What indication is specifically noted during a No Light Off?

During a No Light Off scenario, the most pertinent indication is the absence of an increase in Interstage Turbine Temperature (ITT). This situation typically occurs during the engine start sequence when the fuel is introduced into the combustion chamber, but the engine fails to light off, indicating that the combustion is not occurring as expected.

An increase in ITT is expected when the engine starts properly because this temperature rises as the fuel ignites and begins to combust. If there is no increase in ITT, it suggests that the fuel is not igniting, which can lead to further complications as the start process continues.

The other indications, while they may be relevant in their own contexts, do not specifically signal the problem at hand. For instance, a decrease in N2 speed could be a result of other issues unrelated to an engine start failure, and visual abnormalities in gauges or smoke in the cockpit may indicate a broader range of operational problems rather than pinpointing the No Light Off condition itself. Thus, the absence of ITT rise is a clear and direct indication of a No Light Off event, making this choice the most accurate reflection of the situation.