In a Bell 407 the power assurance check takes into account torque, pressure altitude and MGT (measured gas temperature aka TOT). The flight manual states that if MGT when tested is less than or equal to the charted MGT, engine performance equals or exceeds minimum specifications. So my question is how does MGT factor into the health of an engine. Torque gauge meters off oil pressure, MGT is a result of compressor performance or bleed air systems. So an engine could be a plus engine with let’s say bad compressor vanes, or a bad bleed air valve etc. So my initial question is what is the correlation between MGT and engine health.
Think of it this way. When an engine is temperature limited (as opposed to being torque limited) it means that the power control can be advanced (thus making more power) until the max specified engine temperature is reached. if one engine produces a particular amount of power while at a lower temperature than a second engine when producing the same amount of power then that first engine has more “room” to further increase power than the second one does. Thus, the first one performs better. In other words, the cooler the engine is at a particular power output level the better the engine. The manufacturer puts out engine performance tables that specify what the nominal engine performance should be. If the engine runs cooler than that then it is better than spec.
The difference between the max ITT/TOT/EGT and the measured value is "ITT/TOT/EGT margin". The margin is highest when the engine is new, and declines as the engine wears out. As compressor and turbine blades erode, efficiency declines and more fuel is burned to make the same torque or N1 or EPR, so temps go up. When the margin gets to some value close to zero, that is, the measured temperature is close to or at the engines limit, the engine is worn out. The power assurance run is mostly to monitor and track the margin over time.