A fan shaft includes a first end, a second end, and an axis, the fan shaft and configured to be coupled to a gear assembly of a gas turbine engine at the first end and to a fan more proximal to the second end than the first end such that in response to being coupled, the fan shaft can transfer torque from the gear assembly to the fan. At least one axial portion of the fan shaft satisfies the relationship 0.55 ≤ T * C J * τ ≤ 0.95 , where T represents peak torque during fan blade off, C represents a distance from a centerline of the gas turbine engine to an outer fiber of the fan shaft, J represents a polar moment of inertia of the fan shaft, and τ represents yield stress in shear of the fan shaft.