Which of the following is NOT a reason to abort an engine start?

The correct answer is that observing a normal N1 increase is not a reason to abort an engine start. During the engine start sequence, one of the key indicators of a successful start is the increase in N1. N1, which represents the rotational speed of the low-pressure compressor and fan in a turbojet engine, generally increases as the engine begins operating normally. If N1 is responding appropriately, it indicates that the engine is successfully starting up, which suggests that the combustion process is functioning as intended and everything is progressing normally.

In contrast, the other options indicate potential issues that could jeopardize the safety and functionality of the engine during start. An ITT (Interstage Turbine Temperature) rate exceeding 1000 °C suggests excessive heat, which could lead to engine damage or failure. The absence of an ITT rise after 10 seconds may indicate that the engine is not igniting or is in a failed condition. Lastly, if the BAT BUS annunciator illuminates, it signals a problem with the battery bus system, which could affect engine operation or instrumentation. Each of these issues could lead to severe consequences if not addressed, thus warranting an abort of the engine start to ensure safety.