Since November, 2019, many operators have been required to remove STC information and deferral capability from their MEL due to the recent issuance of Policy Letter 109. See http://fsims.faa.gov/PICDetail.aspx?docId=PL-109. In this letter, the addition of STCs within the MMEL/MEL framework will be allowed only if the STC holder applies for and receives an STC relief approval letter from the responsible AEG (Aircraft Evaluation Group). Some within the Principle Inspector community understood this to mean all previously included STCs were to be removed prior to their evaluation under the approval letter process. This was going to have dire consequences for the entire aviation community, if not solely for the reason of recently added/required ADS-B equipment.

Thankfully, the FAA released a National Policy Letter N 8900.528 to clarify that previously included STCs in the MMEL do not have to have approval letters to be used by the operators. Certain active producers of STCs may transition to approval letters over time, but there is no required timeframe, as of this time. Many STCs included in the MMEL which no longer have active companies supporting them can continue to be used within the MMEL, as they are considered previously evaluated under the FOEB (Flight Operations Evaluation Board) process prior to their inclusion into the MMEL.

Bottom line: Any STC within the MMEL is fair game to use as an operator (unless your PI specifically sites safety concerns). Any new STCs will have to go through the approval letter process and be added to the MMEL by the operator desiring that relief.