A gas turbine engine for an aircraft includes a fan system having a reverse travelling wave first flap mode, Fan RTW, and including a fan located upstream of the engine core; a fan shaft; and a front engine structure arranged to support the fan shaft and having a front engine structure nodding mode comprising a pair of modes at similar, but not equal, natural frequencies in orthogonal directions; and a gearbox. An LP rotor system including the fan system and a gearbox output shaft arranged to drive the fan shaft has a first reverse whirl rotor dynamic mode, Rotor RW, and a first forward whirl rotor dynamic mode, 1FW. The engine has a maximum take-off speed, MTO. A backward whirl frequency margin of: the ⁢ ⁢ lowest ⁢ ⁢ frequency ⁢ ⁢ of ⁢ ⁢ either ⁢ ⁢ mode ⁢ ⁢ Fan ⁢ ⁢ RTW ⁢ ⁢ or ⁢ ⁢ Rotor ⁢ ⁢ RW ⁢ ⁢ at the ⁢ ⁢ MTO ⁢ ⁢ speed the ⁢ ⁢ MTO ⁢ ⁢ speed may be in the range from 15 to 50%.