What is the maximum Np% with the PMU off?

• A. 100 +/- 2
• B. 102 +/- 2
• C. 104 +/- 3
• D. 100 +/- 5

Answer: (A)

The correct answer is 100 +/- 2, which represents the maximum Ng% or Np% (engine output power) when the PMU (Propulsion Management Unit) is turned off.

When the PMU is disengaged, the engine's control system reverts to a more manual, pilot-controlled mode. This mode allows for certain parameters to be controlled without the limitations imposed by the PMU; however, it still has maximum thresholds to ensure safety and performance reliability.

In this specific case, having the PMU off allows the engine to operate effectively up to 100% Np with a tolerance of plus or minus 2%, which accounts for small variations in engine performance that can occur under different operating conditions. This tolerance ensures that pilots can still operate effectively within safe limits, without exceeding what is considered optimal for engine performance, thereby reducing the risk of damage or failure.

Understanding why the other percentages are incorrect is critical; they suggest higher thresholds that would not align with the safe operational limits when the PMU is off. Therefore, 100 +/- 2 ensures pilots have the correct margin to safely manage engine power output without risking excess that could lead to complications during operation.