Which of the following may indicate a malfunction in a turbine engine?

Unusual exhaust smoke color is a significant indicator of a malfunction in a turbine engine. The color of the exhaust smoke can reveal a lot about the engine's performance and condition. For instance, blue smoke often indicates oil burning, while black smoke can suggest rich fuel mixtures, and white smoke might indicate fuel dilution or water intrusion. These atypical colors can signal underlying issues, such as a malfunctioning fuel control system, issues with lubrication, or problems related to combustion efficiency.

This contrasts with regular engine vibrations, which may be common during normal operation due to various factors, or stable engine noises, which typically indicate that the engine is functioning within expected parameters. Similarly, increased fuel efficiency is usually a sign of good performance or optimization, rather than an indication of a problem. Therefore, observing unusual exhaust smoke color provides critical information that could point to serious mechanical issues needing immediate attention.