GE115 Engine question
Join Date: Feb 2005
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Usually there are two idle speeds for a turbine engine:
1) Ground Idle - a low idle speed for taxi ops where excess thrust is not wanted, to avoid undue brake usage. The engine will be slow to respond to a throttle advance, but it's assumed no rapid thrust increase is needed.
2) Approach Idle (sometimes flight idle) - a higher idle speed for inflight use, where rapid throttle response may be required. Usually it's selected by some combination of flap setting, u/c up/down, or w/o/w logic.
Certainly not unique to the 115.
1) Ground Idle - a low idle speed for taxi ops where excess thrust is not wanted, to avoid undue brake usage. The engine will be slow to respond to a throttle advance, but it's assumed no rapid thrust increase is needed.
2) Approach Idle (sometimes flight idle) - a higher idle speed for inflight use, where rapid throttle response may be required. Usually it's selected by some combination of flap setting, u/c up/down, or w/o/w logic.
Certainly not unique to the 115.
Join Date: Jul 1999
Location: Big Sky Country
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just to carry on from barit's explanation, another purpose is to reduce the spool-up time of the engine...eg. from ground idle to TO power might take 6 seconds, from flight idle to TO would take considerably less than that. Some engines eg. CFM56 also had issues with flaming out in heavy rain at idle settings, hence the introduction of flight idle for some of those models.
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Thanks for the responses so far chaps. Really appreciate it.
The background for the question is that when the EEC is in this approach idle mode, descending through cloud at approx altitude of 20Kft, an ice threat is detected by the engine - due to the TAT and altitude icing envelope (note: EAI valves remain closed as no actual ice has been detected). When the ice threat is detected, N1 increases slightly. Any idea why this happens? Something to do with the BAI valve perhaps?
Thanks again
Mc
The background for the question is that when the EEC is in this approach idle mode, descending through cloud at approx altitude of 20Kft, an ice threat is detected by the engine - due to the TAT and altitude icing envelope (note: EAI valves remain closed as no actual ice has been detected). When the ice threat is detected, N1 increases slightly. Any idea why this happens? Something to do with the BAI valve perhaps?
Thanks again
Mc
Join Date: Feb 2005
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I can think a possibility: It may be that the bleed air is not sufficient (or not hot enough) at normal approach idle to do the job, and the automatic idle "bump" could be to bring the temp. & pressure up a bit. On the other hand, simply maintaining the "bump" could degrade the plane's descent rate needlessly.
Just a guess.
Just a guess.
