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The Snecma-Larzac 04-C6 idling RPM ranges from approximately 53% on the ground to 85% at high altitude. Why is that?

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For one there is less resistance to the fan and compressor due to the decreased air density -> more rpm out of the given power. On the other hand, the mass flow needs to be high enough to sustain the operation of the engine -> both lead to a higher rpm...

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  • $\begingroup$ There is also the ram air effect in flight at altitude. $\endgroup$ – Michael Hall Oct 28 at 16:09
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Ground idle and in-fight idle are different. In-flight idle would move aircraft on the ground and would require a bit of brakes to keep the aircraft stationary. Additionally, landing roll would be longer. Therefore, ground idle activates on most jet aircrafts after touchdown. This activation is usually occurs after a small delay, taking into account that pilot may be doing a touch and go.

Of course there is also density effect on both ground and in-flight idle rpms. Both increase with altitude to provide some engine stall margin.

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    $\begingroup$ Would "ground idle" be different for an airfield at MSL vs one at MSL + 5000'? $\endgroup$ – FreeMan Oct 28 at 18:09

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