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In a turbofan engine, when the thrust lever is at maximum and is then suddenly pulled back to idle, the engine will shut off and lose power.

How can this be the case as the engine should be at idle when the thrust lever is brought to idle? But in this case, the engine shuts off because of the sudden movement. What is the specific reason for this?

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The rotational inertia of the fan will continue to push air through the engine after the fuel supply has been reduced. Reduce the fuel supply too quickly and the result is like holding a match in a gale: the wind blows the flame out.

To prevent this, modern engines use digital controls (FADEC) to ensure that the engine is kept within its operating parameters regardless of what the pilot might do with the throttle.

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This was only a problem on the 1st generation of turbojets, where the fuel controller was effectively a needle valve controlled by the thrust lever and the "fuel control" was the pilot's brain. These engines also had the reverse problem; you couldn't "slam accelerate" them without overtemping and/or flaming out.

Once hydromechanical fuel controllers came about in the early 50s you could move the thrust lever all you wanted (within reason) because the lever provided a mechanical "command" and the fuel controller actually managed fuel flow based on continously measured engine parameters to achieve the commanded setting.

The next step from that was electronically trimmed hydromech fuel controllers in the 70s, then FADEC in the 90s. But it was only in the 40s and early 50s that you had to carefully move the thrust lever to that degree.

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