I remember flying in an Ansett Lockheed L188 Electra (civilian version of the P3 Orion) with the two inboard engines near idle speed. The reason was a design fault, subsequently rectified, but in the interim Electras cruised with the inboard engines at or near idle:
Three aircraft were lost in fatal accidents between February 1959 and March 1960. After the third crash the FAA limited the Electra's speed until the cause could be determined.
After an extensive investigation, two of the crashes (in September 1959 and March 1960) were found to be caused by an engine mount problem. The mounts were not strong enough to damp a phenomenon called "whirl mode flutter" (analogous to the precession of a child's top as it slows down) that affected the outboard engine nacelles. When the oscillation was transmitted to the wings and the flutter frequency decreased to a point where it was harmonic with the outer wing panels, violent up-and-down oscillation increased until the wings would tear themselves off.