbery. Update/upgrade/fix tech computers so we can open the files we need.


Synopsis: Aircraft Maintenance Technician reports that component was installed on aircraft without proper signoff.