Thrust degradation from dirty fan

[orion1210]

We are all aware that gas turbine engine performance can be reduced as a result of compressor and turbine deposits in terms of increased fuel burn resulting in increased turbine temperatures.

Aside from this, I wondered if anybody could put any figures on potentially reduced thrust output or the resulting cruise speed penalty as a result of a dirty and therefore a less efficient fan, assuming that a given N1 can still be obtained without being turbine temp limited?

Thanks

[lomapaseo]

We are all aware that gas turbine engine performance can be reduced as a result of compressor and turbine deposits in terms of increased fuel burn resulting in increased turbine temperatures.

Aside from this, I wondered if anybody could put any figures on potentially reduced thrust output or the resulting cruise speed penalty as a result of a dirty and therefore a less efficient fan, assuming that a given N1 can still be obtained without being turbine temp limited?

Thanks

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.

[VinRouge]

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.

[orion1210]

Thanks for your replies.

I’ve seen ITT increases as a result of dirty fans and I understand the reasons behind this. In my mind the fan is essentially multiple aerofoils, which when dirty create more drag which in turn requires more energy to reach the desired N1, hence increased fuel burn.

What I’m really interested in is for a given N1, is the resultant thrust less?

When I think about this deeper, a dirty/contaminated wing creates more drag/less lift, to maintain level flight, in simple terms is counteracted by an increased AoA which in turn is balanced with an increased energy input in the form of a higher engine power setting. So in in the case of a dirty engine fan, this additional energy input to maintain target N1 is additional fuel, but again surely this results in less thrust... or are the aerodynamics involved in the fan far less susceptible to these kinds of losses and are therefore overall negligible?

[orion1210]

Mods: please move to techlog, apologies.

[VinRouge]

Mods: please move to techlog, apologies.

Orion, it may well be that compressor degredation due to caccrued grunge is accounted for in the Measured to Gross performance factors applied to performance charts and further accounted for in Gross to Net as part of the safety factors applied. It is logical that a contaminated/dirty compressor will provide less total thrust for a given N1 than one fresh out of the shop. How much, i guess only the manufacturer will know through in service life. May have the same issue with EPR controlled engines which use the pressure ratio at the front of the fan to the hot efflux as a ratio, rather cold fan input/output.

[tdracer]

When the engine/aircraft combination is certified, there are 'factors' applied to address normal engine to engine variability (often referred to as 'minimum engine'), and the aircraft power setting charts take into account thrust uncertainties due to external effects (humidity is a big one at higher temps - high humidity reduces the air density which in turn reduces thrust at N1). As a general rule, there is less variability in thrust at EPR than for thrust at N1, so N1 engines end up with larger 'min engine' factors (it's also why, when operating an EPR in alternate N1 mode, you may have a performance penalty).
All that being said, there are cases where engines start moving outside the 'normal' variability range. That happened a while back on the PW2000 - it was discovered that the way the leading edge of the fan blades abraded on high time engines caused a significant loss of thrust at EPR. As a result, a performance penalty had to be applied until the fan blades could be re-profiled to regain the original performance.

[lomapaseo]

What I’m really interested in is for a given N1, is the resultant thrust less?

For the large by-pass machines the dirt, grundge builds up on the slow surface speed part of the fan which is the inner concave surface nearest the blade root and directly in line with the compressor inlet. This suggests that of it that gets sucked in at low taxi speeds will also partly accumulate behind the fan as well.

Obviously at higher fan speeds under power most of this will be centrifuged out of the fan discharge.

Meanwhile any accumulation on the working parts of the blade will affect the mass flow rate through the engine for a given RPM.

The control functions for the engines may vary between different types and add more fuel (higher EGT) and/or increase the RPM or hold RPM and decrease flow rate that then equals less thrust

Not a big deal unless you are paying the end of the month fuel bill on a large fleet

Now some types of grundge (fine sand/dust and even lots of rain per month) will actually erode/wear the fine leading edges of blades. This could also affect the engine cycle and in some cases the fan blades themselves. It's all in the accumulated rates and time on wing.

Manufacturer's Service Bulletins should cover any concerns

[vapilot2004]

EGT is the primary indicator of engine health. The leakage caused by compressor blade and seal wear are the main causes of rising EGT as it relates to engine maintenance, which in turn reduces delivered air to the combustors. Typically, increased fuel flow will mirror the incremental reduction in EGT margins for a given thrust setting.

Dirt can cause problems in the turbine section, leading to wear either directly via abrasion, or indirectly due to clogged cooling passages causing higher local operating temps. For extreme cases, such as desert and dusty environments, the HP compressor is also prone to blade wear from the abrasive action of dirt and sand.

Other than volcanic ash events, dirt, FOD, and sand ingestion damage occurs on the ground during taxi, takeoff, and landing. From what I've been told, some operators observe a rigid cleaning schedule based on flight cycles, while others include EGT margin data in their scheduling. A logged event, such as sand ingestion or operation at certain known locations will also influence the intervals.