Originally Posted by
lomapaseo
Pretty hard to notice pure thrust differences of such a small magnitude beyond what's auto-controled
What's noticeable and documented is the EGT and fuel burn (over hours) with a dirty fan.
Most effects are due to bug spatter which on some routes and days is quite noticeable on walk-arounds. Even quite difficult to tell from a small bird-strike with no damage.
Normally, results only in increased % burn rather than a decrease in output. My type, we set an EPR on takeoff, which is computed from a lot of sources, including FADEC engine specific data tables tracking each engines historic performance. We should get the rated thrust in identical conditions for a dirty engine over a clean one, only difference will be a hotter EGT/MGT. May be different for types using an N1 value for departure.
The health and particularly the fuel efficiency is trended, once it crosses a threshold, its time to get the ground coconut (that's what they use to clean them btw) out to give the donks a comp wash.
Different story if the engine is getting long in the tooth. As the efficiency drops, the EGT required to hit a specific EPR will climb, this is do with lots of things, turbine wear, corrosion, blade creep, sub-optimal clearances. This means often the engine will be monitored on wing and you may get cautions for a high EGT on takeoff, particularly if hot n high. The manufacturer publishes different limits for these engines as they are under constant monitoring by ACARS or download to ensure that there is nothing seriously wrong with the donk.