Turbojet/turbofan powered airplane critical engines are any LH or RH outboard engines... Number 1 or 4 for a 747... Vmca-Vmcg computations are based on the loss of any such engines. The "critical engine" is strictly an aerodynamic consideration...
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Also consider another thing. Power ON or OFF stall speeds are the SAME in a jet airplane, since no propeller moves air over the wings, thereby no change in stall speeds.
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Hope this answers your question...
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Happy contrails -