This paper examines viscosity solutions to a class of fully
nonlinear equations on Grushin-type planes. First, viscosity
solutions are defined, using subelliptic second order
superjets and subjets. Then, a Grushin maximum principle is
proved, and as an application, comparison principles for
certain types of nonlinear functions follow. This is
accomplished by establishing a natural relationship between
Euclidean and subelliptic jets, in order to use the viscosity
solution technology of Crandall, Ishii, and Lions (1992). The
particular example of infinite harmonic functions on certain
Grushin-type planes is examined in further detail.