Originally Posted by
k.swiss
Some points to note:
- 10 minutes TOGA//takeoff-thrust is a hard limit for OEI, you can damage a good engine if you exceed. It is not wise even by a few minutes.
- If you encounter an engine failure on approach there is no EOSID, you have to follow the missed approach procedure. The question is good one, does it take into account the climb gradient to ensure obstacle clearance?
Rest I agree with you John, great analysis. Great question also from the poster very curious about this one.
What you say is almost the full story, but not quite, it arises out of §33.27.... and that is the way the world has been. Until now... the modification I am doing an STC on large engines, coming out of the TFE731 tests gives max rated TO thrust levels while the engine is below MCT RPM, EGT and fuel flow. (Turns out, there are nice things that can be done with aero).
Can you run the engine at full rating in an emergency for longer than 5 minute or 10 minutes? The qualification of the engine required demonstration to show there is zero damage in such a case, and if there was any damage, then the engine has to operate for another 5 minutes after the time that damage occurs. For the demonstration, it is not known and is not required to be known that the engine was run for more, so at that time limit, it can only be said that it is the required test point, after which, you are on your own. However, the test point is not at the RPM and EGT and thrust of TO/GA, it is at 120% RPM related to that point, and that is not a trivial matter; that is, your 100% N1 limit, the engine has to do the test at 120% N1, and that then means the thrust output is between 1.4-1.44x the rated thrust, the EGT will increase by the relationship it has to N2 (N3) and so has fuel flow, that is, your EGT will be far higher than the limit EGT that the engine has been tested to.
Will the engine fail at 5:01 at MAX TO/GA thrust? No. Will it need to be inspected, yes, it exceeded a limit. Some engines have a designated time exceedance inspection program, some do not. Coming off the stop is desirable though as you have one engine between your beer and being a glider.
watch the video on
www.delta-burn.com which shows a very simple but repeatable test. The ground test matches the flight test quite accurately.
So, in this case, the time limit for the thrust level which is predicated on the margins that arise from the § 33.27 (a) Turbine, compressor, fan, and turbosupercharger rotor overspeed. requirement. From that, the question is what can you run a modified engine at... as modified, we are always around 10% lower RPM and 140C lower EGT, and 30% TSFC better than when the engine came out of the factory. If that sounds odd, know that for the pleasure of paying for MSP GOLD, the engine in this video came back after overhaul with a ZERO EGT margin, thanks! In the video, it is putting out more thrust than the standard engine... so... what is it's max TO thrust time limit? It never gets to Max EGT, (not because of the efforts of an MSP GOLD MPI). If run at normal target RPM for the rated thrust, which is say, 98.1%, 3,700lbf, it gets 5,250lbf out of it, which is a nice bump but not for normal operations. When pulling the RPM back equal thrust occurs with a >9% lower N1, which is lower that normal cruise RPM, an EGT that drops by 140C, 740C vs 885C and so what is the time limit on the engine? Outcome? This engine gets sea level TO thrust to 9000' sea level max CLB to 16,000' and above that it has 2 times the excess thrust that the standard engine gives. Alternatively, and as they are my engines, they run slower, colder and cleaner, and with lower fuel burn, and the range of the aircraft goes up. 50% up.
The STC program for the first engines has started up, this is straightforward on the engine, it is an irritant for Boeing architecture, but mainly simple for the Airbus, and the EPR target aircraft. N1 target is fine for manual thrust systems, otherwise G/A can be interesting.
For a normal case, 33.27 gives the time constraints and the qualification that has to be met, it just happens that the engine STC that I am doing kind of messes with the mind somewhat.
