View Full Version : Turbofan Flat Rating

14th May 2006, 16:07
EGT is proportional to the Turbine Inlet Temperature and the TIT indicates how much energy there is to turn the Turbines and hence rotate the Compressors.

Operate the turbofan engine at a particular EGT at ISA conditions now and we get 50,000 lbs Thrust.

Years down the road, we operate it at the SAME EGT and ISA, we still get 50,000 lbs Thrust ?

Mad (Flt) Scientist
14th May 2006, 16:38
Probably not; most engines deteriorate with age, so even though you're using the same energy and getting the same EGT you probably wont get the same thrust (you may also be using more fuel to get to the same EGT, if the combustor efficiency has degraded)

14th May 2006, 17:28
So if we use extra fuel to get the same EGT at ISA, theoretically we still get same thrust as 10 years ago?

Mad (Flt) Scientist
14th May 2006, 18:05
I doubt it; even for the same energy input to the turbines, I doubt they'll extract energy as efficiently as when new (gaps and seals have probably degraded, causing the turbine (and compressor) to lose efficiency.

IIRC, the Speys of UK Phantoms - which were very old by the end of their life - had fuel flow AND thrust penalties large enough to be accounted for in aircraft operations.

Old Smokey
14th May 2006, 23:20
Typically, as a jet engine ages, we experience increasing EGT to deliver the same thrust as in 'new' condition. All of which probably explains why we refer to EPR or N1, not EGT, when setting a 'target' thrust.


Old Smokey

15th May 2006, 14:07
Looking at a high-bypass engine:

As the LPT seals and airfoils deteriorate, N1 will decease over time when running at constant LPT inlet temperature (that's called EGT on many engine models, even though it's not the exhaust temperature). The N1 decrease means that the fan pumps less airflow, so fan thrust is DOWN a bit.

Conversely, the LPT is extracting less energy from the core stream, so the LPT exhaust out the core nozzle is a bit hotter; ergo core thrust is UP a bit.

So it's a tug-of-war; Which will win - core thrust or fan thust? My bet for most engines the total thrust will be down, but not much.

(NOTE where I have specified the EGT measurement point; it's sometimes called ITT or Inter-Turbine Temp. Not all engines are the same!)