Discussion
A four-engine aircraft is re-engined with more powerful units at the same spanwise stations.
What is the most likely consequence for the empennage?
Answer: D
Correct answer: The fin and rudder may need enlarging for the engine-out case
Asymmetric yawing moment is thrust times the engine's distance from the centreline. The stations are unchanged but the thrust is greater, so the moment to be balanced after a failure grows in proportion.
Since the fin and rudder were sized against the original figure, the minimum control speed rises unless they are enlarged - which may be unacceptable against the take-off performance the new engines are meant to buy. The tailplane is unaffected, being sized by trim and stability rather than by thrust, and the tail arm is fixed by the fuselage.
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