Aircraft operating under 14 CFR Part 135 that are required to have a third gyroscopic bank-and-pitch indicator installed are:
1. Multi-engine aircraft with six or more passenger seats.
2. All helicopters, regardless of the number of passenger seats.
According to the Federal Aviation Regulations (FARs) Part 135.105, all multi-engine aircraft with six or more passenger seats are required to have a third gyroscopic bank-and-pitch indicator installed. This is to ensure that the aircraft has redundancy in its instrumentation in case one of the indicators fails.
Additionally, FARs Part 135.144 requires all helicopters to be equipped with a gyroscopic bank-and-pitch indicator. This is because helicopters have a unique flight envelope and require specialized instrumentation to ensure safe operation.
It is important to note that these requirements may be subject to change, and it is the responsibility of the aircraft operator to ensure that their aircraft meets all applicable regulations and requirements.