Above what altitude does the PMU raise N1 to maintain Np above 80%?

The primary purpose of the PMU (Performance Monitoring Unit) in the T-6A is to optimize engine performance under varying flight conditions. As altitude increases, the air density decreases, leading to a drop in engine performance, specifically in terms of thrust and turbine RPM (Np).

To ensure that the Np remains above 80%, which is critical for safe flight operations and avoiding undesirable engine behavior, the PMU adjusts the N1 (the engine speed) when flying above a certain altitude threshold. At 10,000 feet pressure altitude, the PMU starts increasing N1. This adjustment helps to maintain the engine settings sufficiently to support the required Np, keeping it within operational limits for safe flight.

Thus, the PMU's algorithm is designed specifically such that this transition occurs at 10,000 feet PA, ensuring that the aircraft remains responsive and within safe operational parameters as it ascends into thinner air where engine performance typically wanes.