What is the maximum duration for which Ng can be at 104%?

The maximum duration for which Ng can be at 104% is indeed classified as having no limit. This means that the engine can operate at this Ng level continuously without a specified time constraint. It's important to understand that operational limits for various engine parameters, including Ng (the gas generator speed), are designed based on the engine's design and operational capabilities.

While there may be guidelines and recommended practices for engine operations, running at 104% Ng is acceptable indefinitely under the specific operational conditions outlined for Part 135 operations. Continuous operation allows for flexibility in certain flight profiles, ensuring that the aircraft can perform optimally without the need for strict adherence to a time restriction for that Ng level.

This approach to engine performance management is grounded in the manufacturer's recommendations and the established operational limits for the aircraft type being used. Therefore, knowing that there is no limit presents a clear understanding for pilots and operators, emphasizing the importance of adhering to these operational specifications for safety and efficiency in flight operations.