An aircraft gas turbine engine includes a fan system having a reverse traveling wave first flap mode, Fan RTW, and including a fan upstream of the engine core; a fan shaft; and a front engine structure to support the shaft and having a front engine structure nodding mode FSN including two modes at similar, but unequal, natural frequencies in orthogonal directions; and a gearbox. The engine includes a gearbox, and a gearbox output shaft to couple output of the gearbox to the fan shaft. An LP rotor system including the fan system and the gearbox output shaft has a first reverse whirl rotor dynamic mode, Rotor RW. A frequency margin of: ⁢ the ⁢ ⁢ frequency ⁢ ⁢ difference ⁢ ⁢ between ⁢ ⁢ mode ⁢ ⁢ ⁢ ⁢ FSN ⁢ ⁢ and ⁢ ⁢ the ⁢ ⁢ highest ⁢ ⁢ frequency ⁢ ⁢ of ⁢ ⁢ either ⁢ ⁢ ⁢ ⁢ synchronous ⁢ ⁢ Fan ⁢ ⁢ RTW ⁢ ⁢ or ⁢ ⁢ synchronous ⁢ ⁢ Rotor ⁢ ⁢ RW ⁢ the ⁢ ⁢ lowest ⁢ ⁢ natural ⁢ ⁢ frequency ⁢ ⁢ of ⁢ ⁢ ⁢ ⁢ the ⁢ ⁢ front ⁢ ⁢ structure ⁢ ⁢ nodding ⁢ ⁢ pair ⁢ ⁢ of ⁢ ⁢ modes is in the range from 5 to 50%.