Under which condition can instruments be unserviceable in part 135 operation?

In the context of Part 135 operations, instruments can be deemed unserviceable under specific conditions that allow for flexibility in operation while ensuring safety and compliance. When considering the correct answer, it is essential to understand the rationale behind each of the conditions provided.

Firstly, approval by CASA (Civil Aviation Safety Authority) is significant because regulatory agencies like CASA establish standards and practices that govern aircraft operations. If CASA has granted permission for certain instruments to be unserviceable, it provides a regulatory framework ensuring that safety is not compromised while allowing operators to maintain serviceability.

Secondly, a Minimum Equipment List (MEL) is critical in this context as it specifies the instruments and equipment that must be operational for a flight to commence. If an instrument is listed in the MEL, and its unserviceability does not compromise the safety or legality of the operation, then the flight can proceed with that equipment inoperative.

Lastly, items classified as being for passenger convenience only can be unserviceable without affecting the safety or operational capability of the aircraft. For instance, features that enhance passenger comfort do not directly impact the flight's safety protocols, and their failure does not necessitate grounding the aircraft.

Given this understanding, each of these conditions aligns with regulatory standards and operational flexibility established