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Detonation in the context of aircraft engines refers to an uncontrolled, explosive ignition of the fuel/air mixture within the combustion chamber. This phenomenon occurs when the pressure and temperature reach levels that cause the fuel to ignite abnormally, resulting in a rapid and violent release of energy. This explosive type of combustion can lead to severe engine damage, increased cylinder pressure, and potential engine failure.

In conventional engine operation, combustion is expected to happen in a controlled manner, where the fuel/air mixture ignites smoothly and progressively. However, when detonation occurs, it disrupts the normal cycle of combustion, leading to an increase in temperatures and pressures that the engine components are not designed to handle, which is detrimental to overall performance and safety.

The other options describe different combustion phenomena rather than detonation itself. Controlled ignition represents the ideal combustion process, while normal combustion refers to the expected operation of an engine. Ignition caused by an external spark source is distinct from detonation, as it pertains to how the fuel/air mixture is initiated rather than the nature of its burning. Thus, recognizing detonation as an uncontrolled event is critical for understanding engine operations and preventing potential damage.